Modern warships (VI) - other topics

This part is about several smaller topics that are not as much driving the general design and idea of warships. Most of them are 'back office' kind of things that should be mentioned.

Ballistic missile defence

BMD is an exception on this page; it's not back office at all, but instead a hype - and has been a hype for a long time. The days of SDI are over, but conventional ballistic missiles had the West's attention since they were used as propaganda munitions in 1991.

There are several ways of mitigating the ballistic missile threat that are essentially preventive or soft kill defences; they seek to avoid that the missile aims at the (correct) ship and most of them are being used anyway in order to avoid being targeted by ordinary cruise missiles. It appears to be fairly easy to defeat the sensors of a ballistic missile even if it was aimed right at the convoy; it may have nothing but a radar that's looking down at the convoy, no sensor fusion - and it has mere seconds to choose what to attack for the one and only approach. There's no turning around after being fooled by some chaff or decoy.

The hard kill methods can be separated into two distinct categories;

• missiles with dedicated seeker and warhead for exo- or upper-atmospheric employment. These missiles may potentially deal with really long range ballistic missiles that were aimed at targets a long way behind the warship. Such missiles are hardly suitable to defeat aircraft or cruise missiles because of their specialisation for the conditions of the upper atmosphere (hardly any air to heat up an infrared sensor's window even at thousands of kph speed, little use for rudders in the thin atmosphere). These missiles may play a role against the satellites in very low orbits (all satellites are very short-lived in such orbits and thus need to be cheap, but the relative proximity to the ground makes less sophisticated sensors viable than with most satellites).
• And then there are also missiles that may deal with ballistic missiles in the lower atmosphere as if they were hypersonic missiles coming from above.

Lance short range ballistic missile as seen by imaging infrared sensor

I am not aware of any aircraft- or shipborne ballistic missile designed to engage ships at sea. The only ballistic missiles capable of hitting a moving ship at sea appear to be the Chinese DF-21D which got a lot of press a few years ago because the USN worries that it might not be able to bomb China at will with its carriers if the Chinese have effective land-based long range ship-killing capability in them. Another example was the Soviet R-27K which was cancelled decades ago.

The purpose of convoy defence at sea doesn't require exoatmospheric intercept missile, and the same type of missile (SM-6) as for long range area air defence could be used for other BMD purposes.

I should also mention that the terminal approach speed of a missile is mostly dependent on the range of the missile; the longer-ranged ones are moving faster on their descent. Hence there are sometimes descriptions such as "can intercept ballistic missiles of up to 500 km range" given for area air defence missiles. A "BMD-capable" missile isn't necessarily able to stop all kinds of ballistic missiles, even if it can stop the short-ranged ones.

You also need a proper radar for the fire control if not the early warning.

Security in ports

Many great powers suffered severe losses of warships in port during the Second World War. The Americans were hit hard in Pearl Harbour, the Germans were hit hard in Brest, Tromsø and various German ports, The Italians were hit hard particularly in Tarent, the British were hit hard in Alexandria and Scapa Flow, the Russians were hit hard at Leningrad. The Italian and Russian capital ship losses in port even exceeded their losses of capital ships at sea, while the German losses of capital ships were about 50/50 in port and at sea. The security and safety at port is thus an important issue as proven by military history.

Classic port security was mostly about defensive minefields, blocking ships, the port's own air defences, anti-torpedo nets, maybe some smoke against bombers and certainly also sailors standing guard. This was insufficient (even though anti-torpedo nets helped a lot), was impotent against the Italian frogmen attacks and would help nothing against incoming missiles. Modern port security & defence needs to be better, much better. A € 2 billion warship is not much less vulnerable to most attacks in port than a container barge.

The air defences and BMD capabilities of warships at port should be ready if not active in wartime (though one search radar in active mode would be enough per convoy, ready to react to incoming threats in time). AEW cover makes sense even in port. Air defences need to be capable of protecting against terrain-following missiles.

