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Effects of biofouling development on drag forces of hull coatings for ocean-going ships: a review.

Abstract This review presents a systematic overview of the literature and describes the experimental methods used to quantify the drag of hull coatings. It also summarizes the findings of hull coating's drag performance and identifies the main parameters impacting it. The advantages and disadvantages of the reported methods listed in this review provide an assessment of the most efficient methods to quantify the drag performance of hull coatings. This review determines that drag performance of hull coating technology varies depending on whether the coating condition is newly applied, after dynamic or static seawater exposure. The summarized data reveal that, while several methods have attempted to quantify drag performance of hull coatings, other methods must be explored in order to accurately measure the long-term drag performance of hull coatings in conditions mimicking those that ship hulls encounter during actual voyages.

Keywords Skin friction, Fouling control coatings, Biofouling, Marine ship hull coatings, Test methods

Introduction

The fuel efficiency of ships is paramount, due to the high cost of fuel and the environmental concerns connected to fossil fuel consumption (e.g., emission of greenhouse gases, S[O.sub.2], and N[O.sub.x]). Commercial ships typically experience dry-docking intervals of 3-5 years, although extended dry docking of up to 7.5 years can be granted for certain larger ship classes [e.g., reference (1)]; during that time, substantial biofouling can occur. Biofouling is defined as the accumulation of micro- and macro-organisms, such as the settlement of bacteria, algae, slime, weed, or barnacles on man-made structures. (2) Biofouling adversely affects ships through the loss of speed, decreased maneuverability, increased fuel consumption and thereby increased emissions of harmful gases, increased frequency of dry dockings, and translocation of invasive species. Frictional resistance represents a considerable part of a ship's resistance. For example, frictional resistance causes 70-90% of the total resistance for slow trading ships (e.g., bulk carriers and tankers) and often less than 40% for faster trading ships (e.g., cruise liners and container ships). (3) The remaining resistance can be attributed primarily to wave-making. Air resistance above the waterline only makes up a minor portion of the total resistance, that is, typically 2% or less for slow trading ships and 10% or less for fast trading ships. (3) The hull coating has a high impact on the fuel consumption of ships. Milne [as cited in reference (4)] reported, for instance, that the world fleet could save a potential $720 million USD (value in the year 1989) in fuel costs due to the reduced frictional resistance of ships with the introduction of the so-called ablative hull coatings. An additional $2289 million USD could be saved due to the extended interdocking periods and, consequently, lower dry-dock costs. The introduction of a new, improved hull coating (in this case, the ablative coating) can largely benefit the economy and environment. It is, therefore, essential to improve the drag assessment tools of hull coatings in order to reduce fuel consumption. Despite the impact of hull coatings on fuel efficiency, environmental issues, and climate change, this topic did not gain much attention until the early 1990s, when an increase in publications took place. (5) Researchers have used various experimental methods to attempt to predict a hull coating's performance with regard to frictional resistance. Nevertheless, large uncertainties exist regarding the correct identification of the optimal coating and its impact on the frictional resistance of ships over time. This article focuses on the topic of hull coatings and their associated frictional resistance. Furthermore, it presents a summary of the most relevant experimental methods and research results regarding the frictional resistance of hull coatings. The article also explores the impact hull coatings have on fuel consumption, the main parameters that affect drag performance, the drag results for various types of hull coatings, and the methods and correlations used to predict a hull coating's drag impact on full-size ships. For more information introducing the topic of hull coating mechanisms and their impact on drag forces, see reference (6).

Drag performance of hull coatings

The primary role of hull coatings is to minimize the drag of the ship. Primarily, drag results from a combination of the coating's surface roughness, potential mechanical damage, and accumulation of biofouling (note that these parameters are interrelated). This section presents a short introduction to the most important hull coatings and their mechanisms with respect to reduction of friction and biofouling. In this article, the term hull coating refers to any type of underwater topcoat that has an antifouling potential (i.e., both fouling release and conventional biocidal antifouling coatings). The terms fouling control coatings or antifouling coatings are also evidenced in the literature to describe the entire coating category used for biofouling prevention.

An unprotected ship hull will become biofouled after a relatively short immersion time. (7) The development of biofouling drastically limits the speed and/or increases the fuel consumption. For this reason, mankind has tried to prevent or limit biofouling for centuries. Indeed, the oldest sources are dated to 700 B.C. (8) The two main hull coating technologies commercially available today are conventional biocidal antifouling coatings and the so-called fouling release coatings. For a thorough review of the principles and mechanisms of biocidal antifouling coatings, see references (2, 9, 10). For fundamental principles and mechanisms of fouling release coatings, see reference (5).

An optimal hull coating must meet strict requirements (11):

* Prevention (limitation) of biofouling, regardless of a ship's operating profile;

* Environmental soundness;

* Economic viability;

* Longevity;

* Strong adhesion with the underlying coating (tie-coat);

* Mechanical strength;

* Long-term durability;

* Low drag; and

* Specific target (i.e., targeted only for species that can attach to ship hulls).

Recent developments in the area of fouling release and biocidal hybrid antifouling coatings are excluded from the generic descriptions since they represent novel working mechanisms [e.g., references (12, 13)].

Principle of biocide-based antifouling coatings

The principle and mechanisms behind biocide-based antifouling coatings are fairly well characterized by the incorporation of active ingredients (biocides) into a film-forming organic matrix. Upon immersion, the active ingredients are released in a controlled manner to prevent or limit biofouling. Various kinds of biocide-based antifouling coating systems exist, such as control depletion polymers (CDP) and self-polishing copolymer (SPC) coatings, and they are often grouped according to the type of binder. This article excludes older biocide-based antifouling coatings such as soluble and insoluble matrix-coating technologies, as they have little market share for larger commercial ships.

Self-polishing copolymer coatings

SPC coatings are characterized by several distinct properties. (2) They exhibit a controlled biocide release rate, which allows a fairly constant leaching rate over time at constant seawater conditions. Also, the polishing rate typically increases linearly, not exponentially with speed. As a result, at low or no speed, a slow polishing rate will occur, providing biofouling protection. The polishing rate of SPC coatings can be up to 15 [micro]m per month, although they are often much lower at typical conditions for a traveling ship [e.g., references (10, 14)]. Another property is a thin and stable leached layer thickness established between two distinct moving fronts: the dissolving pigment, or leaching front; and the eroding polymer front. (9) The leached layer thickness is fairly stable and low, with values typically ranging between 10 and 20 [micro]m. (2)

The term self-polishing has, traditionally, been associated with a progressive thickness depletion confined at the outer surface of the SPC coating, which leads to a continuously renewed bioactive surface. (15) The exact definition has, however, been an issue of intense debate over the years. Almeida et al. state that the efficiency of SPC coatings has allowed for drydocking intervals of up to 5 years. (8) However, marine paint companies often claim efficiency up to 7.5 years for their best-performing SPC coatings. (16)

Control depletion polymers

CDPs, also known as ablative or erodible coatings, are similar to the traditional soluble matrix technologies. The difference stems from the fact that CDPs use synthetic organic resins in addition to rosin derivatives. Once in contact with seawater, the biocides dissolve together with the soluble binder, and the dissolution process-controlling ingredients are removed from the surface. A key difference between the CDP and SPC coatings is the fact that the binder dissolution mechanism is driven by hydration and dissolution, not hydrolysis. CDP technologies are effective for up to 36 months. (2)

Principle of fouling release coatings

The working mechanism of traditional fouling release coatings relies on a dual mode of action, that is, nonstick properties and a biofouling (fouling) release behavior. The principle of fouling release coatings is to use the exerting force of water against the hull of a traveling ship to remove or limit biofouling. Biofouling organisms have difficulty adhering to the smooth, low-energy surface; thus, either they move away to find a more favorable site for settlement or the seawater flowing over the hull removes them during the voyage. (17) Fouling release coatings are, therefore, particularly suited for ships that travel at sufficiently high velocities and remain in port for only brief periods of time. In contrast, these coatings are less likely to benefit ships, such as pleasure boats, traveling at low velocities and remaining in port for long periods of time.

Present fouling release coatings are based on cross-linked poly-dimethylsiloxanes (PDMS), and they usually contain oil additives to enhance their slippery properties. Fluoropolymer fouling release coatings can contain oils with fluorinated acrylate polymers, per-fluoropolyether polymers, and poly (ethylene glycol) fluoropolymers. (5) Studies have shown that surface properties do not solely dictate the fouling release properties, however; coating thickness also plays an important role in bio-adhesion. For example, if fouling release coatings are too thin (below [approximately equal to] 100 [micro]m dry film thickness), barnacles can adhere to the underlying coatings, thereby establishing firm adhesion. A thin film does not have the ability to absorb the cutting force of the barnacles as they attach to the surface, and once they penetrate down to the underlying hard primer, their adhesion strength increases greatly. (18) The efficacy of fouling release coatings relies solely, if sufficiently thick, on the special nature of the surface. As a result, one drawback of current fouling release coatings is their susceptibility to scraping or gouging during for instance mooring, due to their elastomeric nature. Conventional biocidal antifouling coatings suffer the same fate, but since they do not rely solely on their surface properties for their efficacy, the issue is not considered to be as significant. (18)

Summary

Table 1 displays the active lifetime, limitations, and advantages associated with biofouling protection and drag of hull coatings used in today's commercial market.

Table 2 presents a distribution of the volume of applied fouling release coatings and conventional biocidal hull coatings for the year 2011. Presently, the use of fouling release coatings is fairly limited, covering about 6.0% of the market. However, the market share of fouling release coatings is increasing, especially in the sector of the marine market comprising larger industrial ships with larger fuel consumptions. (19) Furthermore, the volume of coating needed per area for fouling release coatings is typically less than that needed for conventional biocidal antifouling coatings. Consequently, the percentage of square meters of fouling release coatings would be higher than the volumetric percentage shown in Table 2.

Fundamental fluid mechanics related to flow of water over ship hulls

Frictional forces act against the forward direction of any moving object, relative to the fluid velocity. In other words, frictional forces arise due to the fluid flowing over the surface. The properties of a hull coating system are, therefore, essential for the friction of a ship, as they impact the forces acting in opposition to its forward motion. Typically, the drag forces from seawater are divided into form drag forces and skin friction (i.e., frictional forces or frictional drag). In the field of hull coatings, the objective is to minimize the drag caused by water flowing over the surface (skin friction).

Drag on a ship hull

Fluids flow over rough and smooth surfaces in a wide variety of situations, in both natural environments and engineering applications (e.g., vehicles, pumps, wind turbines, pipelines, and ships). As a result, extensive research has been devoted to the drag forces induced by flow over surfaces with varying roughness. In the case of a ship, the theoretical effective power (towing power), PE (in Watt), necessary to move a ship through water is, equation (1):

[P.sub.E] = [F.sub.T]U = 1/2[rho]][C.sub.T][U.sup.2]S, (1)

where [F.sub.T] is the total resistance, U is the speed of the ship, [C.sub.T] is the total resistance coefficient, S is the wetted hull area, and [rho] is the seawater density.

