Medium-sized ship's steam.

As navies sail deeper into the 21st Century they are facing a growing range of propulsion options for warships from medium-size--those of less than 8000 tonnes full-load displacement--to large patrol vessels. The basic building blocks of propulsion remain firmly entrenched in two technologies; diesels and gas turbines, but it is the selection of applications that is arousing growing interest.

The century-old propulsion technology of engines, transmissions and drive shafts is now being challenged.

The Diesels

The diesel remains the backbone of all naval propulsion either as a prime mover or as a power generator. Its combustion efficiency ensures greater return for the fuel consumed. Diesels operate on a two-or four-stroke cycle and, in the former, fresh air is blown into the cylinder and burned gas is driven out as the piston passes the exhaust point. Although every down stroke provides power, the length of the effective stroke is limited because hot gas is ejected through the exhaust points and is also diverted to power the blower.

In the four-stroke cycle the first down stroke sucks air into the piston, which is compressed by the up stroke. The second down stroke provides the power and the burned gas is expelled by the second up stroke. It is this technology that is primarily used for warship propulsion.

Yet the amount of power that each cylinder can deliver is finite because the power stroke twists the crankshaft. Increasing the number of cylinders makes it more difficult to keep the crankshaft rigid, a problem that is aggravated by the fact that the pistons do not operate simultaneously. This creates vibration and, at a certain level known as the resonance speed, the crankshaft can break.

Because warship engine rooms are limited in height they need diesels with shorter strokes. As cylinder power output depends upon the volume swept out on each stroke, then the engine must produce more strokes to reach a given power output. But this brings it nearer to its resonance speed yet because of their intrinsic low specific fuel consumption, their low operating costs and sustained operation producing long cruising ranges, diesels have proven suitable for a wide variety of vessels. They have proved especially valuable for vessels whose missions require patrolling: including offshore patrol vessels, while their reliability in producing sustained levels of power has meant they are invaluable for vessels such as mine countermeasures vessels and auxiliaries.

But diesels can be adapted with superchargers to provide high bursts of speed. These compress combustion air before it enters the cylinder and are driven either mechanically from the crankshaft or in the case of turbo-chargers driven by exhaust gases.

Four-stroke high-speed engines using lighter F-75 standard fuel diesels have powered fast attack craft and high-speed patrol boats since the 1960s, although their fuel consumption is higher and life cycles shorter than that of their medium speed counterpart.

Diesels are produced by a wide variety of companies in the western world. Leading the pack are Caterpillar (which also owns Perkins Engines and Sabre engines) and Detroit Diesel Corporation in America, and CRM, MTU Friedrichshafen, MAN B&W (which owns the former Alstom which produced Mirrlees, Blackstone, Paxman and Ruston), Isotta Fraschini Motori, Semt Pielstick and Wartsila (which owns GMT and Sulzer) in Europe. Their engines have been used as the sole propulsion of medium-size warships such as destroyers and frigates, notably France's Cassard and Lafayette classes that use the Semt Piel-stick 18PA6 V280 and the 12PA6 V280 respectively. These vessels would have previously used steam turbine systems, which require large engine room crews. The installation is so efficient that some navies continue to seek Codad (COmbined Diesel And Diesel) configurations as exemplified by Singapore, which selected four MTU 20V 8000 M90s for its Formidable class frigates.

Gas Turbines

Nevertheless, a majority of navies use gas turbines. These frequently are marine versions of aircraft jet engines. In the gas turbine air is drawn into a compressor, mixed with fuel and ignited in a combustion chamber or combustor. Part of the power is used to support the compression process through the compressor turbine but most of the hot gases from the combustion chamber are used by the power turbine to convert the thrust into a rotary motion that is fed to the transmission through reduction gears before being vented through the exhaust.

The market is dominated by General Electric and Rolls-Royce, although the Norwegian Skjold class are now being powered by Pratt & Whitney turbines (see Water Jet subtitle). Sweden's Visby class corvettes have four 4 MW Vericor Power Systems TF50A gas turbines backed by two MTU 16V N90 diesels. While attention is largely focused upon western gas turbine design, mention should also be made of the Ukrainian organisation Zorya-Mashproekt, which produced 65 per cent of the Soviet Navy's 3 to 16 MW gas turbines. With the breakup of the Soviet Union the company has focused on the export market, developing 10 MW and 25 MW engines for this purpose, although it still works closely with Russian yards and has provided the UGT1600 (also known as DT59) installation for the Indian Navy's Delhi class destroyers and Talwar class frigates.

Zorya-Mashproekt also supplies the high-speed UGT6000 (also known as DP71) and UGT3000 cruise turbines for the Tarantul class corvette's M15 Cogag and M15-1 Codag installations--and a series are being built for Vietnam. The company now offers a new version of the M15 installation with DS76 gas turbines replacing the UGT3000s.