Defence against sabotage by hijacked cargo ships intentionally ramming anchored warships is possible by using a (floating) ship as blocking ship. The tug crews in port should also be ready and motivated to intervene.

Small boat attacks in port are rather unlikely in a conventional war between nations.

There's a surprisingly comprehensive wiki article about security against sabotage by divers or attack by tiny submarines.

  
Mine countermeasures

Dedicated MCM ships are not the subject of this article series. It does make sense to have limited MCM capabilities onboard an ordinary warship, though. A mine avoidance sonar capability makes sense on frigates and larger warships for self-protection and some divers are necessary to inspection , maintenance and potentially repairs underwater as well. A minehunting drone for object identification could be used, and might be of some use for battle damage assessment in ASW as well. A warship may thus have a limited capability against stationary naval mines, including clearing them with divers, with little extra equipment.
Mobile naval mines (such as self-deploying, torpedo-like mines) should be handled just as the heavyweight torpedo threat should be handled. 

Boarding

Warships need to be able to inspect ships to identify blockaderunners and auxiliary cruisers (cargo ships turned into raiders). This has to be done at a safe distance (see HMAS Sydney), and only a helicopter should thus be exposed to the great risk of closing in with a suspect ship in order to keep the warship itself safe. Some volume, some seats, and a winch capable of pulling up two grown men should help a lot. Additionally ropes for fast rapelling may be used during boarding. The extra training requirements affect but a few sailors and the extra equipment requirements (helmets, small arms, inspection tools) require but negligible expenses.

Boarding by boat / dinghy is unacceptable in wartime and the zenith of incompetence if done at wartime unless done close to port, period. The warship would need to come too close and be too much at risk to shorten the boat's cruise to and from the suspect ship to an acceptable level

Evacuation

Warships depend heavily on inflatable marine life rafts for survival of the crew of a sunk ship. The capacity of the boats carried by a warship is but a tiny fraction of the crew size. Navies mandate a certain excess evacuation capacity (as far as I know 110% capacity in the case of the USN), but the ability to actually make good use of this capacity after taking multiple hits, experiencing a fire and heavy seas is questionable. Frequent evacuation training is risky and may actually injure and even kill people, especially when done in cold weather and high sea states.

Everything that's inflatable is highly susceptible to damage by fragmentation, and the metal shells of inflatable marine life rafts are not protected against the fragmentation effect of missile hits.

Evacuation by helicopter is possible in principle, but has a very low throughput (personnel evacuated divided by time) and is thus of little help in case of a torpedo hit (which sinks a ship quickly). Severe fires onboard or stormy weather may impede evacuation by helicopter as well.

A simple way to improve the capacity for evacuation would be to mount many additional evacuation devices on vertical walls of the superstructure as was done during WW2 with Carley floats. This would go counter the polygonal radar stealth trend in warship construction, though.

enough orange +
reflexive tapes on a survival suit

Navy personnel often wears flash (fire) protection equipment when the ship is combat-ready or at war, but hardly ever do they wear survival suits. These are mostly reserved for those who are in boats or helicopters. Such suits might be a necessity for making good use of inflatable marine life rafts in cold weather, though. The transition from warship to inflated raft is the tricky part, and may easily go wrong regardless of technique used.

blue, black, white or grey uniforms are idiotic for shipboard use

The absolute minimum required for a sane approach for crew survival is to not give them uniforms that camouflage in no other environment than the sea. A rational uniform has to have a bright orange top part. It doesn't need to be exposed all the time, but the user has to be able to expose this easily detected colour with little force and one hand only. A bright orange hood might suffice. This isn't really about uniforms: Navies have to do their job and prepare for real battle - and this includes the nigh-certain events of ships being burned or sunk, and crews being forced to evacuate under difficult conditions. It's a mindset issue.

Firefighting for other ships

Water cannons can be used to assist with firefighting on another ship (at least cooling down the outside of a hull if nothing else). Some water cannons should be present in a convoy, albeit not necessarily on a warship.