The effective power necessary to move a ship through water is proportional to the ship's speed cubed and is also proportional to the total resistance coefficient. An effective (and popular) way to decrease fuel consumption over a given distance is, therefore, to decrease the speed of the ship. However, this increases the traveling time. The ship powering requirement in terms of shaft power (SP) is impacted by the overall efficiency of the propeller and shafting, equation (2):

SP = [P.sub.E]/PC, (2)

where PC is the propulsive coefficient, representing the overall efficiency of the propeller and shafting. (20) The value of the propulsive coefficient usually ranges between 0.5 and 1 [derived from reference (3)].

The total drag coefficient consists of a form drag resistance coefficient--CR, an air resistance coefficient--CA, and a skin friction coefficient--[C.sub.F], equation (3):

[C.sub.T] = [C.sub.R] + [C.sub.F] + [C.sub.A]. (3)

The form drag results from wave and wake-making, while the skin friction is due to the tangential shear stress on the ship's hull, caused by the fluid flowing over it. The air resistance force, [F.sub.A], is given by equation (4):

[F.sub.A] = 1/2[rho][C.sub.A][U.sup.2][A.sub.air], (4)

where [A.sub.air] is the cross-sectional area of the ship above water. (3)

The form drag force, [F.sub.R], is given by equation (5):

[F.sub.R] = 1/2[rho][C.sub.R][U.sup.2]S. (5)

The skin frictional force, [F.sub.F], is given by equation (6):

[F.sub.F] = 1/2[rho][C.sub.F][U.sup.2]S (6)

The total drag coefficient for a hull, [C.sub.H], is equivalent to equation (3), excluding the air resistance, (21) equation (7):

[C.sub.H] = [C.sub.R](Fr) + [C.sub.F](Re). (7)

The form drag is a function of the Froude number, ([C.sub.R](Fr)), defined as, equation (8):

Fr = U/[square root of gL], (8)

where g is the gravitational acceleration and L is the water line length of the ship.

The Froude number is used to determine the form drag resistance of ships with different lengths and velocities. A high Froude number indicates a large form drag. The skin friction, in turn, is a function of the Reynolds number, ([C.sub.F](Re)), defined as, equation (9):

[Re.sub.s] = UL/v, (9)

where v is the kinematic viscosity.

The Reynolds number of a ship relates to its surface friction, so a large Reynolds number results in a high surface friction. The opposite is true for the skin friction coefficient, which typically decreases slightly when the Reynolds number increases. Equation (8) clearly shows that, as the Froude number decreases (i.e., the form drag decreases), the length of a ship increases when the speed is constant. Equation (9), in turn, reveals the fact that the Reynolds number increases (i.e., the surface friction increases) when the length of a ship increases, if a ship is maintaining a constant speed. (22) In general, a larger ship has a larger total resistance than a smaller ship. However, when the total resistance divided by the length of the ship is evaluated, the resistance per hull meter decreases as the ship increases in length. Therefore, the use of larger ships is beneficial when moving goods, as the power per mass of goods decreases compared to smaller ships. Practical constraints such as the size of the Suez Canal and the structural integrity of larger ships do, however, limit the size of ships. (23)

The relative contribution to the total resistance from the form drag and skin friction of a typical naval ship is shown in Fig. 1. At low to moderate velocities (Fr < 0.25), the skin friction is the largest component of the total drag. (24) At higher velocities, however, the form drag, designated by the sum of form and wave-making in Fig. 1, becomes dominant. Figure 2 shows the resistance in tons as a function of speed (knots) for the destroyer Yudachi. Noticeably, both the skin friction (frictional resistance in the figure) and form drag (residual resistance in the figure) increase with increasing speed, although the skin friction increases relatively slowly, as opposed to the form drag.

In short, the hull resistance is the summation of the forces in the direction of motion of the tangential stresses that the water produces on the hull; apart from the viscosity and density of water, the hull resistance primarily depends on the area, length, and roughness of the wetted area of the hull, as well as the speed of the ship. (26) It is significant to note, however, that an increase in mechanical roughness or biofouling exerts a greater relative influence at low to moderate velocities than at high speed. (20) Since mechanical damage or biofouling causes an increase in skin friction, leading to a higher total drag, the development of low-friction hull coatings is essential, especially the development of hull coatings that can prevent or limit biofouling at these lower speeds where marine organisms can attach more easily.

The operation of ships at maximum speed can be estimated from a ship's Froude number. Most conventional ships do not have sufficient power to operate at velocities associated with a Froude number greater than 0.4. (27) A ship equipped with the power to operate up to a Froude number of 0.4 and having a water line length of 300 m would, therefore, have a maximum moving speed of approximately 22 m/s ([approximately equal to]40 knots). Figure 3 provides an example of a wake (at stern) and wave (along hull) pattern, which is responsible for the form drag, for a model of the KRISO container ship (KCS) with a Froude number of 0.26. (28) The Cartesian coordinates (X, Y) are used to display the data, where X denotes the downstream direction, and Y the starboard. The origin of the coordinates is located at midship on the free surface. All the coordinates are non-dimensionalized by the length between perpendiculars of the model ships, that is, the approximate water length of the ship.

Figure 4 presents a schematic flow regime along a ship hull which determines the magnitude of the skin friction. From the bow and along the first part of the hull, the flow will be laminar, followed by a short transition region which, further downstream, develops into a highly turbulent region with a steadily increasing boundary layer thickness. The turbulent flow region covers the vast majority of a hull at typical traveling velocities. Turbulent wake flows start occurring at the separation point, a certain distance before the stern.

The form drag and skin friction are the primary resistances responsible for the fuel consumption where only the skin friction is significantly affected by biofouling on the hull surface. The increased annual fuel cost of a fouled hull compared to a clean hull can be estimated. Several factors must be assumed: (a) the form drag, air drag, and skin friction contributing to 28, 2, and 70% to the total drag for a clean hull of a bulk carrier, respectively; (b) for a biofouled hull, the form and air drag remains constant, while the skin friction increases by 40% due to biofouling (3); (c) a bulk carrier in clean condition uses 30 tons of fuel oil per day when traveling (29); (d) the fouled and clean bulk carriers travel for 80% of the year; and (e) the fuel oil price for both carriers is $600 per ton. Given these assumptions, the price of fuel oil per year for a bulk carrier with a clean hull would be 10.51 million dollars, and the price of fuel oil per year for a bulk carrier with a fouled hull would be 11.98 million dollars per year. Therefore, based on this example, a fouled hull costs an additional 1.47 million dollars annually, compared to a clean hull. Clearly, this example demonstrates the necessity of developing and applying hull coatings that limit or prevent biofouling.

Parameters impacting total drag

From a design point of view, the ship design (ship geometry), propulsion system (propeller and engine), and surface condition--which is related to the choice and condition of the coating system--are the three main parameters that influence the overall fuel efficiency of a ship. For newly built ships, all three parameters must be considered in order to minimize fuel consumption. For existing ships, the hull coating system is the only one that usually influences their efficiency, because ship geometry and propulsion are rarely subject to change after a ship has been built. Therefore, apart from retrofitting, the only means to enhance fuel efficiency for the existing fleet is by reducing the skin friction. The main parameters identified to affect skin friction are as follows:

* Speed;

* Seawater parameters (i.e., temperature and salinity of seawater);

* Weather parameters (i.e., wind, waves, and currents); and

* Coating surface conditions (e.g., the presence of biofouling, roughness, and potential mechanical damage).

The following sub-sections briefly detail the influence of the first three parameters with respect to skin friction, while the coating surface conditions are addressed in the "Impact of the coating surface condition on drag forces" section.

Speed

A ship's speed largely impacts (a cubed dependency) its necessary towing power, as noted earlier. However, the speed of a ship also influences the degree of biofouling. The speed distribution (e.g., maximum speed, average speed, frequency of idle periods, and duration of idle periods) influences the degree of biofouling development and, subsequently, the skin friction over time. Idle periods will cause more biofouling, compared to periods when a ship is traveling. In fact, the fouling organisms normally encountered on ship hulls cannot colonize on ships traveling at velocities above 4-5 knots. For this reason, it is considerably easier to protect the hulls of high-speed ships and those that are rarely in port than the hulls of slower, more idle ships. (8)

Temperature of seawater

The temperature of the seawater is a critical parameter because fluid properties and biofouling activity change with temperature. Table 3 displays the dynamic viscosity and density of standard seawater. (30) As the temperature increases from 1[degrees]C to 30[degrees]C, the dynamic viscosity of water reduces by more than 50%, while the density decrease is very limited ([approximately equal to]0.5%). Lackenby stated that, for a smooth turbulent flow, the net effect is a reduction in resistance of approximately 2.5% for a rise in temperature of 10[degrees]F (=5.6[degrees]C), which is due primarily to changes in the viscosity. (31)

Shear stress consists of viscous and turbulent shear stress. The viscous shear stress is proportional to the dynamic viscosity, whereas the turbulent shear stress is proportional to the density of the fluid. The density of seawater changes only slightly within the temperature range expected to be encountered by ship hulls. Ships typically operate in the transitional or rough regime, causing the turbulent shear stress to be much greater than the viscous shear stress. Therefore, the temperature changes impacting the turbulent shear stress are of minor importance. Furthermore, since the viscous shear stress contribution is relatively small in the transitional or rough flow regime, the temperature changes that would affect it are of little significance to the total shear stress. It must be stated that this relationship is complex and the dependency of the total shear stress on temperature must be verified experimentally. However, it is well known that the total shear stress will decrease slightly as temperatures increase.

The temperature is also important for ship performance over longer periods of time because it affects various coating properties, such as the rate of chemical reactions in the binder and the dissolution rates of biocides. (9,10) In addition, a high temperature typically leads to higher biofouling intensity. The temperature is rarely considered in the performance analyses by ship owners or operators, although the temperature does have some influence on drag. (32) The lack of interest in temperature influence on drag might also be because it is given by the traveling route and therefore not subject to be changed by design.

