1). As I understand it, current boats use a disiel engine to charge their batteries and the engines themselves (connected to the shaf(s)) are completely electric. Older designs, specifically those intended to opperate largely on the surface and only dive when under threat or stalking a target, had the disiels coupled directly to their shafts with the ability to de-clutch them and engage electric motors while under water. German type XXI I believe actually had two sets of electric motors, fast cruise and quiet creep (why another engine set would be necessary I can't imagine). If all the above is true, what is the reason for moving away from directly engaging combustion engines? Automotive complexity? Inefficiency of the diesel at high or low speed compared to a constant 'cruise' for generating electricity? Space saving?

2). I've been lead to believe that the diesels on most D/E can't support the top sprint speed of the boat (usualy ~20knt?) and that for long range transit they would effectively have a cruise speed, or at least an average speed, in the mid to lower teens. If true, why? Is this because most boats of this design aren't envissioned making long range transits? Even aside from that I would have thought the lower the power of the diesel engine, the longer the charging time, the greater the exposure when snorting?

3). Realistically what *is* the top speed of D/E designs? I've been led to believe most modern designs are capable of just above 20 kts, with one or two supposedly being capable of more like 25. I realize that probably all info on top speeds is classified, but is there any general wisdom or rule of physics physics that would say, prevent a DE from travelling at 30knts? (I do realize that exponentially more power is needed for even a few extra knots at those speeds which might make it basically impractical)

4). Environmental control: its been suggested to me that maintaining air quality and temperature on a submerged sub is not trivial even for a nuke. Temperature in very cold or very hot water apparently is challenging and takes up some of the hotel load. Air is a big deal but not specifically for the production of oxygen but more for the removal of contaminants, apparently CO being the most dangerous. To what degree is a DE limited by environmental conditions compared to a nuke? I've heard of a German boat making a two week transit underwater under AIP...but does this translate into a univeral ability to opperate for two weeks without snorting or is this dependent on environmental conditions?

1. Not US diesel boats of the WW2 fleet type:

"1D1. Submarine diesel engine installations. Figure 1-14 shows a typical main and auxiliary engine installation aboard a modern, diesel-electric drive, fleet type submarine. Each engine is coupled with a generator to form a generator set. Through the main control cubicle, the current supplied by main generator sets may be directed to charging the batteries or powering the main motors. The auxiliary generator set may be used directly either to charge the batteries or to power the auxiliary equipment. It may also be used indirectly for powering the main motors. Main motors are used for propulsion and may be powered either by the batteries or by the main generator sets. "

This description is kind of roundabout, but it seems to indicate that U-boat diesels drove electric motors, which were in turn either coupled to the propeller shafts:

"At greater depths, the U-boat is driven by electric motors using electricity drawn from giant internal batteries. When these are depleted, they must be recharged. This is done by traveling on the surface, or while snorkeling at periscope depth. The diesel engines are used to turn the electric motors so that they act as dynamos (generators) to recharge the depleted batteries. For rapid recharge, both diesel engines would be clutched onto the electric motors. For maximum range, one diesel engine would be clutched to an electric motor – which in turn acted as a generator to drive the other electric motor. Both propellers were turning in this way with only one diesel running."

Which is kind of what you were talking about, but not exactly. Can you provide any examples?

2. Power constraints on the size of engine you can put inside the pressure hull, plus fuel constraints.

3. Hull form has more to do with it than anything else. The original teardrop shape was tested in the US with a diesel-electric boat (USS Albacore) and actually achieved 33 knots submerged, acording to Wikipedia. Once you get to certain plateau of hydrodynamic efficiency, the real constraint is applicable power.

4. Don' know anywhere near enough about life support systems to give an opinion, except to say that as an engineering principle, anything that does work needs power. So whatever your power budget is, if you factor in greater and greater amounts of life support functionality, something else has to give.

It should be noted here that AIP systems are fuel powered, not rechargeable like batteries. They are an auxiliary capability intended to give the boat commander an alternative to battery/snorkel operation when that would be too dangerous. They do not replace such systems, which are also installed.