Search and Rescue

SAR is a noble task and purpose for any ship or boat, and most warships would be particularly well-equipped for it. The combination of naval helicopter (FLIR used for identification of ships helps with searching and the boarding equipment helps with rescue), boats with outboard motor and ship-mounted infrared sensors (needed to improve the detection of sea skimmers) is a powerful one.
 
The problems with SAR are that it may divert helicopters from submarine search efforts and slowing a convoy down to care for survivors of one or multiple ships might endanger it. To detach a warship for the rescue effort would put the remaining convoy at increased risk and it might take a long time to catch up afterwards if the convoy's cruise speed is high. It may for this reason be impossible to just send small boats to pick up survivors without any ship staying behind for the effort. Even worse; those boats may be wanting for an evacuation effort if their ship gets hit in the meantime.
In the end it may be advisable to have a few commandeered civilian helicopters with the convoy, ready for rescue missions only (large cabin, equipped with winch).

Replenishment at sea

Warships can be replenished at sea, and this is done primarily with fuels and food. Replenishment of munitions at sea is possible as well, but the effort was widely judged to not be worth it. An exception to this rule was the rare torpedo resupply efforts for German submarines at sea in the Second World War. Missiles and torpedoes are so expensive that the national stocks don't exceed the inventories on board the ships by much (if at all) anyway.
 

Endurance

Warships can be resupplied at sea, but it would still be favourable to possess the endurance for a transatlantic (NY-Rotterdam) crossing at 17...19 kts with about 20% fuel reserve. This is more than just about fuel (including for helo operations) and cold storage capacity: It's also about crew quarters/morale, spare parts (especially for helos) and munition stocks (in particular lightweight torpedoes, and avoiding dependence on "expendable" sonobuoys).

Warships should have little trouble with such a requirement, but especially the fuel supply of and for drones may be an issue. Smaller craft are much less fuel-efficient than larger ones, and drones should be small. This is not an issue with towed decoys, but freely moving surface drones on picket duty for early detection of sea skimmers and such would have a fuel consumption on a 15...25 kts transoceanic cruise that's out of proportion to their size. Their systems would not be maintained by a crew since drones are by definition unmanned, and they would likely have little if any redundancies in order to keep them cheap and small. Drones would need to be recovered, maintained and refuelled by the warship's crew to achieve the endurance and appropriate cruise speed for a transoceanic convoy action. They would also be recovered ahead of high sea states, so the ship would need to be able to carry all its drones at the same time. To keep drones outside during a storm leads to high drone attrition and/or slows the convoy down as small surface craft become slow in high sea states (albeit wave-piercing hull shapes may help to maintain speed at medium sea states).
 

Speed and agility

Many convoy escorts used to be fast ships in order to catch submarines before they dove away, but many successful convoy escorts were slow designs (see Flower class ASW corvettes). To run away from torpedoes is impossible once they got close anyway, so there's little reason to emphasise the top speed much.

During the Cold War there was an argument that ASW frigates needed to be fast in severe weather to keep up with nuclear attack submarines trying to escape. That's now less of a concern since so much about ASW has become helicopter-centred. A submarine that's running away from a convoy before having done harm is a submarine that was protected against anyway.

The top speed of a warship doesn't look terribly important nowadays. The cruise speed on the other hand should keep the pace with the large cargo ships, which may cruise at 12...25 kts. A design for 17...19 kts cruise speed (with towed sonar array and towed decoys deployed) would thus make much sense. Towed sonars are still be usable at 17...19 kts at most, depending on type.
A higher cruise speed is very difficult to achieve with a small hull (relative to the much bigger container ships and tankers) and a given set of equipment requirements. Engines cannot be run on 100% output for days, so more power would be installed to achieve 20 kts of cruise speed than is actually necessary to reach 20 kts. Even more power may be installed for redundancy; marine engines may be shut down for maintenance during a cruise, and having more engines at hand means that a ship repairing one of its engines would not necessarily slow down a convoy or fall behind.