Salinity of seawater

Capurro defined salinity as "the total of solid materials in grams in 1 kg of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine, and all organic material completely oxidized." (33) In other words, the concentration of the dissolved salts is designated as a single solute. The salinity of the world's oceans is fairly uniform, with an average of approximately 3.5 wt%, corresponding to 0.6 M. However, local variations in the salinity may exist, for instance, near river mouths. The relative amounts of various salt components in seawater are practically constant, with NaCl being the most abundant compound. (33) The fluid properties affected by salinity are density and, therefore, viscosity, (30) which influence the drag of an object moving through water. Since larger ships are expected to spend the vast majority of their time in oceans, where the salinity does not differ substantially from 3.5 wt%, the small variation in salinity is not expected to influence the total drag over longer periods. The salinity does, however, affect self-polishing rates and biocide release rates and, consequently, the ability to prevent or limit biofouling. It was, for instance, found that when the salinity decreases from 0.55 M to 0.28 M, the leached layer thickness decreases from 7.2 [micro]m to 4.9 [micro]m in steady-state conditions for an SPC coating. Furthermore, that same decrease in salinity corresponds to a decrease in the release rate of [Cu.sup.2+] from approximately 120 [micro]g/([cm.sup.2] x day) to 45 [micro]g/([cm.sup.2] x day). (10) The influence of salinity on drag for fouling release coatings has yet to be investigated in depth. Fouling release coatings are, ideally, chemically inert; therefore, the influence of salinity is expected to be of minor importance, as the chemical surface structure is expected to remain identical, even in cases of varying levels of salinity typically encountered by traveling ships. From a drag point of view, the variation in salinity has not been given much attention, most likely due to the parameter's small variation as well as the fact that the salinity is given by the traveling route and therefore not subject to be changed by design.

Weather parameters: wind, waves, and currents

Wind, waves, and currents vary along traveling routes. Although they influence drag, the parameters are not subject to design influence. Weather routing of ships can, however, be used to establish the shortest time route or the most economical route by applying available information about the weather conditions with regard to the wind, waves, and currents [e.g., reference (34)]. However, because the performance of hull coatings has little, if any, significant impact on the drag arising from wind, waves, and currents, these parameters are not factors worthy of consideration in the hull coating design process. For an introduction to the topic of wave resistance, see references (35-38).

Experimental setups for determination of skin friction

Full- and small-scale measurements are used to determine the drag impact of the parameters listed in the "Parameters impacting total drag" section. Experimental setups applied to measure skin friction on hull coatings described in the following sections include rotating disks, rotating cylinders, towing tanks, water tunnels, static and dynamic panel exposure tested on a boat, pipes, and optical methods. The task of accurately converting small-scale skin friction coefficients ([C.sub.F]) into full-scale ones for either clean or biofouled hull coatings is very complex; in fact, to the authors' knowledge, no small-scale test has ever been conducted with subsequent full-scale testing. Furthermore, even if the conversion of small-scale results was considered accurate for biofouled hull coatings, the actual fouling pattern on a ship would likely be much more heterogeneous, which would complicate the up-scaling. Differences in the intensity and type of biofouling over a hull can be caused by various factors, including light-shade effects, larval zonation in the water column, differences in the flow patterns and stresses along the hull, and the color of the coating system. (39) The drag comparison of hull coatings via small-scale measurements can, however, be useful, as they can indicate differences in fuel-saving capabilities, although estimations may be linked with some uncertainty. Small-scale measurements are advantageous because they are more economical than full-scale experiments; they allow experiments to be conducted faster and they are capable of varying only one or a few parameters at a time, which is useful when determining the hull coating's drag performance. Static experiments have been conducted where, for example, flat plates and disks have been immersed in oceans or rivers with subsequent "true" biofouling. However, although relevant, such methods only realistically resemble a static body, such as a stationary ship at port conditions, without consideration of the mechanical shear stress encountered by a moving ship. Zargiel and Swain stated that dynamically grown biofouling, which better mimics ship conditions, should be used when addressing biofouling's influence on drag, because static and dynamic exposures differ in the diversity of the species and the mass of biofouling attached to hull coatings when compared at otherwise similar conditions. (40)

Rotating disks

The friction disk machine (FDM) has been used by several authors to measure the drag induced by surfaces [e.g., references (41-44)]. Granville used rotating disks to determine the skin friction coefficient of rough surfaces. (41) The indirect method described involves measuring the torque and rotations per minute (RPM) of a rotating disk uniformly covered with the roughness under investigation. Figure 5 illustrates an FDM, including the most important parts therein. The FDM used by Holm et al. employed disks of 22.86 cm in diameter and 0.3 cm in thickness. (43) The disks were immersed in a cylindrical test chamber of 25 cm in height and 33 cm in diameter. The disks were then spun from 700 to 1500 RPM, with changes of 200 RPM. The hull-coated disks were statically exposed for a time period ranging between 21 and 24 days. In all cases, the hull coatings experienced an increase in drag. An ablative antifouling coating resulted in the lowest increase, 9%, whereas three fouling release coatings experienced increases of 17%, 27%, and 29%, based on their skin friction coefficient development (Fig. 6). (43)

As depicted in Fig. 7, along the disk surface, three regions can be distinguished: a laminar flow region, a transient region, and a turbulent flow region.

Several authors have successfully applied the FDM to correlate roughness with drag. An interesting feature of the FDM is that the shear stress on the investigated disk changes as a function of the radius. The FDM test method will, therefore, determine results for the investigated surface with different shear stresses applied, as opposed to, for example, the sides a rotating cylinder in a Coutte flow (see "Rotating cylinders" section). This can be considered an advantage because it will allow for the evaluation of the drag performance of the hull coating under a wide range of shear stresses. The FDM is also beneficial because the coating layer is simple to apply due to the flat geometry and the drag for the investigated surface can be quickly measured. Furthermore, a visual inspection is possible for an investigation of the surface. The varying shear stress could, however, be considered a limitation because it prevents the application of a uniform shear stress to the disk surface. The shear stress in the center of the disk is small, and it increases as the radius increases. Additionally, the constrained flow in a tank could potentially cause uncertainties regarding the actual speed of the disk relative to the fluid. This uncertainty can be limited by either a setup with unconstrained flow (e.g., a very large tank), or the determination of the swirl factor, which accounts for the wall effect.

Rotating cylinders

In order to measure the drag induced by surfaces, several authors have used a rotary setup, in which an inner cylinder rotates inside a static cylinder [e.g., references (45-49)]. A rotating cylinder system and its most essential parts are shown in Fig. 8. In this case, the inner (rotating) cylinder, where test samples were applied, had a diameter of 30 cm and a height of 17 cm. The diameter of the outer, static cylinder was 38 cm, and its height was 17 cm. The water tank's volume was 400 1, and its temperature could be controlled using a heat exchanger. The tangential velocity of the cylinder surface ranged between 10 knots and 35 knots for the experiments conducted by Weinell et al. (49)

Weinell et al. determined the skin friction coefficient of surfaces by means of a rotating cylinder. (49) This indirect method described involves measuring only the torque and the RPM of the cylinder under investigation. The rotating cylinder setup has proven to be effective in measuring differences between the resistance of smooth and rough surfaces; indeed, this type of setup is used in the hull coating industry and at scientific institutes [e.g., references (45, 49)]. The advantages of the apparatus are its low operating cost, easy maintenance, compact size, and simple construction. In contrast to friction planes (e.g., flat plate tests in towing tanks), the rotating cylinder setup offers a distinct advantage because it does not suffer from complications associated with the development of a boundary layer along the length of a test section. One disadvantage of the rotor is caused by the end effects, which occur at the top and bottom, where the flow regime makes the transition from "cylinder" to "disc" flow. A further disadvantage could potentially be related to the difficulties of obtaining the same hull coating smoothness as that of plane surfaces, which bear a closer resemblance to hull surfaces than cylinders do. The rotor setup could therefore possibly measure a rougher surface than flat plates would measure. (45) Furthermore, the rotating cylinder system produces results that are difficult to interpret accurately, mainly due to the formation of the TaylorCouette flow. The term Couette flow describes the flow between two surfaces that are in close proximity, such that the flow is dominated by viscous effects, and inertial effects are negligible. When considering flow in an annulus, the Navier-Stokes equations can be solved exactly by analytical techniques, but when subject to a number of significant assumptions, particularly that of laminar flow, the application of the resulting solution is severely limited. In practical applications, there is a need to quantify the power required to overcome the frictional drag of a rotation shaft at angular velocities, which far exceed the velocities at which laminar flow occurs. When it is not possible for the radial pressure gradient and the viscous forces to dampen out and restore changes in centrifugal forces caused by small disturbances in the flow, the fluid motion is unstable, resulting in a secondary flow, named the Taylor-Couette flow. Although the Taylor-Couette flow occurs at sufficiently high rotational velocities, it can also occur at lower rotational velocities if the gap between the cylinders is too large. The interpretation of the Taylor-Couette flow's impact on the skin friction coefficient is complex; although it already has been subject to a vast number of scientific studies, further studies are needed in order to determine its impact (Fig. 9). (52)

Towing tanks

Of all the test facilities, towing tanks are one of the most frequently employed to estimate ships' resistance to motion. Indeed, many authors have used towing tank tests to investigate friction [e.g., references (53, 54)]. Essentially, the setup consists of a long, open-topped tank of rectangular cross section containing water. A rail runs along the top of each side of the wall for the total length of the tank. A carriage, supported by wheels running on the side rails, usually spans the water. The carriage, to which either a flat plate or ship geometry is attached, can be driven either by motors situated "onboard," driving the wheel, or by a motor located next to the towing tank driving a long cable, fixed to the carriage. Between the carriage and the flat plate or ship geometry, a drag force gage is placed to measure the drag forces in the longitudinal direction. Figure 10 provides an example of a towing tank with a ship model. Typically, a flat plate is used when determining skin friction, and a ship model is used when determining the form drag of a specific ship geometry.

There are several advantages in using a towing tank with a flat plate. For instance, the flow over a flat plate has been studied a great deal; therefore, many correlations exist for this geometry. Furthermore, a flat plate, as compared to a profiled body, has a minimal form drag effect (wake and wave); thus, these effects are reduced to a minimum causing the effect of skin friction to stand out. Compared to a profiled body, a flat plate more accurately simulates large ship hulls with respect to friction. On a flat plate, there are no pressure gradients parallel along the flow direction, affecting the boundary layer. Plates are relatively cost-effective, easy to manufacture, and simple to handle. Finally, hull coatings can easily be applied to a flat plate. However, experiments carried out in a towing tank also do suffer some drawbacks. A long tunnel is, for instance, needed to obtain reliable results, due to the development of steady flow scenarios. The development of a boundary layer along the length of the test section will only form as the initial part of a hull, without including the effect from further downstream of the hull. Applying roughness at the leading edge can, however, provoke a turbulent flow, thereby mimicking flow conditions further downstream of a hull. Hydrodynamic similarities between the size of the models and the ships are widely applied [e.g., reference (20)] to minimize this drawback, although uncertainties in the up-scaling are bound to occur. Finally, end effects at the edge of the plate, where the flow regime makes the transition, cause changes to the local skin friction coefficients at the edges. However, a wider plate results in a less significant transition influence from the edges.

