Warship design trends: warships are changing. For some years now, emphasis has been put on reducing a ship's signature.

Warship design trends: warships are changing. For some years now, emphasis has been put on reducing a ship's signature. But new hull forms are also materialising, such as the British Triton trimaran. Interest in surface-effect ships also seems to be reviving, and yet, in the United States, the DD-21 programme may create a new kind of surface combatant. This metamorphosis is driven in part by technological developments as well as evolving needs. (Naval Warfare)

In the past, the two greatest drivers for new hull forms were the desire for higher speed (preferably with reduced power consumption) and the need for decreased motion in rough seas, usually to accommodate helicopter operations. In the second case, the objective was to give a small, presumably inexpensive ship the steadiness associated with a large hull. Much of a ship's resistance is associated with the way in which her waterline cuts through the seas. The vessel's waterline also determines the extent to which she is affected by wave motion. The broader the waterline, the greater the resistance, which is why fast ships have long narrow hulls. Unfortunately, the narrower the hull, the livelier it is in any sea. The obvious solution is to run several long narrow hulls in parallel. The result gains beam (for steadiness) at a limited cost in resistance. The simplest example is a catamaran, which offers considerable deck area at a limited cost in displacement, hence resistance. Given modern lightweight engines, a catamaran can make very high speeds.

The Hull Issue

The catamaran design has its drawbacks, of course. For example, each of the sub-hulls generates its own wave train, and the wave trains interact in the space between hulls. The situation may worsen in a heavy sea. At one time the US Navy built a pair of catamaran salvage tugs, prizing their stability in rough weather. It later found the wave-interaction problem serious enough to discard the ships.

A greater beam is attractive because deck area is now so important. The obvious application is helicopter takeoff, landing and stowage. The capacity of vertical missile launchers also depends, not so much on the overall weight-carrying capacity of the ship (missiles are relatively light), but on the deck area available for launch cells. Indeed, it is possible to trace a design evolution from ships that were weight-critical through those that were volume-critical and finally to those that were deck-area critical. In other words, the warships of the gun era carried heavy but concentrated weights and their hulls thus had to be voluminous enough to support those weights. That is why battleships of the two World Wars show such small superstructures since there was not much need for volume above the main deck. In addition, the hull had to be massive to house all the needed ship functions, including accommodation.

With the advent of missiles, radars and computers, hull volume became far more important. Hence the boxy appearance of the warships of the 1950s through the 1980s. Incidentally, the need for volume made it almost impossible to provide any sort of armour, since the areas that had to be covered were so large. Now that deck area itself often seems to be the critical commodity, the appropriately designed ship of the future would be some sort of multi-hull craft offering a larger overall deck area without the associated hull drag increase.

Unfortunately, in some seas catamarans have a distinctly uncomfortable motion, because one hull tends to find itself far more deeply submerged than the other. However, in the case of the British Triton, a pair of short outriggers stabilise a long narrow hull. Because the outriggers are relatively small, variations in their buoyancy have less effect than in a true catamaran. Most of the contents of the ship are located in the structure connecting main and auxiliary hulls. The trimaran's designers claim that by choosing lengths properly they can suppress most of the bow wave the ship creates, thus materially reducing the power needed to achieve a given speed. Sceptics point out that even trimarans can suffer in some seas, particularly quartering ones. A commercial trimaran would probably be largely immune to this problem, since it could avoid uncomfortable conditions, but that would hardly apply to a warship. Tests with the Triton may show whether or not this is a serious issue.

Another issue is scaling. It is easy enough to draw a sketch of a trimaran aircraft carrier, but in reality the deck connecting the sub-hulls feels considerable stresses as those hulls try to move independently. Beyond a fairly small size, stresses are probably such that the ship would tear itself apart in any sort of sea. The upper size limit is still matter for conjecture.