That extra power may help to quickly turn around the ship in response to an incoming missile if it's available within seconds. It's often preferable to be hit at the stern by a missile (above water!) than elsewhere. Warships of all sizes are pretty good at such emergency turns.

Medical support

A convoy at high seas should have the facilities for proper emergency surgeries. It's but one room that's required, and the two or three medical personnel (with more tasks than only medical affairs) that should be onboard any warship of 200+ crew anyway, so this is no extraordinary requirement. In worst case the helos may be able to shuttle additional medical personnel in from cargo ships or other warships.

NBC

Not all warships have a proper NBC filtration system, and it appears to be out of fashion due to the reduced threat of nuclear warfare. A washdown system that might clean a ship from fallout using seawater on the other hand may be most useful as camouflage against both infrared and radar sensors. Its spray helps to conceal and the water could also almost equalise the temperatures of the ships' surfaces and the sea.

A CV90120 light tank showing the use of mist as countermeasure to IR sensors (and likely laser ranging).

Naval intelligence

It's questionable how much the trick of guiding convoys around mobile threats still matters. Surface threats should be eliminated by air power easily, and subsurface threats would typically have unknown locations.

Naval intelligence is more relevant for identification of signals and echoes; noise profiles, radar and radio emission patterns and wavelengths et cetera. 

Missiles that may aim at particular parts of a target after identifying its shape with an imaging sensor would also need to know where to aim at for best effect.

Cargo ships would have to switch off AIS transponders in a convoy, and maintain radio discipline as ordered by the convoy leader. Many cargo ships are flying the flags of Panama and Liberia while being crewed by Filippinos. It's unclear how this would affect the availability and discipline of cargo ships in a future war. Most likely insurance schemes that compensate for the loss of cargo ships would be necessary, as previously during the world wars.

I may still underestimate the importance of naval intelligence. I do simply not believe that all important intelligence may be collected ahead of a war, so I expect a navy to focus more on being prepared to deal with the unknown than on trying to know as much as possible about threats.

Orbital satellites

Orbital satellites may be used to detect and even identify surface ships, but those would rarely be a threat anyway, as mentioned before. It's in theory possible to detect submerged submarines with orbital radar satellites, but this requires fairly fast movement at little depth, so it's not really relevant for all I know.
 

Hollow force syndrome

You may have read about low readiness of warships in one or another navy. This is sometimes due to teething problems, sometimes due to severe design flaws and sometimes due to modernisation programs.

There's one all-too common reason for low readiness that's easily avoidable: Insufficient funding for repairs at the industry and spare part stocks. These are either "hollow force" symptoms or symptoms of a ship being very old (and thus likely already of little military utility save for the training of inexperienced sailors).

A navy should simply not buy more ships than it can operate at a good level of readiness, and it should modernise or get rid of ships that have spare part supply problems because of obsolete parts that no-one still supplies or maybe even no factory of the world is still capable of producing (this happens a lot with electronics). The "hollow force" symptom is not a consequence of politicians insisting on budget cuts; it's a consequence of naval top brass and "defence" politicians insisting on keeping the quantity of active hulls high instead of adapting the force size to the cuts properly. It's better to mothball a ship or two than to have six ships in poor readiness.
 

Upgrading warships

The easiest ways to upgrade a warship is to give it better munitions (and to integrate them in fire control) or to improve the crew quarters. It's more demanding to upgrade the sensor equipment, and there's rarely a major hardware upgrade for radars or sonars despite the often 30 year-long career of most frigates and destroyers.