Water tunnels

Water tunnels are widely used and, in principle, they provide identical results to the towing tank. Towing tanks necessitate the movement of the test section through the water, whereas a water tunnel requires only that water move over the test section. Barton et al. provided an example of a water tunnel used to measure drag, shown in Fig. 11. (56)

The test setup consisted of a closed loop, recirculating water in a tunnel built specifically for the controlled and detailed measurement of local skin friction, drag, and general boundary layer research. A number of components are needed to be specially designed to achieve controlled and uniform flow conditions within the working section, which is where the friction and flow properties were measured. The bulk flow velocity through the working section of the tunnel ranged from 0.3 to 2 m/s, yielding a Reynolds number ranging from approximately 5 x [10.sup.5] to 2 x [10.sup.6] [equation (9)] with the length parameter being the length of the working section. (56) In the water tunnel setup provided by Barton et al. (2011), the working section was 220 cm long, 20 cm high, and 60 cm wide. Additionally, the test surface within the working section measured 99.7 cm long, 59.7 cm wide, and 0.3 cm thick.

A water tunnel offers several advantages, compared to a towing tank. It is often much smaller, and it allows for the incorporation of components that will obtain a controlled flow, such as a honeycomb structure, thereby creating a laminar flow, even at high velocities. Furthermore, optical equipment (see "Optical methods for drag measurements" section) can be installed at a fixed position in order to investigate the flow patterns for the hull coatings under investigation. The relatively small working section and relatively low Reynolds number are among the few disadvantages of using a water tunnel when compared to a towing tank.

Static and dynamic panel exposure tested on a boat

The setup described in this section was developed by Swain et al. at the Florida Institute of Technology, USA. (57) The test consists of a combined static and dynamic exposure of panels followed by drag measurements using a small boat. Panels measuring 25 x 30 [cm.sup.2] were exposed in natural seawater conditions at the static immersion site located in the Indian River Lagoon, FL. Figure 12 depicts the dynamic test setup, where up to 4 panels could be exposed simultaneously. A rotating stirrer was installed in the bottom of a deep circular tank, measuring 5 ft in diameter and 4 ft deep. The stirrer was maintained at a rate of 60 RPM, resulting in a velocity of water of approximately 5 m/s moving over the panels.

Figure 13 illustrates the hydrodynamic test facility, consisting of a wet well built into the aft section of the hull of a modified, 9 m Chris Craft Commander.

The boat was capable of testing at velocities of up to 17 m/s and local Reynolds numbers of approximately 5.5 x [10.sup.7]. The test panels were statically and dynamically exposed prior to drag testing. They were mounted on a through-hull instrument. The instrument was equipped with a floating element force gage that measures drag forces on the hull coating; it is also equipped with a video camera, which could observe the hull coating surface condition and biofouling communities (Fig. 14). Figure 14 displays a drag meter installed in the well of a boat, where drag forces are applied to the test panels.

In the study by Swain et al., the coatings were subjected to one cycle of 60 days' static immersion and 15 days' dynamic immersion. (57) One important advantage of this setup is that the panels were exposed to natural seawater during every part of the test. The static immersion resembled the conditions a ship is exposed to in port, while the dynamic exposure, providing water velocities of up to 5 m/s (=10 knots), resembled the shear stress some ships experience along their hulls, although it can be argued that the velocity is an insufficient representation of the majority of the larger commercial ships. The drag measurement facility could provide velocities of up to 17 m/s (=33 knots), providing a wide range of dynamic testing possibilities. The setup was also capable of varying the static and dynamic immersion periods, resembling in-port and traveling conditions, respectively, which offers a large degree of flexibility. This method provides an excellent side-by-side comparison, as the panels are exposed to very similar conditions, which is useful when determining the optimal choice of hull coating.

Pipes

Flows in pipes are relevant to the hull coating industry because resistance in pipes is related to surface properties and, therefore, related to the frictional resistance of hull coatings. Experimental tests and flow modeling in pipes have been conducted by several authors [e.g., references (58-60)]. Figure 15 shows an example of a pipe system used to determine the skin friction of hull coatings, as described by Leer-Andersen and Larsson. (59) The equipment consisted of a tank, which was a large vertical PVC cylinder; a cylindrical pipe connected smoothly to the tank; and a pumping system capable of pumping water from a large basin into a tank. The pipe test section could be split axially in order to coat its inside and join it afterward. Before the pressure drop of the 1.5 m long test section was measured, 3.0 m of coated pipe was used to attain a fully developed flow. The pipe in the setup had a diameter of 6.7 cm. Leer-Andersen and Larsson (2003) measured the skin friction coefficients for three types of coated surfaces, known as fiberflocs, and ten coated surfaces with barnacles. (59)

There are several advantages to the test setup used by Leer-Andersen and Larsson. 9 The setup is simple with minimal construction and maintenance cost. Large increases in the skin friction coefficient can be measured quickly and accurately via simple pressure drop measurements. Furthermore, extensive literature exists regarding the flow in pipes related to mechanical roughness. However, some disadvantages of the test setup have been identified by Leer-Andersen and Larsson. (59) For instance, in this setup, the outlet is too close to the test section. A fully developed flow is not attained at the test section because the pipe length is too short. A poor alignment of the pressure taps influences the differential pressure gage measurement causing skin friction coefficient uncertainties. A non-uniform pipe diameter causes pressure differences and, therefore, uncertainty. Additionally, accuracy of the measuring equipment is limited, due to low diameter tolerance. PVC material was chosen partly because only pipe material that can be split and joined is applicable. The tolerance of the PVC pipes used was 0.6 mm and the diameter was 6.7 cm; a substantial portion of the uncertainty in the measurements can be explained by the diameter tolerance. Finally, a reduction in the available tube cross section was evidenced due to biofilm, barnacle, or other material accumulation. General disadvantages occur for hull coatings, with regard to any pipe setup such as the closed system creates difficulty with the examination (e.g., roughness, biofilm growth, and the velocity profile inside the pipe); the hull coating cannot easily be applied in the same manner as on a ship or flat plate, which causes uncertainties in the upscale to full-scale or the comparison with other experiments where flat geometries are used.

Optical methods for drag measurements

Laser Doppler Anemometry (LDA), also known as Laser Doppler Velocimetry (LDV), is a non-intrusive technique used to measure fluid flow patterns in a high resolution. One advantage of LDA is the fact that precise and reliable flow properties such as boundary layer measurements can be obtained and then used to determine frictional drag over hull coatings. Furthermore, measured flow patterns combined with surface roughness might prove useful to discerning the relationship between surface roughness and drag. The fundamental principle of LDA revolves around the Doppler Effect, which causes changing wave frequencies for a moving body relative to a stationary body. Particles that scatter laser light (e.g., polystyrene in water, with a typical size interval between 5 and 100 pm) can be immersed in a fluid, and then the reflected light can be used to obtain the velocity profile. If three different lasers are applied, all three velocity components can be obtained. Although several authors have employed LDA to investigate surface friction [e.g., references (61-63)], its application has only recently been applied to determine hull coating's drag performance. In fact, Candries and Atlar stated that, to the best of their knowledge, theirs work was the first publication in open literature regarding LDA measurements on hull coatings. (61)

An LDA setup has several advantages. It allows systems involving complex flows to be easily studied, as the method is non-intrusive. It also prevents flow disturbance, as compared to a pitot-tube, for example. Furthermore, a vast amount of information regarding the flow can be obtained (e.g., boundary layer thickness, displacement thickness, momentum thickness, and instantaneous velocity components in all three directions). The LDA setup also allows very localized measurements of the mean velocity and fluctuating flow properties to be determined, even for a turbulent flow. The flow measurements are extremely precise with LDA, although some sources of errors do exist, as with any measurement technique. One main disadvantage of the LDA setup is the high cost of its commercial instruments. Also, LDA only measures velocity in smaller volumes of fluid; thus, larger flow patterns are impossible to detect. Table 4 presents the advantages and disadvantages of LDA for the determination of hull coating's drag performance. Hopefully, this technique can be applied in the near future to further enhance the knowledge of the drag performance of hull coatings.

Summary

In short, the ideal properties of a method to determine the drag performance of hull coatings include:

* Accurate skin friction coefficient determination;

* Correct and feasible conversion to full-scale;

* Potential short- and long-term drag performance determinations;

* Investigation of several hull coatings under identical and ship-like conditions;

* Low cost;

* Easy use;

* Low maintenance and manual work; and

* Reliable.

Every method of determining the drag performance of hull coatings has advantages and disadvantages. No clear definition of the best method exists, as each method can be viewed from various perspectives; nonetheless, Table 5 indicates advantages and disadvantages of the methods explored.

Impact of the coating surface condition on drag forces

The results found in the literature from both full-scale ship trials and small-scale tests document that biofouling and mechanical damages lead to significant increases in the frictional drag [e.g., references (42, 49, 64)]. Skin friction caused by the hull condition can be divided into three main categories (Fig. 16): smooth, mechanically rough, and biofouled (micro-fouled and macro-fouled). A smooth hull condition is typical after a dry-dock operation; the duration of this condition varies, however, depending on the quality of the applied hull coating and the conditions it encounters during operation. The boundaries between smooth (unfouled), micro-biofouled, and macro-biofouled are vague, leading to unclear definitions.

Mechanical roughness is a relatively rigid surface roughness, which can occur due to structural roughness (welds and plate waviness), a poor coating application (e.g., overspraying, runs, and sagging), mechanical damage, corrosion (pitting and rust), and coating failures (e.g., blistering). Biofouling is substantially different than mechanical roughness in both origin and form; it is caused by the accumulation of microorganisms, plants, or animals on man-made structures, such as a hull or other subsea structure. Biofouling typically consists of complex biological species from the following main categories: bacteria, diatoms, tunicates, bryozoans, tubeworms, algae, barnacles, and mussels. Micro-fouling forms the primary film of biofouling on subsea structures. The life forms responsible for micro-fouling generally consist of bacteria, protozoa, fungi, and diatoms (Fig. 17). Macro-fouling, on the other hand, occurs when biofouling organisms that are sufficiently large to be seen with the naked eye attach to subsea structures. These usually consist of green, brown, or red algae, or animals such as barnacles (Fig. 18).

Biofouling, and in particular marine biofilms, is quite different compared to mechanical roughness, as they are neither uniform nor rigid. They consist of microbial cells that attach to a substrate and then proceed to grow, reproduce, and synthesize extracellular polymeric substances (EPS) dominated by polysaccharides. (65) Their coverage ranges from fairly uniform to patchy, and the scale of the component organisms ranges from bacteria ([micro]m) to filamentous algae (cm). The complexity of marine biofilms is obvious. A surface coated with a heterogeneous, compliant polymer with entrapped organisms is much more complex than a homogeneous sand roughness.