True for German boats too. The U-505 was kept afloat by breaking the connection between the diesels and the generators, allowing the props to turn the shafts and charge the batteries/run the pumps while under tow.

Not quite the same thing. German boats had their entire propulsion train hooked up in-line. On the surface, the diesels were directly driving the propeller shafts through a mechanical clutch linkage to the electric motors, which acted as generators for ship's service power and battery recharging as well. For submerged electric drive only use, the diesels were shut down and unclutched from the electrics, which then drew power from the batteries.

US fleet boat diesels were directly coupled to generators, which then delivered power to whatever electrical utility they were connected to by cable--batteries, propulsion motors, and/or ship's service power. The diesels had no mechanical linkage to the propeller shafts:

Is the tear drop hull not adopted primarily because of issues fitting the machinery to that form? Seems to me a 30 knot sprint would still be desirable. Do nuke boats achieve their speed through better hydrodamics (much bigger length: width ratio) or through brute power?

Another random question I thought up; are D/E's snorts limited by sea state? At some point can the weather be to rough to take on air without flooding the snorkle? If so, at that point do they just have to wait it out?

EDIT: Wikipedia seems to indicate that the US made the switch to the diesel-generator arrangement pre-war but that pretty much everyone else kept the diesels directly coupled to the shafts until post war. So the practice isn't as modern as I suspected.

A high length:width ratio is detrimental to hydrodynamics once passed certain point, and all the Western subs, with their cylindrical middles are above that point.

The pressure hulls were cylindrical and relatively narrow. So fitting the machinery was never that big an issue. The real problem was that prior to the invention of the snorkel, the outer hull had to be optimized for surface running, because that's where the sub spent most of its cruising time.

Hydrodynamics don't hurt, but the real answer is that nuclear plants can generate a lot of power.



Well, you can only raise the snorkel so high out of the water, so there has to be a point at which the inlet valve is closed more than open and you choke the engine. Subs tend to want to be fairly deep in bad weather anyway. If you have a battery charge emergency, you could theoretically operate with the sail and snorkel surfaced. (Well, the hull would breach a lot too.) In that kind of weather, it's not likely you'll be spotted by visual means, or even radar.



The arrangement had a lot to recommend it--the diesel engines could be run at maximum efficiency, the electric motors could be as well, electricity generating capacity wasn't governed by propulsion constraints, etc.

To clarify on machinery arrangement, that of vast majority of all subs in all navies through WWII with exception of USN, was two shafts each with diesel-coupling-motor/generator-coupling-prop; with due note of all exceptions like steam subs, more than one diesel in a row, etc. the foregoing was the overwhelmingly common standard. In USN the pure diesel electric arrangement defined the true 'fleet boats' starting with Porpoise class ca. 1934, though some WWII-built fleet boats had a composite arrangement where the aft two diesels were connected to a reduction gear along with electric motor, and only the fwd pair of engines drove independent generators (some of the boats w/ notorious license built MAN engines by HOR had this arrangement, which they retained even when re-engined with GM engines). Also US boats until late in WWII had relatively high speed electric motors running the props through reduction gears; the late war evolution and postwar standard in all navies was direct connected slow speed electric motors (single or tandem motors in the tear drop hull era).

The pre WWII standard had fewer elements of machinery, with the dual purpose motor/generator, driving the boat as motor when disconnected from the diesel, charging the battery as generator at full power of the diesl when the prop was disconnected, or trickle charging while the engine also drove the prop, at lower than max speed. The 4 engine DE arrangement had a lot more flexibility to run the engine(s) needed for whatever purpose(s) each at optimal output, flexible physical arrangement, and no need to worry about the design rpm of the engine relative to optimum prop speed. For the same technology of diesels it would be heavier and less efficient at full output (more losses in the electric drive), but in USN it coincided with introduction of higher rpm higher power/weight diesels (than previously, and v. other navies in the same period, generally speaking).

2. The snorkeling speed is also limited by flow around the snorkel, strength, depth keeping near the surface and lack of concealment with huge snorkel wake above a certain speed. And, the tear drop hull sub will have excessive wave making resistance on the surface, so the boat couldn't go as fast when surfaced as submerged even if the diesel output was equal to that of the motor.