Upgrading of major weapons systems and even reconfiguring for different mission sets is made more easily if one makes use of standardised modules such as the Mk 41 VLS or modules from the MEKO system. One shouldn't fall for the "modularity" buzzword wholly, though: A navy that is in full marketing drive to sell to the nation the idea that it should get ten frigates and talks all the time about how easily it could reconfigure these for new mission needs thanks to "modules" would probably buy 12-15 modules for 10 ships, if it completes the module development at all. The naval bureaucracies are interested in having hulls, not in having all those peripheral things (save for the USN, which thinks it's an air force and spends more on aircraft than on ships these days).

Crew concept

The German navy is touting a 2-crew concept for the oversized, overpriced patrol ship that's the F125. The F125 is supposed to be more at sea per year by having one crew that recovers on land and one crew on duty. They did sell this idea by claiming that a single crew would be really small due to automation, so two crews would be affordable. They will predictably fail to recruit and train enough personnel for a 2-crew concept, and the crews are small because those ships have little sensor equipment and no more firepower than a 30 years old 300 ton fast attack craft. Moreover, the entire idea of having multiple crews is nonsense - even for embargo enforcement missions. NATO has well over a hundred warships. Let them rotate the next time some unfortunate small power faces the wrath and is embargoed by the United Nations. There's no need for one specific ship to stay on blockade duty for 60-80% of a year.

Transoceanic convoy operations would sure be exhausting to some degree, but a single crew with decent quarters and comfort onboard could still execute more such missions in a row than any naval bureaucracy would ever buy enough munitions for (remember the false contact problem). Bad leadership could change this for the worse, but a 2nd crew is no answer to that either.

To have a 2nd crew almost doubles the operating expenses of a warship for negligible gain in deterrence or defence value.

The average warship crew is composed of amateurs compared to the average cargo ship crew. There's more division of labour for tasks in navy ships (this is part of the reason for the way too many collisions at sea) and crewmembers are not very versatile because poor retention rates reduce the average time on sea in a lifetime of a navy sailor to a few years (if the sailor is member of a ship crew at all). Personnel policy and systems go beyond the scope of this article series, but really good policies would enable very different, vastly superior crew concepts with a bit smaller, but first and foremost much more versatile crews. A part of the reason for the current division between AAW and ASW ships is that to train for high proficiency at both takes too much time - the crews are not stable enough. Imagine a team that's 90% officers and sailors who have served on that one ship for the past five to ten years together, with 40% time at sea. Both their ASW and their AAW competence would be superior to anything known to any NATO navy.

Reserve displacement, volume and deck area

Reserve displacement is how much additional mass the ship can take in before being too heavy (to float). It used to be all-important to allow a ship to survive the leak caused by a torpedo hit, but nowadays it's especially of interest regarding upgrades. Good upgrades may add much weight, and an insufficient reserve displacement (or too much top heaviness) may prevent such an upgrade. Deck areas and volumes need to be reserved or used for low importance purposes or else one might not find places where to install the upgrade hardware and where to berth the additional crewmembers required to operate it.

There is an evil twin to such foresight; "equipped for, but not with" warship designs. These are yet another ugly way how naval bureaucracies try to get more and more toys (ship hulls) to play with; they buy hulls, but don't equip them fully. That's how underarmed ships like the La Fayette class (3,200 tons, but no more powerful than a 1,300 ton Sa'ar 5 class corvette) came into being. The bureaucracies then sometimes praise the "room for growth". One shouldn't fall for this. "room for growth" is a good idea exclusively if the basic equipment status is already fine.

Construction standards 

Much was written about how only shipyards specialised on warships can build proper warships, and I call B.S. on that. That's disinformation by shipyards that specialised on 'marketing' to naval bureaucracies (and politicians) and have lost competitiveness on the civilian market after decades of cushy contracts and having bureaucrats & politicians as their primary or only customers.

Warships have different construction standards that make a 4,000 ton warship much more likely to survive a hit than a 4,000 ton civilian ship, but much of this could be retrofitted, even an improved watertight compartmentation. In fact, a cargo ship with a cargo of secured hollow steel spheres could easily be rigged to stay afloat after hits that would sink any 10,000 ton warship.