The frictional drag increase due to biofouling depends on many parameters, including the type of organisms, their location on the ship, the percentage of the hull covered with biofouling, the type of ship, and the ship's speed. Likewise, the biofouling settlement on a hull coating varies according to many parameters, including the ship's speed, hull geometry, intensity of sunlight reaching the hull's surface, water depth, and water temperature. Hull treatment in dry dock is known to reduce friction due to the surface treatment. For instance, a fully sandblasted hull and a new coating scheme establish a relatively smooth surface while eliminating biofouling and mechanical damages and potentially restoring the ship's hull to a new-build condition. Figure 19 provides an example of the decrease in resistance due to dry-docking treatment and the post period in which resistance steadily increases for a container ship. The hull and propeller developed an added resistance of less than 0.5% per month in this case. (32) The increase in resistance is primarily explained by the increase in roughness from damages and biofouling, which developed over time.

Mechanical surface roughness

Mechanical roughness impacts skin friction and is commonly correlated with ships' drag. Furthermore, it influences the settlement and attachment of biofouling, deeming it a significant parameter in the evaluation of the expected biofouling attachment to hull coatings. Several authors have investigated the topic of roughness and its impact on skin friction [e.g., references (54, 67-69)]. Roughness is usually defined as the texture of the surface, for example, highest peak to lowest valley, kurtosis, and skewness. Several researchers have attempted to correlate drag with surface structure; the most common for ships is the [R.sub.t](50) roughness, which is the maximum peak to valley over a sampling length of 50 mm, as exemplified in Fig. 20.

Other roughness parameters, such as the arithmetic average roughness, [R.sub.a]; the root mean square roughness, [R.sub.q]; the skewness roughness, [R.sub.sk]; and the kurtosis roughness, [R.sub.ku], have also been applied to correlate with drag [e.g., references (54, 61, 69)]. The arithmetic average roughness is a mean of the surface roughness, which gives a general description of the height variations without being sensitive to small deviations from the mean line. The root mean square roughness represents the standard deviation of the distribution of the surface heights. Compared to the arithmetic average height, it is more sensitive to large deviations from the mean line. The skewness of a profile is sensitive to occasional deep valleys or high peaks, and it can be used to distinguish between two profiles that have the same [R.sub.a] or [R.sub.q] but different shapes. The kurtosis roughness describes the sharpness of a surface profile. If [R.sub.ku] < 3, the distribution is said to be platykurtoic, with relatively few high peaks and low valleys. Conversely, if [R.sub.ku] > 3, the distribution curve is said to be leptokurtoic, with many high peaks and low valleys. For a thorough introduction to the various roughness parameters, see e.g., references (70, 71). Table 6 displays an example of the roughness parameters for 220-grit sand paper (rough surface), a copper-based biocidal antifouling coating, and a fouling release coating. The fouling release coating was found to have slightly less skin friction, although this determination did not correlate well with [R.sub.q]. However, the smoother surface, which, in this case, is the fouling release coating, generally exhibits lower skin friction.

Measurement of surface roughness usually requires removing the waviness portion from the measured profiles in order to focus on the surface features of interest through the selection of different cut-off lengths for filtering. In general terms, waviness represents the larger irregularities, whereas roughness comprised the smaller irregularities of the surface. The cutoff filter is a short-pass filter that allows the high wave-number components through it, thereby separating the waviness from the roughness. More waviness features are included in the surface roughness profiles when a longer cut-off length is used in the filtering. The values of the investigated roughness parameters and their correlation with respect to drag are affected by the selection of the cut-off length. Unfortunately, many of the published works regarding surface roughness and hull coatings do not mention the cut-off length; therefore, their results cannot be compared. (73) Although Schultz (54) stated that waviness has little effect on drag, Townsin (4) asserted that induced waviness caused by the flow over a pliable surface, that is, a biofouled coating, must be considered. However, neither Schultz (54) nor Townsin (4) addressed the absolute impact of mechanical waviness and induced waviness on biofouled surfaces. No universal relationship has been found that is capable of linking a geometric surface description to the frictional behavior, although several attempts have been made (e.g., Schlichting's equivalent sand roughness (68)). For all such methods, the surface friction will diverge increasingly from the surface friction obtained for a given surface structure as the surface geometry changes. Therefore, for surfaces with large deviations in surface geometry compared with known measurements, experiments are still the only adequate solution. (59) One of the fundamental difficulties in this respect is the fact that, from a hydrodynamic or geometric point of view, a non-smooth surface cannot be described solely by a single parameter such as the average roughness height (peak to valley); therefore, surface optimization through modeling is complicated. Several other descriptive parameters, such as [R.sub.a], [R.sub.q], [R.sub.sk], and [R.sub.ku], are needed

to describe the roughness; indeed, even with these parameters defined, predicting the drag has still proven to be a complicated matter. (59)

In addition to impacting drag, surface structure impacts mechanisms whereby fouling organisms attach to a substratum. This process occurs via a bioadhesive, which flows into surface imperfections and cures in order to create a secure mechanical lock. (74) In this way, surface roughness can promote organism settlement. (75) For instance, Aldred et al. (76) stated that cyprids not only actively choose to settle on surfaces with the maximum number of attachment points, but it also increased the attachment strength. Theoretically, therefore, rougher coating surfaces are more likely to be biofouled than smooth surfaces, (43) and their biofouling release requires higher traveling speeds compared to smoother surfaces. The least preferred topographies with the lowest number of attachment points offer a diminished settlement and growth of the propagules and larvae of fouling organisms (77); in addition, they favor the release of biofouling from surfaces. (76) It has also been suggested that microtopographies might influence fluid dynamics very close to the surface and thus micro-hydrodynamically prevent settlement. (73) In fact, Scardino et al. (78) found that the roughness skewness has a higher impact on biofouling prevention than the mean waviness, which has a higher impact than the mean roughness. While topographical studies show that the surface structure influences the settlement and attachment in the marine environment, the underlying mechanism responsible for reduced biofouling still remains unclear.

Newly applied hull coating condition

The effect of surface roughness on the frictional drag of clean or newly applied hull coatings has been investigated by several authors [e.g., references (54, 61, 80, 81)]. Candries et al., (45) Mirabedini et al., (48) Candries and Atlar, (61) and Flack et al. (69) investigated the initial (unfouled) drag performance of hull coatings based on two rotor setups, a towing tank, and a water tunnel, respectively. The investigated hull coatings were fouling release coatings and tin-free SPC coatings. A summary of the findings is presented in Table 7 for various test methods. Table 8 displays the results obtained with LDA derived from both the Hama (82) and Reynolds Stress (e.g., reference (62)] methods for determining the skin friction coefficients when hull coating systems were compared to smooth, uncoated surfaces.

Flack et al. detected only a minor difference in the skin friction coefficient for the initial friction of fouling release coatings and biocidal antifoulings; indeed, the difference was within experimental uncertainty. (69) Weinell et al. found that the investigated hull coatings (2 fouling release and 4 biocidal antifouling coatings) displayed similar frictional drags at lower velocities (10-20 knots), but at higher velocities (25-35 knots), the frictional drag of the coatings became significantly different. (49) The fouling release coatings with lowest micro-roughness showed the lowest drag, and the biocidal antifouling coatings showed a slightly higher drag. Mirabedini et al. compared the skin friction coefficient between a fouling release coating and three biocidal antifoulings. (48) They determined that the skin friction coefficient for the fouling release coating was between 9% and 22% lower (depending on the Reynolds number). Furthermore, the largest difference in the skin friction coefficient for a newly applied hull coating condition for the SPC coatings was 3.5%. (48)

Weinell et al. tested two SPC coatings in a biofouling-free environment with respect to skin friction coefficients over a period of 5 months. (49) Both coatings showed a tendency to smoothen until a steady state was reached after approximately two months. The initial friction of the SPC coatings might, therefore, decrease slightly for a period of time following dry docking, if they remain free of biofouling.

Biofouled hull coating condition

Biofouling starts from the moment a ship is immersed in sea water. The hull surface rapidly accumulates dissolved organic matter and molecules such as polysaccharides, proteins, and protein fragments. (83) This accumulation is typically followed by the accumulation of micro-organisms (biofouling). Figure 21 presents an example of a hull with severe macrofouling.

Presently, the fluid mechanisms affecting the drag of a ship suffering from soft macro-fouling (e.g., slime) are not well understood. (21) A significant amount of research has been conducted to investigate the effects of soft macro-fouling on frictional drag [e.g., references (44, 56, 84, 85)]. Slime may be thought of as constantly varying stream wise roughness, not only in height but also in morphology [e.g., references (21, 60)], which is one of the difficulties of modeling slime skin friction. Researchers have also conducted substantial research regarding drag predictions related to hard macrofouling (e.g., barnacles and shells) [e.g., references (54, 59)]. Unfortunately, there is currently no method available to accurately predict the drag for a particular ship based on its trade route, choice of hull coating, biofouling distribution on the hull, type of biofouling, or mass of biofouling. Additionally, to the authors' knowledge, there is presently no means of accurately measuring and characterizing micro-fouling or soft macro-fouling, which correlate well with frictional drag. Therefore, a ship operator cannot accurately estimate the impact that an eventual removal of soft macro-fouling via underwater hull cleaning will have on the fuel efficiency of his ship. Table 9 presents the literature data describing the differences in skin friction increases ([DELTA][C.sub.F]) due to various soft macrofouling conditions. The evidence clearly shows that slime formation significantly contributes to an increase of skin friction.

Table 10 presents data from the literature, showing differences in skin friction ([DELTA][C.sub.F]) due to hard macrofouling for full-scale ships. The literature shows that macro-fouling offers a vast contribution to skin friction; generally, the [DELTA][C.sub.F] is significantly higher for hard macro-fouling than for soft macro-fouling. However, Tables 9 and 10 are not directly comparable because the exposure and test conditions were not identical.

Table 11 displays the data from the literature regarding differences in skin friction ([DELTA][C.sub.F]) and shaft power ([DELTA]SP) due to different biofouling conditions for full-scale ships. The percentage increases of shaft power are not directly comparable to those of the skin friction, since the skin friction constitutes a different relative contribution to the total resistance compared to the shaft power. Full-scale ship trials show that the frictional drag changes due to biofouling represent an important parameter with respect to the total resistance of a ship and the need to minimize a ship's drag increase.

