Propulsion systems selection is an important part of naval ship design. These systems have always had to satisfy speed and ship-service power requirements. Nowadays, increasing fuel costs are moving such decisions towards more fuel-efficient solutions. Unlike commercial ships, naval ships operate in a variety of speeds and electric loads, making fuel consumption optimization challenging. Propulsion System choices for modern Naval Vessels depends on some criteria:
Traditional criteria
• ship speed requirements
• shock capability
• noise reduction
• infrared signature
Additional criteria
• change in operation requirements
• greater range of speed
• added mission flexibility
• longer periods away from base with smaller crews
• cleaner emissions
An emerging global macro-trend in naval engineering over the last fifteen years has been a decisive migration to ‘more electric’ propulsion due to progressively increasing electrical load demand and a focus on affordability. All-electric and hybrid electro-mechanical arrangements are routinely demonstrating merit since 1990 in the Royal Navy Type 23 ASW frigate and subsequent modern frigate programs. These include the Royal Navy Type 26, German Navy F125, Spanish Navy F110, Italian and French Navies’ FREMM, Republic of Korea Navy FFX-II frigate and Finnish Navy SQ2020 frigate programs.
Types of Propulsion Systems
Combined diesel or gas (CODOG)
Combined diesel or gas(CODOG)is a type ofpropulsionsystem forshipsthat need a maximum speed that is considerably faster than their cruise speed, particularlywarshipslike modernfrigatesorcorvettes.
For everypropeller shaftthere is onediesel enginefor cruising speed and onegearedgas turbinefor high-speed dashes. Both are connected to the shaft withclutches; only one system is driving the ship, in contrast toCODAGsystems that can use the combined power output of both. The advantage of CODOG is a simpler gearing compared to CODAG, but it needs either more powerful or additional gas turbines to achieve the same maximum power output. The disadvantage of CODOG is that thefuel consumptionat high speed is poor compared to CODAG.
Combined diesel and gas (CODAG)
Combined diesel and gas(CODAG)is a type ofpropulsionsystem forshipsthat need a maximum speed that is considerably faster than their cruise speed, particularlywarshipslike modernfrigatesorcorvettes.
Pioneered byGermanywith theKöln-class frigate, a CODAG system consists ofdiesel enginesfor cruising andgas turbinesthat can be switched on for high-speed transits. In most cases, the difference of power output from diesel engines alone to diesel and turbine power combined is too large forcontrollable-pitch propellersto limit the rotations so that the diesels cannot continue to operate without changing thegear ratiosof theirtransmissions. Because of that, special multi-speed gearboxes are needed. This contrasts tocombined diesel or gas(CODOG) systems, which couple the diesels with a simple, fixed ratio gearbox to the shaft, but disengage the diesel engines when the turbine is powered up.
For an example the new CODAG-propelledFridtjof Nansen-class frigatesof theRoyal Norwegian Navy, the gear ratio for the diesel engine is changed from about 1:7.7 (engine:propeller) for diesel-only to 1:5.3 when in diesel-and-turbine mode. Some ships even have three different gear ratios for the diesel engines— one each for single-diesel and double-diesel cruises, and the third when the gas turbine is engaged.
Such a propulsion system has a smaller footprint than a diesel-only power plant with the same maximal power output, since smaller engines can be used and the gas turbine and gearbox don’t need that much additional space. Still, it retains the highfuel efficiencyof diesel engines when cruising, allowing greater range and lower fuel costs than with gas turbines alone. On the other hand, a more complex, heavy, and troublesome gearing is needed.
Typical cruising speed of CODAG warships on diesel-power is 20kn (37km/h; 23mph) and typical maximal speed with switched on turbine is 30kn (56km/h; 35mph).
Combined diesel-electric and diesel (CODLAD)
CODLAD(COmbinedDiesel-eLectricAndDiesel) is a naval propulsion system in which an electric motor and a diesel engine act on a single propeller. The transmission system takes care of making one or both motors act on the propeller shaft.
The CODLAD propulsion system is based on the use of electric motors directly connected to the axes (generally two) of the propellers. The electric motors are powered by diesel generators and to have higher speeds, as happens inCODADpropulsion systems, the higher power diesel engine is inserted which is disconnected from the transmission system to return to cruising speed.
This system that uses diesel engines for both propulsion and for the production of electricity for on-board services significantly reduces costs, as the number of diesel engines for the various ship services decreases and the electric motors need less maintenance. Furthermore, since electric motors can work more effectively in a larger number of revolutions, and being directly connected to the propeller axis, the transmission systems for coupling and decoupling the diesel-electric systems with the diesel engines used to have higher speeds.