Swath

Another approach to such a solution is the Swath (Small Waterplane Area Twin Hull), which uses a pair of torpedo-like underwater hulls connected by struts to a bridging deck. Because the buoyant bodies are well submerged, the Swath should feel little stress in a seaway. Since the struts produce only small wakes, there is only limited interaction between the hulls. The Swath has usually been associated with steadiness rather than speed, probably because until very recently it has been difficult to package much power in the submerged bodies. However, with the emergence of lightweight electric motors, Swath design may once again be a viable way of achieving high speed coupled with substantial deck area. The US Navy is currently trying an alternative, the Slice, which is, in effect, a Swath with four rather than two submerged bodies. Propellers are mounted on the two forward bodies, which are closer together than the after pair, to limit wake effects.

Yet another possibility is a single submerged body, for buoyancy, using hydrofoils for steadiness. If the foils are deep enough, they should not be affected by the sea through which the ship runs. By placing most of the buoyancy well below the surface, the ship drastically reduces the waterline area which creates most resistance -- and which also responds most strongly to waves. This Hyswas (Hydrofoil Small Waterplane Area Ship) concept is currently being tested in prototype form by the US Navy.

Catamarans and Swaths are conventional in the respect that they support the ship's weight by buoyancy. There was no alternative until the advent of the hydrofoil, the air cushion and the exploitation of surface effect. Hydrofoils seem to have fallen out of favour, but surface-effect craft or air-cushion vehicles are appearing in some numbers. The US Navy, for example, has almost one hundred air-cushion landing craft. Norway has air-cushion mine counter-measures vessels and is beginning a series of air-cushion fast attack types. Unlike the buoyancy-supported, an air cushion vessel continuously produces the layer of compressed air on which it rides; thus its carrying capacity depends upon its powerplant. Although very large air-cushion craft have been sketched, the practical size limit seems to be a few hundred tons. Interestingly though, it is the catamaran that has recently pushed the hovercraft off its throne.

Wave Drillers

The conventional displacement hull, however, is far from having breathed its last. There has been considerable interest in wave-piercing displacement hulls, for applications as diverse as fast transAtlantic yachts, fast attack boats and the new US Navy DD-21. A truly conventional hull rides over waves. Just how fast it bounces up and down depends on how fast it strikes the waves. At some point the vertical motion becomes intolerable, and the ship has to slow down. With wave piercing, the limit lies in the amount of water one can tolerate sweeping along the deck. Clearly there comes a point at which the waves are just too high, but wave-piercing yachts have managed to cross the Atlantic at average speeds in the 40-knot range. To do that, they combine wave-piercing hulls with extremely lightweight engines. Whether such hulls offer realistic military capability is another question. Presumably they are most effective when meeting waves bows-on, and often warships cannot choose their courses so clearly. On the other hand, with the advent of vertical launchers, ships may be free to tolerate much more deck wetness than in the past.

Stealth

Then there is stealth, most prominently the designs with a view to reducing radar cross-section. Apparently the main contributors to a ship's radar cross-section are the corner reflectors created by decks (or the sea) and vertical elements of the ship, and clutter on the deck and in the superstructure. The usual solutions are to angle the sides of the hull outward and the sides of the superstructure inward, and to enclose as much of the superstructure as possible. The French Lafayette and the Swedish Visby typify this approach.

The Arleigh Burke class is a less extreme approach to the same objective. Recent American work on enclosed masts was also intended to reduce radar cross-section. The masts are largely radar-transparent, some elements being wrapped in radar-absorbing material, and the antennas inside are tuned so that they reflect only over a very limited range of frequencies. A similar tuning concept is being applied to missiles such as the Exocet Block II. In both cases the hope is that hostile radar will not be operating at quite the specific frequency for which the antenna or radome is tuned, hence will not see the antenna as a major contributor to radar cross-section.

Current stealthy ships have their sides canted outwards (flared) because that hull form also carries sea-keeping and survivability advantages. However, it is better, from a radar perspective, to cant the hull sides inward and to curve the deck edge, an above-water form called tumble-home. French battleships of the turn of the century had tumble-home, and so do submarines. As a conventional or flared hull sinks deeper in the water, waterline area grows, and the ship sinks less for every ton of water taken on board. A tumble-home hull has the opposite behaviour. It seems likely that tumblehome is acceptable (from a sea-keeping point of view) only above a certain tonnage, which suggests that the US DD-21 is workable whereas a tumble-homed fast attack boat might not.