Gas turbines are widely used in destroyers and frigates because they have the advantage of being extremely compact while capable of producing substantial power, up to one kW of power for one kg of engine mass. This is because they run hot and fast, yet they are also quieter than diesels, which makes them suitable for anti-submarine operations. Increased combustion temperatures and lower specific fuel consumption mean that the gas turbines can also be used to provide cruising speeds over long periods although they are not as fuel-efficient as diesels. The drawback is that they are expensive to run and the Spruance (DD 963) class destroyers, which are now being paid off, can burn up to $ five million worth of fuel a year.

Separate engines for the high speed and cruising functions are used in what are called Cogog installations such as are found in the British Type 42 destroyers, which run Rolls-Royce Olympus TMN3B and Tyne RM1C. Alternatively, as in the Arleigh Burke (DDG 51) class the same GE LM 2500s are used for both functions in Cogag (Combined Gas turbine And Gas turbine) layouts incorporating four gas turbines. Here, half the turbines are run when the destroyer is cruising and the other two are used when high speeds are required.

However, this is expensive in terms of fuel, and in many destroyers and frigates gas turbines are combined with diesels to exploit the technical advantages of each type of engine for lower operating costs. Norway's new Fridtjof Nansen class frigates, for example, will have a Codag (COmbined Diesel And Gas turbine) plant using LM 2500 gas turbines and Izar (formerly Bazan) Bravo 12V diesels, while Germany's Sachsen (F124) class frigates have LM 2500s operating in conjunction with MTU 20V 1163 TB93 diesels. An alternative is to use the two types of propulsion in sequence to exploit their respective strengths; the Franco-Italian Horizon destroyers will both feature a Codog (COmbined Diesel Or Gas turbine) installation based upon LM 2500 and SEMT Pielstick 12 PA6B diesels.

Diesels can be 20 to 30 per cent more efficient than gas turbines, but an attempt has been made to improve the latter with a view to reducing fuel consumption through the application of Inter-Cooled Regenerative (ICR) technology. This involves diverting exhaust gas via a regenerator and through an intercooler to boost the temperature of the air entering the combustion chamber/combustor, which increases overall thermal energy. Not only does this reduce costs but also it improves low-power performance, possibly to diesel levels with a 30 per cent cut in fuel consumption.

A multi-national project began in the 1990s to develop an ICR propulsion unit for application in a variety of platforms. The project involved Northrop Grumman (formerly Westinghouse) and Rolls-Royce, with the participation of DCN, and used a 21 MW gas turbine based upon the RB211 which became the WR-21.

This was selected for future British warships, it was evaluated by the Japanese Maritime Self Defence Force and was considered for the US Navy's DD-21 programme until this became the 14,000 tonne leviathan DD(X), rendering the engine inadequate for the task of moving the ship whilst providing the greater demands for hotel service power.

The WR-21 provides excellent fuel consumption at both low- and high-power levels as well as meeting the requirements for reduced infrared signatures and exhaust emissions together with low ambient noise levels. It is being installed in the Duke (Type 45) class destroyers, but Rolls-Royce states that they currently have no plans to apply the technology to other gas turbines, including what must be regarded as its successor, the 30 MW (+) Marine version of the Trent 800 aero-engine, which is designated MT-30.

The engine is actually an extremely modular 36 MW unit with a high efficiency power turbine and gas generator that exploit advances in blade cooling concepts (this helps achieve 42 per cent thermal efficiency), and an annular combustion chamber configured to reduce emissions. With improved fuel efficiency the MT-30 can be used for high speed and cruising while the unit, which includes single crystal blades in the turbine, has a longer life with reduced maintenance. It has been selected for the early DD(X) as well as for Lockheed Martin's first Littoral Combat Ship (LCS) while also being a contender for the European Fremm multiple-mission frigate.

Its competitor here was not the 25 MW General Electric LM2500 but its successor, the 30 MW LM 2500+. This is a more powerful version of a well-proven engine thanks to the addition of a new wide-chord bladed disc stage of compressor blades in front of the first stage blading leading to a 20 per cent increase at full power. All first stage blades are wide-chord designs and there have also been improvements in the compressor to improve efficiency.

Tweaking the Diesels

Diesels are also being steadily enhanced and are able to match many features of the gas turbine in terms of high-speed bursts, which is why they are selected even for smaller warships that have such a requirement. For Greece's Super Vita (Roussen) class fast attack craft MTU is providing the 16V 595 TE90, which can give speeds of 34 knots or a range of 1800 nautical miles (3300 km) at 12 knots. The MAN RK 280, which evolved from Ruston's RK270, has been supplied for the Brunei class corvettes and gives these 1940-tonne ships a maximum speed of 30 knots.

To help achieve this there are alternate approaches. Double-stage sequential turbocharging is used by the market leaders such as MTU and Semt Pielstick, but a 'smart' electronic fuel injection system is a solution from Rolls-Royce Diesels. The latter is also important to meet environmental demands that are of growing importance.