The dilemma with firefighting at sea is that you want to cool down the area on fire to extinguish the fire for good, and the preferred way to do is is to inject water that evaporates (which converts a lot of thermal energy). The problem is that the more seawater you bring into the hull without it evaporating and escaping to the outside atmosphere the heavier the ship is going to be - until it sinks. To extinguish fires is often easy, but to cool down compartments that burned is much tougher. So as long as water is an important means to cool, a good reserve displacement and reliable control of ballast tanks are the way to go. An alternative way of cooling down burning compartments is to inject chemicals that react using the thermal energy as the activation energy. The fire's thermal energy gets converted into chemical energy. This can cool the compartment enough to extinguish the fire. Still, water is an important means to cool because it's plenty, it's free - and chemical agents are not necessarily procured and carried in the necessary quantities because the top brass has different priorities and peacetime isn't punishing such behaviour.

Fires can also be extinguished by covering the surfaces of flammable materials with less flammable materials (which may be super-quick, but is dangerous as the coating may coat lungs and suffocate people), by flushing out the atmosphere (and thus the approx. 21% oxygen - again not necessarily healthy for the crew) preferably with inert gases or by restricting the flow of oxygen to the fire (foam does this in addition to covering the flammable materials).
Automatic fire extinguishing equipment can be retrofitted into all compartments. De facto instant fire extinguishers are even installed on many yachts, at bus and truck engines, and there are even portable types that can be thrown into a fire to extinguish it.

There are mostly traditional ways of dealing with leaks (especially having many watertight compartments, plugging small leaks and protecting components against the electrical and chemical consequences of contact with saline water). You may also fill a compartment with a quick-hardening foam to limit how much additional weight a leak may add to the ship. This can be retrofitted as well.

Shock hardening (important for near misses of mines and torpedoes) is important as well, but it can be retrofitted to some degree.

Hull materials

Steel is relatively cheap and still the dominating material for the construction of ships, including warships. Aluminium got a really, really bad reputation after the USS Belknap fire where the superstructure lost its structural strength due to the heat of the fire and collapsed. An aluminium alloy structure does rather not melt in such fires; it becomes soft when hot and collapses under the weight of the superstructure elements it was meant to support.

USS Belknap after the fire, with collapsed superstructures

Plastic composites have been considered and used (especially for small craft) and fibreglass-reinforced thermoset matrix plastics may become the materials of choice for superstructures. They don't have the same corrosion issues and thermoset plastics don't melt. There are still issues at the interface with a steel hull because the steel and the plastic composite expand at different rates as they are warmed and accordingly contract differently as they are being cooled. The automotive sector despairs over this problem, that's why we don't drive many plastics-skinned cars.

Offboard electronic warfare

A few land forces have pioneered the use of helicopters as electronic warfare platforms, complete with jammers. Most VHF radio jamming is limited in its effectiveness because a line of sight is needed to the receiver that shall be jammed. Jamming vehicles are sometimes armoured, but their employment far forward or in prominent positions (hilltops, for example) is still dangerous and difficult. Raised masts may help with this, but helicopters have it much easier to get into line of sight; they simply climb to the necessary altitude.
Naval helo-based jamming would be possible as well, but the underlying reasoning would be different. Warships that outsourced air search to AEW helicopters would be largely silent; they wouldn't emit much themselves, and this should help their survival. Radio and radar jamming on the other hand requires an active emitter, so outsourcing this as well might make sense. The mobility and altitude of a helicopter could also help deal with the radio horizon issue.

There might be a reasonable case for an offboard radio/radar jammer to deal with attempts to identify ships by long-range SAR (synthetic aperture radar - more or less imaging radar) and for jamming radio datalinks to subsonic cruise missiles (at least jamming the link from missile to platform so the operator could not 'see though' the missile's seeker and help it with decisionmaking). The ASW helicopters would be an obvious platform of choice for this - maybe a jammer kit could be provided alongside an AEW kit? The AEW platform itself might help with jamming in its own radar band.