Probably the most radical feature of the DD-21 hull form is that, instead of riding over waves, it is intended to plunge through them. In this way the ship avoids changing its angle to an observer, and thus maintains its stealthy radar image. Reportedly DD-21 specifications include drastic reductions in radar cross-section under all sea conditions. Sceptics will ask whether any ship running at speed can truly eliminate one of its major radar signatures, that of its wake. There is apparently some hope that the careful shaping of the underwater hull can suppress the bow wave, at least at particular critical speeds, by creating interfering wave trains. This is much of what advocates of the trimaran claim for their design; in its case the interference is created by both the main and auxiliary hulls.

For the future, any attempt to drastically reduce cross-section will entail gross simplification of a ship's superstructure -- which will in turn mean cutting the number of antennas. The new technology required, which is under active development in the United States, is intended to make broad-band multi-function antennas possible. Such antennas must operate both passively and actively. The broad range of signals they detect is usable because the signals are converted into digital form at the antenna. They are then shunted onto a bus, where different elements of the ship's combat system can strip off the relevant signals. Such multi-function antennas are planned for the new Joint Strike Fighter and they exist in very embryonic form in the F-22.

Stealthy Aerials?

In theory, multi-function antennas may solve a worsening problem of allocating shipboard real estate. In the past, the main competitors were radars and electronic support or jamming antennas. Then navies adopted satellite systems for long-haul communications; the satellites operate at radar frequencies. The problem is not merely that radars and satellite dishes compete for the same topside space, thereby creating clutter and interference (via side lobes) -- but that sidelobes from satellite transmissions inevitably leak into a ship's electronic support measures system, which cannot a priori distinguish them from hostile radar emissions. The famous case in point is HMS Sheffield, forced, during the Falklands War, to turn off her ESM suite while she transmitted on her Scot satellite link and thereby made herself unaware of the approaching Argentine Exocet-firing aircraft that eventually hit her. The post-Falklands solution was to burst satellite transmissions, minimizing dead time for the ESM set.

Now, however, satellite systems are increasingly vital, as warships depend more and more on data from remote sensors. The US Navy calls the new kind of operation "network-centric warfare," and the key point is that at times the satellite system is at least as important as the ship's own sensors (see Naval Affairs in the DSEi show report in this issue). That means giving the satellite receivers the sort of prime shipboard real estate which might otherwise be devoted to radars -- or else finding some way to combine the two. Hence the intense interest in multi-function apertures (antennas).

Plastics

Some years ago glass-reinforced plastic (GRP) was introduced for mine countermeasures craft, because it offered distinct advantages over wood as a non-magnetic material. The new Swedish Visby class corvettes have gone one step further by being largely built of carbon-based composite materials, which offer even lighter weights. Similar materials are used in some current stealthy aircraft. The main objection to such materials, in comparison with steel or aluminium, is that not only are they more difficult to repair in the event of damage but also far more complicated to modify at mid-life update stage. Therefore, provisions have to be made at design stage. There may also be questions as to how well they retain their strength in the event of a fire and of whether they produce toxic fumes when they burn. Presumably experience with the Visby will resolve such questions, although Sweden has had considerable experience with its glass fibre Landsort-class mine hunters (more than the fibres, it is the bonding resin that generates the toxic fumes).

Another area of new technology is propulsion. It still seems that gas turbines are the only prime movers capable of producing high power, and are inherently quieter than diesels. However, until now gas turbines, like all turbines, have been efficient only at maximum power -- therefore, existing warships tend to use either secondary gas turbines or diesels for cruising. The RollsRoyce/Northrop Grumman WR-21 offers an alternative. By exploiting some of the heat in its exhaust, it considerably improves turbine efficiency even at fractional power. At least in theory, a ship so powered could dispense with secondary powerplants thereby saving space and weight. This argument has already drawn considerable interest. Certainly no other new gas turbine seems to be on the horizon, although existing simple-cycle engines, such as the LM-2500, continue to be developed.