With greater calls to reduce emissions including nitrogen oxide, sulphur oxide, carbon dioxide and particulates this aspect of marine propulsion technology is attracting special attention in the diesel field (less in gas turbines, which are inherently cleaner than diesels and can be further cleaned through additions to the fuel). Here nitrogen oxide emissions can be reduced by reducing peak combustion temperature; this is achieved by increasing the compression ratio or changing valve and fuel injection timing, but other technologies being pursued include catalytic systems using either a chemical or non-thermal plasma.

Fuel injection itself has benefited from advances in the automotive industry. The mechanical injection system is now being augmented by high pressure or 'common rail' injection systems, some of these employing the latest piezo-electric-controlled injection valves that have fewer mechanical parts.

Moving the Screw

Whichever prime mover is used in a warship the energy it creates tends to be used in the same way to operate great steel drive shafts with fixed or controllable pitch propellers. This is achieved through massive reduction gears or transmission systems produced by a number of specialist companies such as CST Cincinnati, David Brown Engineering, Maag Gears, Renk, ZF Marine and Zvezda in Russia. But the growing use of electrical power may mean the toll for both reduction gears and drive shafts.

Electrical propulsion was developed by the Royal Navy for its latest anti-submarine vessels, the Duke (Type 23) class frigates which featured a Codlag (Combined Diesel-eLectric And Gas turbine) installation with Spey gas turbines, MAN B&W (Paxman) diesels and two 3MW Alstom electric motors for use during sonar searches. By the time these ships began to enter service navies were seriously examining the concept of an integrated full electric propulsion system based on the use of gas turbines and/or diesels acting not as prime movers but as power generators for high-speed permanent magnet propulsion motors.

By then the concept was common in fast ferries and cruise liners and was gradually being introduced into large auxiliaries such as the Dutch Rotterdam class amphibious warfare ships. It is now being applied in the new generation of warships starting with the Royal Navy's Daring class destroyers which will have the WR-21 and two 2 MW diesel generators and two 20 Alstom MW electric motors. It appears that the Iceberg design bureau has developed an electrical propulsion system based upon Kolomna diesels with Zvezda gears but the production status is unclear.

The DD(X) is to have two MT-30 gas turbines and two auxiliary gas turbines generating power for a pair of 36 MW high-speed permanent magnet generators. Both the DD(X) and the Type 45 will retain conventional drive shafts but the advantage of an integrated full electric propulsion system is that it has the potential to offer other options which have largely been confined to smaller warships. In particular it offers the opportunity for a total elimination of transmission systems because the electric motors will provide power via cables directly to the propulsors resulting in massive savings in both weight and space; in addition it might allow designers to distribute the whole propulsion system machinery around the ship, saving space and reducing vulnerability to battle damage.

Water Jets

The conventional propeller, even without a massive drive shaft, may well continue to be used, but other options are available or will soon become available. One which has become established for smaller warships is the water jet, which operates in a similar way to aircraft jets sucking in water and driving it out with great force in steerable units that can then act as rudders.

Water jets can provide high bursts of speed, but other advantages of this installation include a reduced acoustic signature, lower shipboard noise levels and better manoeuvrability. They are especially valuable in smaller warships because they offer a reduced draught (and risk of damage with floating debris) as well as lower maintenance costs.

The concept was used by Sweden's G6teborg class corvettes, which are now being augmented by the Visby class with Rolls-Royce Kamewa 125 water jets. Norway's Skjold prototype fast attack craft has water jets and they have also been selected for both of the US Navy's LCS contestants, but currently they are not really suitable for frigate-size ships or larger. However, Bird-Johnson, a Rolls-Royce subsidiary, is designing and developing the 26 MW AWJ-21 advanced water jet, partly with government funding, and this might be suitable for larger warships. While the first of the Norwegian Skjold class has two R-R Allison 571-K gas turbines individually rated at 3 MW and two MTU 183 diesels, the five subsequent production vessels will have a Cogag system featuring Pratt & Whitney gas turbines: two ST18M and two ST40M with Renk BUS 86/75 transmission system to both water jets. This will be retrofitted into the lead ship.

Motor-in-Pod

An alternative that has already proven itself efficient in the commercial world comes in the form of a self-contained, electrically-driven propeller in a steerable pod. The podded drive, such as the Alstom and Kamewa Mermaid with brushless synchronous motor, has been used in cruise ships and has been selected for the French Navy's Mistral amphibious warfare vessels. They are extremely easy to maintain, for they eliminate the need for the bulky conventional propulsion system, as well as the standard type steering systems, they save space and are easy to remove if damaged or if they require overhaul.

But their exposed position makes them extremely vulnerable to shock, especially from mines, their light weight prevents them producing high bursts of speed and it is difficult to control both their acoustic and magnetic signature. Until these problems can be overcome it is extremely unlikely they will be incorporated in most surface combatants.

Code to Cods and Cogs

CODAD                  COmbined Diesel And Diesel
COGAG        COmbined Gas turbine And Gas turbine
COGOG         COmbined Gas turbine Or Gas turbine
CODAG             COmbined Diesel And Gas turbine
CODOG              COmbined Diesel Or Gas turbine
CODLAG   COmbined Diesel-eLectric And Gas turbine