Full-scale experiments have the obvious advantage compared to small-scale experiments: their hull coatings experience an aging and biofouling process on the entire hull, not only on a small area, which typically occurs for small-scale tests. If the ship is traveling, it also experiences varying biofouling intensities on the hull, instead of a similar intensity, which is often the case for small-scale experiments. Furthermore, fullscale experiments are often carried out over longer time periods, resulting in long-term evaluations. Subsequently, changes in parameters such as roughness, biofouling, and shaft power can be measured, indicating the effect of changes in the coating surface condition, although this process is admittedly complicated and linked with uncertainties. In comparison, full-scale drag prediction via small-scale experiments is certainly bound to be encumbered by limitations and uncertainties. Full-scale experiments are affected by disadvantages, as well. For example, evaluating the impact of a single parameter, such as the choice of a hull coating, is complicated. Furthermore, reproducing experiments with similar conditions is quite difficult due to the ever-changing parameters during voyage (e.g., temperature, biofouling intensity, salinity, and season), resulting in uncertain statistical errors that further complicate the drag performance estimation of hull coatings.

Drag performance development over time

Few experimental methods have been developed to specifically determine the drag performance of hull coating systems over time in conditions similar to that experienced by operating ships, i.e., presence of marine biofouling and partly dynamic exposure of hull coatings. However, Swain et al. did conduct one such study. (57) Four coatings were investigated when they were newly applied (unfouled), after 60 days of static immersion, and again after an additional 15 days of dynamic immersion. The coatings consisted of one copper-containing SPC coating (Cu-SPC), a copper-containing ablative coating (Cu-Abl), and two early silicone-based fouling release coatings (FR-1 and FR-2). The skin friction was measured at a boat speed of 25 knots and a Reynolds number of 5.5 x [10.sup.7] (See "Static and dynamic panel exposure tested on a boat" section for a detailed description of the experimental method). Table 12 shows the performance of the coatings relative to each other. Based on the evidence, the best-performing hull coating varies for each of the conditions (newly applied, static immersion, and dynamic immersion) tested. Clearly, test conditions are of the utmost importance when evaluating the hull coating's drag performance, as the performance rating was largely influenced by these conditions. In this study, fouling release coating-2 was the best-performing coating at the end of the entire exposure cycle.

Comparison between biocidal antifouling coatings and fouling release coatings

Small- and full-scale experiments have generally shown that fouling release coatings can better reduce skin friction compared to conventional biocidal antifouling coatings in newly applied conditions [e.g., references (45, 61, 64)]. However, when the measurements have been performed after periods of static exposure to biofouling, the early fouling release coatings often show a higher drag than the conventional biocidal coatings. This can be explained in part because the primary mechanism of early fouling release coatings is based on the prevention of biofouling attachment through low adhesion strength and the subsequent release of biofouling due to water flowing over the surface; clearly, this mechanism is not fully utilized during static immersion. The biocidal antifouling coatings only partly rely on this principle. No decisive advantage could be obtained for either of these coating technologies tested after combining a static immersion with a dynamic immersion, due to the limited number of published results and the indecisive conclusions of those published. Tables 13, 14, and 15 compare the early fouling release coatings and the conventional biocidal antifouling coatings when they are newly applied, statically immersed, and dynamically immersed, respectively.

Although small- and full-scale measurements have been carried out for conventional biocidal antifouling coatings and fouling release coatings, further investigations are needed to clarify the conditions that determine the optimal choice of coating, that is, the one with the lowest skin friction over a typical dry-dock period and a potential gain of fuel savings. From Table 13, it can be argued that early fouling release coatings typically result in less drag in a newly applied condition. From Table 14, in turn, it can be argued that biocidal antifouling coatings typically result in less drag after static immersion. Candries and Atlar explained that the drag differences for the newly applied fouling release and biocidal antifouling coatings were caused by the differences in surface texture. (61) The surface textures from a fouling release coating and a tin-free SPC coating are displayed in Figs. 22 and 23, respectively. It is significant to note that the fouling release surface texture is much less spiky, and it has a lower peak-to-valley distance. The smoother fouling release coating surface compared to the tin-free SPC antifouling coating is the explanation given for the lower drag of fouling release coatings.

Fouling release coatings are appealing due to both their potential fuel savings and their lack of harmful effects to the marine environment, since only small amounts of toxic components, if any, are released into the surrounding seawater. It can be anticipated, therefore, that fouling release coatings will increasingly capture market shares of the industrial marine coating market, as the potential for drag reduction and biofouling control is considerable. This effect could be further enhanced through the minimization of the primary drawbacks, such as mechanical surface vulnerability, high price, and limited efficacy for slow and idle ships.

The impact of ships' hull coating conditions on drag

Surface friction influences ship operating parameters such as maximum speed, fuel consumption, and maneuverability. This section presents correlations and experimental results between hull coating conditions and their impact on drag for full-scale ships. Various authors have investigated the impact of surface properties on drag for full-scale ships [e.g., references (64, 91, 92)]. Hull performance tests for full-size ships over time can be conducted in several ways. However, one of the major challenges is to isolate the impact of the hull coating surface condition from all other parameters, such as the propeller, machinery, water temperature, and sea state, in order to determine the effect of the changes in hull coating conditions over time. Another challenge with full-scale tests is the inherent difficulty in obtaining a fair side-by-side comparison of different hull coatings, due to the ever-changing exposure conditions that a ship hull experiences. One common method of testing is to directly compare one hull coating over the full lifetime of its in-service ship's performance (e.g., fuel consumption, shaft torque, or number of propeller revolutions per nautical mile) to that of another system. Although this approach is useful, any increase in resistance cannot be attributed solely to the coating condition; therefore, the absolute value of the hull coating's drag performance cannot be determined accurately. The effects of other parameters on the ship's performance can be minimized by propeller cleaning, minimization of machinery changes, optimal machinery maintenance, and tests in calm waters. However, ships are bound to experience different conditions. The parameters impacting drag will practically always be subject to individual variations; consequently, the determination of the hull coating's drag performance is bound to be a complicated matter. Figure 24 outlines a side-by-side comparison of two hull coatings. The comparison is based on the changes in fuel consumption, speed, and shaft torque over two 60-month periods, where a new and different type of hull coating is applied after the first 60-month period. Based on the changes regarding the in-service ship performance (e.g., fuel consumption) between the first and second period, the hull coating performances can be estimated. For instance, the in-service ship performance method resulted in a fluoropolymer fouling release coating, compared to a tin-free polishing antifouling, and acquired a fuel savings of 0.56% for a container ship, 11% for a tanker, and 22% for a bulk cargo ship. (93)

A quicker, simpler method is the widely used [R.sub.t](50) roughness measured at ship yards and laboratories to obtain an initial estimate of the hull coating's drag performance. The [R.sub.t](50) roughness found at ship yards does not, however, offer indications of a hull coating's capability for biofouling prevention, development of mechanical roughness, or, therefore, long-term drag performance. An average hull roughness (AHR) can be determined by evenly distributing a number of mechanical roughness measurements (e.g., 100) over the hull with for instance [R.sub.t](50) measurements. As the roughness height distribution is random, a sufficient number of measurements must be taken in order to obtain an accurate AHR. (92) Townsin presented a method to calculate the skin friction based on the mechanical hull coating roughness measurements for a relatively unfouled hull coating surface. (4) The AHR is ascertained by determining a large sample of [R.sub.t](50) values. Equation (10) shows the increase in the skin friction coefficient based on the [R.sub.t](50) roughness measurements, compared to a smooth surface:

1000[DELTA] [C.sub.F] = 44([(AHR/L).sup.1/3] -[([Re.sub.s]).sup.-1/3]) + 0.125, (10)

where [DELTA]CF is the skin friction coefficient increase, L is the ship length, and [Re.sub.s] is the ship's Reynolds number at a specific speed U. The model is only valid if the AHR is below 230 [micro]m. Diver inspections will typically be used to monitor and evaluate the performance of a coating system when it is out of dry dock. However, this method only evaluates the biofouling that is visual to the naked eye, and therefore, only provides basic indications of the performance in terms of areas covered and approximate degrees of fouling, which prevents accurate drag evaluations. Schultz presented a model to predict the change in a full-scale resistance coefficient for a hydraulically smooth surface, typical as applied hull coating condition, and biofouled hull coating condition (micro- and macro-fouled), based on towing tank tests on a flat plate (Table 16). (20)

Control of biofouling by hull cleaning

When a ship's hull is sufficiently fouled, causing severe increase in drag, removal of the biofouling on the hull by an underwater hull cleaning may be a worthy consideration. Waterborne hull cleaning allows for the removal of biofouling accumulations on hulls and propellers during idle periods, such as mooring and harboring. The appropriate use of these cleanings can improve the drag performance of a hull coating system for a period of time and potentially delay dry docking and its associated costs. Hull cleaning can reduce friction substantially by removing biofouling; indeed, at times it may restore the level of friction to its former condition, when the ship left dry dock [e.g., references (64, 90, 94)]. Admittedly, the long-term effect of hull cleaning on ship performance is not well established. The ship operator, therefore, must balance the cost and inconvenience of underwater cleaning for a biofouled hull with the uncertainty of the performance gain. Biofouling is typically removed by an underwater procedure where divers apply an impeller system with rotating brushes against the hull coating, thereby removing the biofouling. Other methods for biofouling removal include water jets and cavitation systems. However, the majority of these methods use diver-operated machines fitted with rotating brushes (Fig. 25). The extent of the area that receives waterborne cleaning varies; for example, it can include the entire underwater hull, selected areas of the underwater hull, propellers, shafts, struts and rudders, and all openings.

Hull cleaning offers several distinct advantages. The process is relatively fast, often taking less than a day to complete. It can be conducted when the ship is in port or stationary for other reasons. Additionally, the decrease in friction often reaches a level approaching that of the ship when it left the dry dock after being newly painted. A negative result of the process is the fact that the cleaning actions are likely to impact the environment, as toxic compounds from biocidal antifouling coatings often are removed during the cleaning process. Another major disadvantage of hull cleanings is the fact that the coating may be mechanically damaged during the process, especially if hard fouling is present. If a cleaning occurs before the seaweed biofouling is reached, then very soft brushes can be used, and the hull coating system is not likely to be damaged. (32) If mechanical damage does take place, the rate of biofouling accumulation will likely increase substantially faster than it would have before the hull cleaning. Clearly, the decision to employ hull cleaning is often a trade-off between reduced biofouling and the risk of a potentially increased rate of biofouling. Figure 26 provides an example of the effect that hull cleaning (hull brushing) and propeller polishing (two times) has on drag performance (designated as resistance in the figure). In this case, the increase of resistance due to hull cleaning drops from the reference point by an approximate factor of 1.23-1.15. Based on the figure, the drag performance just before the hull cleaning was carried out (5700 days) is reached again approximately at day 6150. Simply stated, within approximately 1.2 years, the resistance returns to the same level as it had been before the hull cleaning.

In short, the physical removal of biofouling by typical hull cleaning methods is relatively efficient and cost-effective, but it is also a relatively short-term method for drag reduction. Therefore, hull cleaning should presently be considered a short-term solution that can be useful during the last stage, prior to dry docking and the application of a new hull coating. The further development of hull cleaning systems, where the hull coating is less likely to be damaged could be a potential means to ensure that biofouling is kept to a minimum over both shorter and longer periods.