Transmission

Although electric transmission has been tried in the past, the only affordable technique for decades was mechanical drive through gearing. Turbo-electric power exacted too high a weight penalty. In a warship, that means considerable space devoted to shafting and gears, and substantial vulnerability to shock which might bend a propeller shaft. Conventional propellers also offer very limited maneuverability at low speed. Recent developments in electric drive seem to offer a viable alternative. Moreover, as surface combatants become larger, the weight penalty inherent in an electric drive seems more affordable. Several recent cruise liners have podded electrically-driven propellers. One of their important advantages is that they can be used to manoeuvre a large ship in very narrow waters, even when she is stopped dead in the water.

For a warship, propeller pods could be mounted along the ship's length. At least in theory, a ship with pods fore and aft could keep going even if her bows or stern were blown off.

Electric propulsion has a broader significance. In effect, electric propulsion separates the prime mover (currently the gas turbine or the diesel) from the propulsor; thus, an electric ship can relatively easily be redesigned to take an alternative prime mover, as long as it will generate electricity. The separation implied by an electric drive also, of course, makes it possible to distribute prime movers around a ship, far from the propulsors, and thus to provide a measure of survivability. American thinking about the DD-21, which is to be electrically powered, envisages multiple paths from generators to propulsors, so that the ship can keep moving after hull damage.

Survivability

There is also increasing interest in combat survivability. During the Cold War Western navies tacitly assumed that any major war would be so short that any ship which could not be repaired within a few weeks would be as good as sunk. In addition, it had seemed that a ship's combat systems could not survive the shock of a major hit. Neither assumption seems valid today. Wars are likely to be limited, and it is well worth spending a year or so repairing a major unit.

On the other hand, it seems unlikely that anything short of an aircraft carrier can accommodate enough armour to stop the warhead of even a small missile. Survivability therefore means dispersion. Since it can be assumed that a single hit will damage a ship over a substantial area, dispersion is useful only if the ship is made quite large. Since ship steel is inexpensive, size need not imply large cost, and it will generally offer advantages in sea-worthiness, even if a conventional hull is adopted. This argument helps explain the substantial size of the DD-21 proposals. One interesting new idea is to capitalise on electric drive. The ship's prime and auxiliary powerplants can be combined, then split into units distributed around a ship. If, as the US Navy believes, wiring can be duplicated so that power can be transmitted around areas of damage, then such a ship would be able to keep fighting despite considerable damage. An all-electric ship would dispense with hydraulic power; one advantage of using electricity exclusively is that it is easier to control using computers. That in turn might make it easier to reduce the number of personnel for damage control -- a most desirable goal, given the high cost of naval personnel, at least in the West. Another argument for amalgamating primary and auxiliary powerplants is that future electric weapons, such as rail guns and lasers, may require a great deal of power when they fire; a ship capable of shunting her entire electric load to the weapon might enjoy distinct advantages.

In short

* "The move away from the conventional hull, as exemplified by sports or commercial designs, is proving more difficult to achieve in navy designs due to the sizes required"

* "following the maturation of catamaran technology, particularly with cruise liners, multiple-hull designs may resolve the hitherto contradictory equation: large deck area + high speed = low drag"

* "catamarans may additionally exploit benefits from the surface effect phenomenon"

* "but slim wave-piercing hulls have not yet had their last word."

Arsenal Ship to Sail Back?

Armada International was recently contacted by Rene Loire, a French Engineer who developed the idea of the Striker missile barge concept after learning that President George W. Bush had mentioned last December in his presidential campaign that he would refloat the Arsenal Ship. "His advising team and the new Secretary of Defense Don Rumsfeld are confronting the existing military staffs by suggesting drastic changes implying better cost-effectiveness, such as putting a stop to the building of very large aircraft carriers (equated to vulnerable dinosaurs), questioning the magnitude of the JSF program, replacement of main battle tanks by wheeled vehicles (some French Army-manned types are being tested over there), etc.. Regarding the Navy, Andy Marshall, is said to favour a fleet of platforms dedicated to guided missile-launching, either submarine or surface. In other words, Missile Barges are back and my Striker is again being appraised by Naval Staffs and at Net Assessment."