Propulsion

Propulsion is linked to speed, range, noise and seemingly unrelated things such as mast designs.
Modern cargo ships are motor ships; they have huge bore, low revolutions per minute diesel engines. They can burn some really low quality fuels, which allegedly they do outside of territorial waters where they don't get into trouble for the emissions. Such a propulsion is optimised for efficient cruise, reliability and repair during a cruise. It's also very loud.

Frigates and destroyers usually have separate engines for cruise and sprint, and typically these are coupled for sprint. The sprint engine is usually a gas turbine (and most advances in aircraft turbofans don't apply to such warship sprint engines because an increased bypass ratio doesn't matter in such an application) and very rarely a diesel engine unless the warship has a modest top speed. The cruise engines are usually either gas turbines or diesel engines. There's usually one cruise and one sprint engine per shaft in AAW warships, but ASW warships sometimes have the engines mechanically decoupled from the shaft to avoid that vibrations (noise) get transferred through the shaft to the sea. Two cruise diesel engines and a single vastly more powerful gas turbine is a common combination on small warships. Electric generators create electricity at and electric motors propel the shaft that drives the screw. This makes them extra silent, and I never quite understood why this wasn't considered just as necessary for AAW-specialised warships - they don't get to choose whether a hostile sub is nearby or not, after all.

Steam turbines had been favoured for about sixty or seventy years for warships, but have fallen out of use in new frigate and destroyer designs because of their inferior fuel efficiency compared to diesel engines and their inferior power density compared to gas turbines. Steam turbines were advantageous for the mast design, though; the absence of hot gasses allowed for a unified mast and funnel design (MACK), which is also possible with all-diesel propulsion, but caused some issues. Gas turbines have hot emissions that require large funnel designs.
Nuclear propulsion is a version of steam turbine propulsion in which a nuclear reactor instead of a heating oil-burning boiler creates the steam, but nuclear power has fallen out of use with destroyer designs and was never used in frigates (too expensive).

Most frigates and destroyers have two shafts and  two screws (sometimes as a controllable pitch propeller instead of a transmission or electric motor).
CODAG = combined diesel and gas (cruise without gas turbine power)
CODLAG = combined diesel electric and gas
CODOG = combined diesel or gas (sprint without diesel power)
COGAG = combined gas and gas
COGAS = combined diesel and steam

COGAG, COGOG, COSAG, CODAD, CODLOG, CODOD, ... - you get the pattern

a CODLAG setup

Integrated electric propulsion has no engine connected to screws by a shaft at all; both cruise and sprint engines (if separated at all) are entirely decoupled from the seawater. The result is the most electric power supply capacity for the ship, and the most silent propulsion. A single running cruise engine could this way drive multiple screws. An IEP could also increase power output at the screws quicker than conventional layouts if it can temporarily draw extra power from batteries. In the long term an IEP might use even more silent fuel cells, but those seem to fall short of their promise (though they are in use in German submarine types).

Auxiliary power units are low power output units for electrical power supply independent from the main propulsion engines and are built into many if not all warships. Such auxiliary power units may actually suffice to move a warship at very low speeds (~4 kts) in emergencies if they can be used to supply emergency propulsion systems (retractable screws with electric engine or Voith cycloidal rudder.

Designs have been pitched in which one or two huge steerable podded propellers with electric motors propel the ship, which is quite a departure from the classic engine-shaft-screw paradigm (civilian azipod example here).

As a rule of thumb, larger propellers can rotate slower for the same thrust and can achieve a higher speed without loud cavitation (this depends on them rotating slowly, and is limited by the low water pressure at few metres depth). Hence the single, huge screws on modern submarines.

Waterjets (ducted propellers) have a reputation for being very silent. The technically similar pumpjets were used on submarines and some torpedoes for this reason. Very few surface warships use waterjets so far, but they may become common.

S O

defence_and_freedom@gmx.de

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