Conclusions

Today's estimations of the drag performance of hull coatings stem mainly from small-scale tests that evaluate the newly applied condition and, occasionally, perform an evaluation after static natural seawater exposure. Only one study analyzed the drag performance of hull coatings through a combination of static and dynamic seawater exposure, which more closely resembles the natural aging process of a ship's hull coating.

Small-scale tests investigating newly applied hull coatings are common and relevant, but they only provide limited information, since they fail to consider biofouling species, mechanical roughness, and other forms of aging experienced by hull coatings on a ship. Therefore, only the drag properties that exist shortly after a ship leaves dry dock are studied; as a result, the evolution of a ship's drag during service is completely ignored. The static exposure tests provide some additional information regarding the impact of aging on hull coatings, but even these are limited, because they only closely reproduce the conditions experienced by idle ships. For instance, the shear stress experienced by a moving ship and its impact on biofouling settlement and growth are not accounted for during static immersion. Furthermore, static tests can potentially provide inaccurate trends in relation to hull coating performance for moving ships, especially as some technologies have a greater dependency on a certain water flow that prevents or limits biofouling (e.g., fouling release technology). Dynamic and static cycles simulating a ship's trading patterns in the presence of biofouling should, therefore, primarily be used when determining the drag performance of hull coatings. It is typically reported that fouling release coatings exhibit a lower drag when in newly applied or clean conditions, while conventional biocidal antifouling coatings (e.g., SPC) exhibit a lower drag when immersed statically in seawater for an extended period of lime. In fact, no decisive conclusions have been made regarding the dynamic exposure of early fouling release coatings and conventional biocidal antifouling coatings in seawater. There simply are no available studies that explore the most recent technologies, which have evolved significantly in the past few years, especially in the area of fouling release coatings. Evaluating hull coatings in newly applied conditions, the difference in drag performance is typically found to be relatively small, whereas it is often much larger after longer periods of static immersion. One major issue that needs to be resolved is the absence of a method that accurately determines the drag performance and associated costs (savings) of specific hull coatings over a given period of time, based on conditions similar to those experienced by moving ship hulls. The prediction of drag performance based on statistical roughness parameters is complicated, since a non-smooth surface cannot be described solely in terms of a single roughness parameter. Several other descriptive parameters, such as the density of the roughness elements and shapes, are needed to describe the roughness; even with these parameters, accurate predictions prove to be difficult. No universal relationship has been found to link a geometric surface description to frictional behavior, although several attempts have been made. Therefore, for surfaces with large deviations in surface geometry being compared to known measurements, experiments are still the only adequate solution. The drag performance prediction due to biofouling is even more complex than that of mechanically rough surfaces, especially for soft biofouling; as such, it is currently not well understood. Fortunately, drag penalties caused by hard macro-fouling are better understood. Systematic studies comparing the drag behavior of hull coatings over time in the presence of biofouling species under realistic speed and activity conditions are limited or nonexistent. Yet, they are essential in determining the optimal choice of hull coating with respect to fuel efficiency. Finally, the task of converting a small-scale measurement to a full-scale ship size is difficult, especially since the literature presents only limited results and correlations verified by full-scale tests, if any. A better understanding of conversion to full-scale drag performance is necessary. One study converted flat plate drag measurements into increased shaft power consumption for a mid-sized naval surface. Compared to a hydraulically smooth surface the following shaft power increase at cruising speed of 7.7 m/s was estimated: (a) 2% for typical as applied on hull coating; (b) 11-21% for deterioted coating or light to heavy slime; and (c) 35-86% for small to heavy calcareous biofouling.

Studies comparing drag differences between hull coatings in conditions similar to those experienced by ship hulls could be useful to evaluate the optimal choice of hull coating and its impact on fuel consumption. Other issues that have yet to be satisfactorily addressed include the quantitative definition of biofouling coverage and, secondly, a correlation between biofouling coverage and simple predictions of resistance and powering. Due to their complexity, many parameters have been ignored to date, including biofouling settlement with regard to exposure time and traveling pattern, the location of biofouling on the hull, and the influence of self-smoothening mechanisms on a ship's drag. Developments in these areas would help to determine the optimal choice of hull coating, the optimal time to conduct underwater hull cleaning (partial or complete), the optimal time between dry dockings with new coating application and benefits of a full abrasive blasting of the hull. The economic and environmental gains associated with the latter knowledge are expected to be enormous.

DOI 10.1007/s11998-014-9651-2

A. Lindholdt, K. Dam-Johansen, S. Kiil ([mail])

Department of Chemical Engineering, Technical University of Denmark, DTU, Building 229, 2800 Kgs. Lyngby, Denmark

e-mail: Sk@kt.dtu.dk

A. Lindholdt

e-mail: Asli@kt.dtu.dk

S. M. Olsen, D. M. Yebra

Hempel A/S, Lundtoftegardsvej 91, 2800 Kgs. Lyngby, Denmark

Acknowledgments Financial support provided by The Hempel Foundation and the Technical University of Denmark is gratefully acknowledged.

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(93.) Corbett, JJ, Winebrake, PJJ, Green, E, Comer, B, "Energy and GHG Emissions Savings Analysis of Fluoropolymer Foul Release Hull Coating." Energy and Environmental Research Associates, LLC, 2010

(94.) Schultz, M, Bendick, J, Holm, E, Hertel. W, "Economic Impact of Biofoulins on a Naval Surface Ship." Biofouling. 27 87-98 (2010)

(95.) Bohlander, J, Zealand, MBN, "Review of Options for Inwater Cleaning of Ships." New Zealand: MAF. Biosecurity. 42 1-34 (2009)

Table 1: Active lifetime, limitations, and advantages associated with
biofouling and drag of the hull coatings used in today's commercial
market

Coating system    Lifetime                  Limitations
                   (years)

Tin-free SPC      3-5 (7.5)   Lower efficiency compared to
                                tributyltin (TBT)-based coatings
CDP (ablative)    3           Poor self-smoothing
                              Increasing leached layers with
                                immersion time
                              Biocide release not constant
                              Little activity during idle periods
                              Higher cost before applying new coats
                                (sealer coating often needed)
Fouling release   5-10        Potential surface damage from in-water
                                cleaning
                              High risk of biofouling under static
                                conditions

Coating system               Advantages               Reference

Tin-free SPC      Lower cost compared to TBT-based    8, 16
                    coatings.
                  Less environmental harm compared
                    to TBT-based coatings
CDP (ablative)    Low cost                            2
Fouling release   Small environmental impact          5
                  Low initial friction

Table 2: Distribution of applied volume of hull coatings in various
geographical regions in the year 2011 (19)

Geographical region      Fouling release    Conventional biocidal
                          coatings (%)     antifouling coatings (%)

China                        3.20                   38.28
Asia Pacific                 1.01                   30.48
Japan                        0.79                   10.16
Western Europe               0.22                   7.30
North America                0.28                   4.32
Eastern Europe               0.39                   3.09
Middle East and Africa       0.028                  0.22
Latin America                0.0056                 0.22
World                        5.93                   94.07

Karunanidhi et al. use the term acrylate instead of conventional
biocidal antifouling coating (19)

Table 3: Dynamic viscosity and density for standard
seawater (30)

Temperature      Dynamic viscosity   Density [rho]
T ([degrees]C)    [mu] (mPa x s)     (kg/[m.sup.3])

1                      1.843            1028.09
5                      1.620            1027.72
10                     1.397            1027.00
15                     1.220            1026.02
20                     1.077            1024.81
25                     0.959            1023.39
30                     0.861            1021.77

Table 4: Advantages and disadvantages of LDA in
determining hull coating's drag performance

Advantages                            Disadvantages

Accurate measurement of flow      High cost of equipment
  velocities
Applicability with direct drag    Extensive knowledge
  measurements                      required
Non-intrusive technique           Indirect estimation of
                                    friction

Table 5: Advantages and disadvantages of experimental drag
performance test methods

Test setup                            Advantages

Rotating disk               Small size
                            Easy application of coatings
                            Fairly low cost
Rotating cylinder           Small size
                            Fairly low cost
Towing tank                 No pressure gradients
                            Resemblance to ship
                              geometry
                            Vast amount of literature for
                              flow over flat plates
Water tunnel                Small size
                            Controlled flow

Static and dynamic panel    Flexible immersion conditions
  exposure tested on a        (static and/or dynamic)

boat Pipes                  Vast amount of literature for
                              flow in pipes
Optical methods for drag    Accurate flow pattern
  measurements                measurements

Test setup                          Disadvantages            Reference

Rotating disk               Varying shear stress over the    43
                              test surface
Rotating cylinder           Complex interpretation of        49
                              friction coefficient
Towing tank                 Large tank necessary             66
Water tunnel                Complicated flow stability to    56
                              two- and three-dimensional
                              controlled disturbances
Static and dynamic panel    Dynamic immersion could          57
  exposure tested on a        deviate some from actual
                              hull conditions of a moving
                              ship
boat Pipes                  Large uncertainty                59
                            Complicated visual inspection
Optical methods for drag    Expensive equipment              61
  measurements              Indirect drag evaluation

Table 6: Roughness statistics for 220-grit sand paper, an antifouling
copper coating, and a silicone fouling release coating (69)

Specimen                                Peak-to-valley      [R.sub.q]
                                     roughness ([micro]m)   ([micro]m)

220-grit sand paper                          305               36.9
Biocidal antifouling coating                 152               20.8
  (copper based)
Fouling release coating (silicone)           135               10.0

Specimen                             [R.sub.sk]   [R.sub.ku]

220-grit sand paper                     0.10         2.84
Biocidal antifouling coating            0.05         2.75
  (copper based)
Fouling release coating (silicone)      0.13         3.13

Table 7: Initial (unfouled) skin friction measurements for fouling
release coating and biocidal antifouling coating

Test setup               Comments about skin friction        Reference

Rotating cylinder   Hull coating skin friction               45
                      coefficient vs. the one of a smooth
                      test cylinder
                    a) 4.3% increase for sprayed fouling
                      release coating
                    b) 5.7% increase for rollered fouling
                      release coating
                    c) 8.0% increase for tin-free SPC
                      coating
Rotating cylinder   9-22% lower skin friction coefficient    48
                      for the fouling release coating
                      (silicone) than the SPC coating
Water tunnel        2-20% lower skin friction coefficient    61
                      for the fouling release coatings
                      (depending on the quality of the
                      application) than the tin-free SPC
                      coating
Water tunnel        The difference in skin friction          69
                      between the fouling release and
                      biocidal antifouling coating was
                      within experimental uncertainty

Table 8: Increase in skin friction coefficients for newly applied
hull coatings compared to a smooth surface determined by use of
LDA (61)