Joe

I don't know about snorkeling but I've read accounts of WWII subs having to go below 150 feet to get away from the wave action in very heavy seas.

Wasn't the system, end-to-end, somewhat heavier per unit of propulsive power output, because it included both generators, motors, and a switching system? Or did the more efficient US diesels compensate for that?

I think it was more a matter of preference than necessity. The boats were seaworthy enough. But waiting a storm out at depth meant that you weren't buring fuel fighting the sea--and probably not making much better speed than you could on batteries, even at low revs--nor were you fatiguing the crew.

High-speed electrical machinery uses to be lighter and smaller per hp than similar units working at low speeds*. There could be a tradeoff there.

But I think the main driver, as in the case of turboelectric power plants for BBs and the like was the greater overall fuel efficiency.

* For instance, alternators for hydro turbines vs. turbogenerators.

Again depends on what diesels. Relative to traditional slower speed heavier diesels, the DE systems on US subs were lighter all in. But, using the similar higher speed diesels in a mechanical or partly mechanical arrangement would be lighter than same ones in DE arrangement. This was the idea behind the composite arrangement in the Salmon and Sargo classes which came after the initial pure DE fleet boats, one engine on each shaft connected to the reduction gear along with e-motors, two engines just driving generators: it saved around 10% in total machinery weight. But there would be an arrangement penalty to connecting all four engines of that general size and type mechanically. And, the greater flexibility and simplicity of all DE won out over the composite arrangement.

Joe

True, but the high speed electric motor sets on the fleet boats equipped with them required reduction gear boxes and had more cooling overhead than low speed direct drive motors. The direct drive motors should have been, in principle at least, more reliable and lower maintenance. There was some reason they became preferred.



I'm sure fuel efficiency and underway maintenance availability were both big drivers.

Do you mean in D/E subs or SSN's?

The other factor in SSK's is that they're pretty small as is, and a lot of that is by design, a tear drop shape means one of two things, you're going to increase the outer hull or decrease available internal space.

Not a small consideration. And given how long most subs can last on battery power alone at flank I don't think the difference is considered that big a deal, especially in the age of ADCAP torpedos.

The reason for going to direct drive motors was not reliability or gear box cooling requirements. USN went to slow speed direct drive motors to improve silencing by eliminating the gearbox whine.

Hojutsuka

I was talking about motor cooling, but otherwise the above makes since as a reason, maybe even the deciding one. But I doubt it was the only perceived or actual benefit. Engineering decisions are seldom about any single thing.

Nobody has addressed this, so I will.

The main electric motors for the Type XXI produced 2,500hp each and drove the propeller through a set of gears. The creep motors produced 113hp each and drove the propeller through multiple belts. The belt drive provided a very quiet way to match the motor speed to propeller speed, but could not handle the power of the main motors, which had to use the noisier gear system.

Hojutsuka

As I understand it the american system gave the Captain the option of running on ONE, TWO, THREE or all FOUR diesels. I could be wrong about this. The odd numbers might only be really useful if pulling maintence or some odd combintaon of surface speed and battery charge while conserving fuel. Or limp home ability after battle damage? One running diesel driving a generator could provide power to both propellors.

It provided the same advantage that captains of multiple boiler steamships had had for a long time. You could tailor your power generation to your needs, meaning you could use fuel more efficiently and minimize wear on your plant.

US fleet subs had a smaller fifth auxiliary diesel that could be put on ship's service power and/or battery charging so that all four main motors could power propulsion:

1. American fleet subs were larger, generally speaking, than the German VII and IX boats, which tended to drive a requirement for higher overall power levels.* The easiest way to achive this was to add engines, which is relatively easy with a pure diesel-electric arrangement. Direct driving the shafts with electric motors and diesels either required more complex arrangements (additional gearing/clutches) or more complex engines (tandem diesels) if you want to use more than two diesels to drive the submarine. It is my understanding that US fleet subs would operate the motors off of one genset, while using the other three to recharge the batteries when in the combat zone in order to minimize the charge time. It also provided the ability to adjust power levels for maximum efficiency when loitering. I think one genset was enough to keep the average fleet boat cruising at around 5-6kts while the batteries were charging. Also, keep in mind that the Germans operated under a tougher ASW environment, so paid more attention in design to silencing before the US did.
2. Fleet subs were called that because they were designed to accompany the battle fleet. To do so required speeds around ~20kts and also tended to set the length & beam requirements. They were faster on the surface than submerged (by about a factor of 2x). More modern diesel electric subs have sacrificed the surfaced performance for submerged maneuverability & speed with the Albacore hull form. They have also sacrificed diesel engine power for larger battery banks. Batteries are heavy and submarines are weight critical by nature. With increased battery capacity, the modern diesel electric can stay submerged for much longer periods than the WWII fleet boats, which means that they have a greater ability to pick and choose when they need to snorkel.
3. Higher speeds drain the batteries faster. Most diesel electrics aren't intended for cross-ocean operations (although some, like Australia's Collins-class, are designed for longer range operations. As noted above, submarines are weight-critical by design. Shaving weight from the power train mechanicals means more battery capacity (endurance), more payload, more fuel, more weight allowance for silencing, or more internal volume for habitabilty. As aevans noted, research subs have been built for higher speeds than 20-25kts. BMT & Rolls-Royce have proposed a gas-turbine powered submarine for high-speed transits: http://media.bmt.org/bmt_media/resources/3...dyformatted.pdf The concept uses fuel cells & batteries for loiter & sprint operations.
4. Nuke boats have the surplus power to run larger/more energy-intensive environmental control equipment, but the submerged endurance of modern AIP-boats will be set by a host of factors, including fuel/oxidizer for the AIP system and the power consumption of the sensors & propulsion equipment, in addition to whatever demands are made by the HVAC system. The systems will be designed to match the intended operational profile of the boat. "How long do you want to stay down? Then that's how long you can stay down. And here's the size of the boat you need to achieve the mission." Obviously, using a smaller crew & automation is also helpful in reducing the loads & increasing the capabilities of the boat, for a given capacity.

With respect to snorkels, they are equipped with cutoff valves that prevent the ingestion of water when waves submerge the intake. When this happens, the engines draw air from the inside of the boat, which is reportedly not the most pleasant experience for the crews--a couple engines sucking down air at the rate of several thousand cfm will drop the pressure inside the boat in a hurry. It also isn't the best experience for the engines, themselves, but it works.

Finally, the teardrop hullform is actually easier to arrange than the old-style WWII sub because it is wider for the same displacement. That means, among other things, less space consumed by passageways & there's the potential to fit 2 or more decks in the wider part of the hull. The teardrop form has better underwater performance (less wetted surface for the same submerged volume) and is more maneuverable. The penalty is greatly reduced surface performance. Most subs tend to have a cylindrical midbody since the performance penalty isn't that great compared to the pure teardrop form--it is much easier to produce & has other benefits, like being easier to dock, having a usable deck when surfaced, etc.

Douglas

*Edit to add: Fleet sub size was driven by the range requirements of the Pacific and the requirement to match the fleet's speed of advance. The high-speed demanded waterline length and a narrow hull. When the pressure hull is only 16' in diameter, the freedom to arrange the engines without being tied to the propeller shafts is a very nice thing to have.

Agreed but the point I was trying to make was that once under, they had to go down fairly deep when in large seas in order to escape the worst effects of the surface wave action.

Interestingly enough, the fleet boats were a failure as fleet boats but the capabilities built into them to try and attain that function ended up making them excellent boats for a cross pacific commerce war, even if it took the navy a little while to figure out how to use them.

The fleet boat concept was what was wrong, not the boats themselves.

Agreed, it didn't work and they were given a new job which they did well in spite of lousy torpedoes and lousy tactics based on a peacetime evaluation that ASW tactics and technology were far more effective than they were in reality. Also, they really hadn't prepared for unrestricted submarine warfare but were given the task on day one. Once they weeded out the older, more cautious skippers and hit on a combination of patrolling and attacking that worked (and eventually dealt with the torpedo issues) things started to click.