The CODLAD system has been adopted in the newVulcano-class logistic support shipunder construction for theItalian Navy.
Combined diesel-electric and gas (CODLAG)
Combined diesel-electric and gas (CODLAG)is a modification of thecombined diesel and gaspropulsion system forships. A variant, called thecombined diesel-electric or gas (CODLOG)system, contains the same basic elements but will not allow simultaneous use of the alternative drive sources.
A CODLAG system employselectric motorswhich are connected to thepropeller shafts(usually 2). The motors are powered bydiesel generators. For higher speeds, agas turbinepowers the shafts via a cross-connectinggearbox; for cruise speed, the drive train of the turbine is disengaged withclutches.
This arrangement combines the diesel engines used for propulsion and for electric power generation, greatly reducing service cost, since it reduces the number of different diesel engines and electric motors, requiring considerably less maintenance. Also, electric motors work efficiently over a wide range of revolutions and can be connected directly to the propeller shaft so that simpler gearboxes can be used to combine the mechanical output of turbine anddiesel-electricsystems.
Another advantage of the diesel-electric transmission is that without the need of a mechanical connection, the diesel generators can be decoupled acoustically from the hull of the ship, making it less noisy. This has been used extensively by militarysubmarinesbut surface naval vessels likeanti-submarinevessels will benefit as well.
Combined diesel and diesel (CODAD)
Combined diesel and diesel (CODAD)is apropulsionsystem forshipsusing twodiesel enginesto power a singlepropeller shaft. Agearboxandclutchesenable either of the engines or both of them together to drive the shaft. Two advantages over simply using a single, larger diesel engine of the same total power output are that (1) diesel engines have somewhat betterspecific fuel consumptionat 75% to 85% max output than they do at only 50% output, and (2) there is a weight and size advantage to using twohigher-speed enginescompared to a single lower-speed engine, even with the slightly larger gearbox system.
Combined gas or gas
Combined gas or gas (COGOG)is apropulsionsystem forshipsusinggas turbineengines. A high-efficiency, low output turbine is used for cruising speeds with a high output turbine being used for high-speed operations. Aclutchallows either turbine to be selected, but there is nogearboxto allow the operation of both turbines at once. This has the advantage of not requiring heavy, expensive, and potentially unreliable gearboxes.
The reason that a smaller turbine is used for cruising is that a small turbine running at 100% power is more fuel efficient than a bigger turbine running at 50% power.
The system is currently used in the 3 ships of the Russian Navy’sSlava-class cruisers, theJapanese Maritime Self-Defense Force’sHatsuyuki-class destroyers, and the Dutch NavyKortenaer-class frigates(on which the Greek NavyElli-class frigatesare based).
Combined gas and gas
Combined gas turbine and gas turbine (COGAG)is a type of propulsion system for ships using twogas turbinesconnected to a singlepropeller shaft. Agearboxandclutchesallow either of the turbines to drive the shaft or both of them combined.
Using one or two gas turbines has the advantage of having two different power settings. Since thefuel efficiencyof a gas turbine is best near its maximum power level, a small gas turbine running at its full power is more efficient compared to a twice as powerful turbine running at half power, allowing more-economical transit at cruise speeds.
Compared toCombined diesel and gas(CODAG) orCombined diesel or gas(CODOG), COGAG systems have a smaller footprint but a much lower fuel efficiency at cruise speed and for CODAG systems it is also somewhat lower for high-speed dashes.
Combined nuclear and steam propulsion
Combined nuclear and steam propulsion system(CONAS) is used on theKirov-classguided missilecruisers.
Complementary to the nuclear component, there are two conventional boilers installed as a backup in case of reactor failure. Both components can drive two geared steam turbines, generating 120,000hp(89 MW), at two prop shafts
Integrated electric propulsion
Integrated electric propulsion(IEP) orfull electric propulsion (FEP)or integrated full electric propulsion (IFEP) is an arrangement ofmarine propulsionsystems such thatgas turbinesordiesel generatorsor both generatethree-phase electricitywhich is then used to powerelectric motorsturning eitherpropellersorwaterjet impellors. It is a modification of thecombined diesel-electric and gaspropulsion system forshipswhich eliminates the need forclutchesand reduces or eliminates the need forgearboxesby using electrical transmission rather than the mechanical transmission of energy, so it is aseries hybridelectric propulsion, instead ofparallel.