Hull coating system                   Skin friction coefficient
                                          derivation method

Tin-free SPC coating                  Reynolds Stress method
Fouling release coating applied by    Reynolds Stress method
  roller
Fouling release coating               Reynolds Stress method
Tin-free SPC coating                  Hama method
Fouling release coating applied by    Hama method
  roller
Fouling release coating               Hama method

Hull coating system                   [DELTA] [C.sub.F] compared to
                                        smooth unpainted reference
                                               surface (%)

Tin-free SPC coating                               13.4
Fouling release coating applied by                 12.8
  roller
Fouling release coating                            10.0
Tin-free SPC coating                               14.7
Fouling release coating applied by                 12.4
  roller
Fouling release coating                            10.1

Table 9: Increases of small-scale friction coefficients due to soft
macro-fouling

[DELTA][C.sub.F]               Comments                Reference
(%)

10                 Pre-roughened disks were tested     44
                     before and after exposure to
                     slime formation
10-20              Soft macro-fouling (slime films)    44
                     on a rotating disk
4-11               Uniformly distributed nylon         Adapted from 45
                     tufts attached to a rough flat
                     plate resembling slime
                     compared to a rough plate
9-29               A friction disk machine was used    43
                   Disks were exposed under static
                     conditions for approximately 3
                     weeks
                   The [DELTA][C.sub.F] increase
                     was 9% for Cu-ablative, 17%
                     for FR-1,27% for FR-2, and 29%
                     for FR-3
58-68              Drag was measured in a towing       64
                     tank using a flat plate. The
                     biofouling coverage on a SPC
                     TBT system after 287 days of
                     exposure resulted in 70% slime
                     coverage in accordance with
                     ASTM D3623
33-190             Non-coated plates were exposed      86
                     for 6, 14 and 17 days in an
                     aquaculture facility
                   The average increase in
                     [C.sub.f] for slime films with
                     a mean thickness of 160
                     [micro]m was 33%; for a mean
                     thickness of 350 [micro]m, it
                     was 68%; for a surface
                     dominated by filamentous algae
                     (Enteromorpha spp.) with a
                     mean thickness of 310
                     [micro]m, it was 190%

Table 10: Increased small-scale friction coefficients due to hard
macro-fouling

Type of measurement         [DELTA][C.sub.F] (%)

Towing tank (pontoon)       85
Flat plate in towing tank   300-400
Flat plate in towing tank   87-138
Rotating cylinder           100

Type of measurement                   Comments              Reference

Towing tank (pontoon)       75% coverage with barnacles     4
                              of 4.5 mm height
Flat plate in towing tank   Fouling release coatings had    64
                              extensive coverage of
                              barnacles
Flat plate in towing tank   Ablative copper and SPC         64
                              copper systems had 1-4% of
                              barnacle coverage
Rotating cylinder           3% coverage of barnacles        49
                              doubled the drag

Table 11: Changes in skin friction ([DELTA][C.sub.F]) and shaft power
(ASP) due to different biofouling conditions for full-scale ships

Increased skin friction                           Comments
([DELTA][C.sub.F]) or shaft power
(ASP)

[DELTA][C.sub.F]: 0.5% per day       Biofouling on full-scale ships
                                       increased resistance by 0.5%
                                       per day while at dock
[DELTA][C.sub.F]: 5% over 40 days    A hull was allowed to foul for 40
                                       days on a coating of bituminous
                                       aluminum which, over a speed
                                       ranging from 5 to 15 knots,
                                       resulted in a frictional
                                       increase in resistance of 5%
[DELTA][C.sub.F]: 8-14%              Added resistance due to slime
                                       formation
[DELTA][C.sub.F]: 25% after 240      The 23 m fleet tender hull had a
  days and 83% after 600 days          non-polishing antifouling.
                                       After 240 days of operation, a
                                       thin slime layer had developed,
                                       and after 600 days, a 1-mm
                                       thick slime film had developed
                                     A 15% reduction in ship speed was
                                       further noted over a 2-year
                                       exposure
[DELTA][C.sub.F]: 100% over 375      Towing tests were conducted at 16
  days                                 knots without a propeller on
                                       Yudachi, a 234-foot Japanese
                                       ex-destroyer. The fouling
                                       condition is not reported,
                                       but fouling is known to have
                                       developed while Yudachi
                                       remained at anchor
[DELTA]SP: 24%                       Power trials on a frigate with a
                                       fouled hull suffering from
                                       incipient tube worm growth with
                                       coverage of 10-20% showed an
                                       increase of 24% shaft power to
                                       maintain a speed of 7.7 m/s
                                       compared to the clean condition
[DELTA]SP: -8% and -18%              A frigate coated with an ablative
  (decrease due to hull                antifouling (cuprous oxide and
  cleaning)                            a TBT- based cobiocide) had
                                       been exposed at Pearl Harbor,
                                       Hawaii, for 22 months. Hull
                                       inspections indicated the
                                       presence of a fairly heavy
                                       slime film with little to no
                                       calcareous biofouling. Power
                                       trials due to the hull cleaning
                                       of the fouled condition showed
                                       a decrease in the required
                                       shaft power of 8% at a ship's
                                       speed of 16 knots and of 18% at
                                       25 knots

Increased skin friction              Reference
([DELTA][C.sub.F]) or shaft power
(ASP)

[DELTA][C.sub.F]: 0.5% per day       Adapted from
                                       reference (21)
[DELTA][C.sub.F]: 5% over 40 days    87
[DELTA][C.sub.F]: 8-14%              88
[DELTA][C.sub.F]: 25% after 240      89
  days and 83% after 600 days
[DELTA][C.sub.F]: 100% over 375      25
  days
[DELTA]SP: 24%                       Adapted from
                                       reference (20)
[DELTA]SP: -8% and -18%              42
  (decrease due to hull
  cleaning)

Table 12: Side-by-side comparison (ranking from 1 to 4, where 1
refers to best performance) in terms of skin friction coefficients at
a boat speed of 25 knots and Re = 5.5 x [10.sup.7] at newly applied,
static exposure, and dynamic exposure conditions

Hull coating system          Newly    60 days   15 days
                            applied   static    dynamic

Fouling release coating-1      1         4         4
Fouling release coating-2      3         3         1
Cu-Ablative                    2         1         2
Cu-SPC                         4         2         3

Modified from reference (57)

Table 13: Drag comparison between fouling release and biocidal
antifouling coatings in newly applied or cleaned conditions

Type of measurement                    Lowest drag

Rotating cylinder      Fouling release coating
Rotating cylinder      Fouling release coating
Rotating cylinder      Fouling release coating
Rotating cylinder      Fouling release coating
Flat plate in towing   Elusive drag conclusion; results
  tank                   depended on the specific type
                         of biocidal antifouling coating and
                         fouling release coating
Flat plate in towing   Fouling release coating
  tank
Water tunnel           Fouling release coating (2-20%)
Water tunnel           Fouling release coating (within statistical
                         uncertainty)
Static and dynamic     Fouling release coating
  panel exposure
  tested on a boat

Type of measurement           Coating condition          Reference

Rotating cylinder      Newly applied                     45
Rotating cylinder      Newly applied                     49
Rotating cylinder      Newly applied                     46
Rotating cylinder      Newly applied                     48
Flat plate in towing   Cleaned condition (after          64
  tank                   287 days of static exposure)
Flat plate in towing   Newly applied                     64
  tank
Water tunnel           Newly applied                     61
Water tunnel           Newly applied                     69

Static and dynamic     Newly applied                     57
  panel exposure
  tested on a boat

Table 14: Drag comparison between fouling release and biocidal
antifouling coatings after static immersion

Type of measurement               Lowest drag

Friction disk machine         Biocidal antifouling
Flat plate in towing tank     Biocidal antifouling
Static and dynamic panel      Biocidal antifouling
  exposure tested on a boat

Type of measurement              Static exposure time      Reference

Friction disk machine         3 weeks, with a subsequent   43
                                removal of hard
                                biofouling
Flat plate in towing tank     287 days                     64
Static and dynamic panel      60 days                      57
  exposure tested on a boat

Table 15: Drag comparison between fouling release and biocidal
antifouling coatings based on full-scale mea surement of static and
hydrodynamic exposure condition

Type of                                Lowest drag
measurement

Static and dynamic   Elusive drag conclusion; results depended
  panel exposure       on the specific type of biocidal antifouling
  tested on a boat     coating and fouling release coating

Fuel consumption     Fouling release coating
  on high-speed
  catamaran ferry

Type of                             Comment                  Reference
measurement

Static and dynamic   One fouling release coating had the     57
  panel exposure       lowest drag while another had the
  tested on a boat     highest. The two biocidal coatings
                       were neither the best nor the
                       worst in performance
Fuel consumption     With same service speed, a lower        90
  on high-speed        fuel consumption was detected for
  catamaran ferry      the fouling release coating
                       compared to the biocidal
                       antifouling

Table 16: Predictions of the change in total resistance
([DELTA][C.sub.T]) and corresponding increase in shaft power
([DELTA]SP) for an Oliver Hazard Perry class frigate (FFG-7) with a
range of representative coating and biofouling conditions at a speed
of 7.7 m/s (15 knots) and 15.4 m/s (30 knots) (20)

Description of condition                 ([DELTA][C.sub.T])
                                       [U.sub.s] = 7.7 m/s (%)

Hydraulically smooth surface                     --
Typical as applied hull coating                   2
Deteriorated coating or light slime              11
Heavy slime                                      20
Small calcareous biofouling or weed              34
Medium calcareous biofouling                     52
Heavy calcareous biofouling                      80

Description of condition                  ([DELTA][C.sub.T])
                                       [U.sub.s] = 15.4 m/s (%)

Hydraulically smooth surface                      --
Typical as applied hull coating                   4
Deteriorated coating or light slime               10
Heavy slime                                       16
Small calcareous biofouling or weed               25
Medium calcareous biofouling                      36
Heavy calcareous biofouling                       55

Description of condition                    [DELTA]SP
                                       [U.sub.s] = 7.7 m/s

Hydraulically smooth surface                   --
Typical as applied hull coating                 2
Deteriorated coating or light slime            11
Heavy slime                                    21
Small calcareous biofouling or weed            35
Medium calcareous biofouling                   54
Heavy calcareous biofouling                    86

Description of condition                    [DELTA]SP
                                       [U.sub.s] = 15.4 m/s

Hydraulically smooth surface                    --
Typical as applied hull coating                 4
Deteriorated coating or light slime             10
Heavy slime                                     16
Small calcareous biofouling or weed             26
Medium calcareous biofouling                    38
Heavy calcareous biofouling                     59


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Author:Lindholdt, A.; Dam-Johansen, K.; Olsen, S.M.; Yebra, D.M.; Kiil, S.
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:May 1, 2015
Words:18226
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