While this was certainly the case on post WWII diesel electric submarines, the U-boats did utilise direct drive.
From REPORT ON THE GERMAN SUBMARINE OF THE U-570 CLASS CAPTURED BY THE BRITISH IN AUGUST 1941, Section II-C-1:
“The drive is direct connected diesel. The chief engineer stated that switching arrangements are provided so that one engine can drive its own propeller direct, and, with its motor operating as a generator, drive the other propeller by electric drive.”
The maximum distance for a depth charge to kill a sub is also still in some debate, as I understand it. Of course part of it is a function of the depth that the sub is operating at, and thus what the ambient pressure the sub is already exposed to at the time. But there’s one rather striking case of a US fleet boat in the Pacific which had a depth charge go off in contact with the deck gun, and while it crimped the pressure hull at that frame, and did very bad things to the boat and crew - she survived to return from her patrol.
The amusing thing about the account from my point of view was that the boat had just gone on patrol after a major refurb at a civilian yard, and the CO had, prior to this depth charge attack, been less than impressed with thier work. He had the grace to include a copy of the apology and appreciation letter he sent the yard in the book he wrote about his experience.
I can’t recall the title, at all, now, but I’ll see if I can find the book on Amazon and will post a link here.
U Boats are presented in a way that would make one think that they could hit and run. They could hit, but they could not run.
Losing 743 U-boats and about 28,000 submariners (a 75% casualty rate) can definitely lead one to conclude that U-Boats were sitting ducks.
To mix it up with someone (Destroyer crew) who was hunting you (the U-Boat that loses 75% of it’s crews) would be equivalent to suicide.
Well, first, the detection aparatus wasn’t all that efficient. Better than nothing, but still pretty primitive. Further, they couldn’t look “back” behind the attacker, so the sub had a chance for a couple minutes to maneuver as the attacker went overhead. For this reason, ASW forces tried to set up sequential attacks by multiple tin cans whenever possible. Next, at slow speed, a sub is very quiet, and can sneak along with a good chance of going undetected unless directly targeted by chance or an unusually skillful attacker. Next, depth charges make a huge racket when they go off, and create a mass of violently disturbed water that’s as good as a brick wall, where acoutic devices are concernted - just the ticket for shielding an evasive maneuver. Next, there are thermoclines, layers of differing water density that play games with acoustic performance, and if you can get a boat on the opposite side of a thermocline from its attacker, it’s like drawing a curtain over the boat’s mauevers.
Lastly, the ocean is a BIG place, and even when creeping along, a couple minutes of undetected sneaking can create a HUGE volume of water to be searched - all the boat has to do is create an opening of a few minutes, and they can increase the size of the search area into a volume so large that only random chance or extreme perseverence and skill would lead to re-detection.
Multiple depth settings was common. Trying to “bracket” the boat above and below was a standard thing. Mind you, the chances of any one depth charge being directly responsible for a kill was very low, but if you could batter a boat hard and long enough with near misses, you could tear it apart slowly, or cripple it to the point where it was forced to surface. Some boats survived attack by depth charges counting in the many hundreds. Four hundred charges, or more, were not unknown, and those were the attacks from which boats returned!
Elite service. In the German (WWI, WWII) and American navies (early part of WWII), the submarines were the only part of the naval force actually putting any real hurt on the enemy. For the Germans, it would mostly remain so; The American surface fleet finally got its act together. But in the those days, boats were where it was at, if you wanted to take the fight to the foe. Sailors, like any fighting man, want to think that they’re doing something, accomplishing a goal. The boats were getting it done, and so had their pick of the fleet personnnel. That allowed the sub force to be choosy, making the units ‘elite.’ Once a unit is established as ‘elite,’ the story builds upon itself, making the unit that much more desireable, leading to more and higher-quality applicants, further allowing choosiness by the fleet, and reenforcing the ‘elite’ image.
Except that this is highly misleading.
Of 684 U-Boats that were sunk in action, 264 were sunk by ships, and not all those were destroyer sinkings, that’s just over one third of all sinkings. That surely is not a case to make that subs were sitting ducks, not by any means, especially if you consider just how many ships were sunk by subs.
Others were lost to mines, accidents such as fires etc.
Around 330 subs were sunk by aircraft, these would usually catch them at night using radar when the subs were on the surface charging batteries. A diving sub trying to avoid air dropped depth charges was highly vulnerable. Given the more limited theatre for subs to be attacked by aircraft, this is very impressive, since subs did their best to keep out of aircraft range and certainly didn’t hang around where air cover was available.
There’s a good reason why subs avoided certain areas and would gather in the Icelandic and Greenland Atlantic channels as these were out of range of aircraft.
Subs along with surface vessels of all kinds proved to be vulnerable to air power, so this is not much of a surprise.
You should also note that the 75% casualty rate was not an overall rate either since a large number were surrendered or scuttled to prevent capture. At my count this is around 460 or so.
My numbers are not 100% because some subs were lost to unkown causes, almost certainly the majority will be unconfirmed sinkings by both ships and aircraft.
Many subs were destroyed in port and were uncrewed at the time, further affecting that 75% crew loss rate.
Sweet Og! That was harder than I’d imagined it would be. Once again I’m reminded that Amazon’s search engine sucks donkey ass. (Putting in the search terms “Submarine History World War Two” should not have accounts of air crew histories as the top result, dammit.)
Here’s what I believe to have been the account I was talking about. The Wiki article about the Halibut makes a vague reference to the excellent resource Silent Victory’s comment about the Halibut having survived a depth charge attack that was, "“one of the most devastating of the war”. Galatin’s account goes into considerably more detail.
On preview:
Actually, I believe you’re underestimating the effect of the jeep carriers. Granted summer 1943 the mid-ocean areas were a haven for the U-boats, but even as early as late 1943 the jeep carriers were closing the noose by extending air cover out into areas the U-boat crews had been taking to be completely safe for surfaced recharging. A lot of those U-boat sinkings by aircraft were from planes being flown off the ASW hunter-killer groups.
The no-fly areas you’re alluding to were quite real, for the first half of the US involvement in the war. But they were closed off rather firmly by mid-1944, I believe.
It wasn’t that one-sided. I remember reading some battle statistics that showed that one of the most important factors was the number of attacking ships. A u-boat had a decent chance of escaping a single destroyer. Increase that to two or three destroyers, and the survival rate plummeted. The reason being that multiple ships allowed one ship to make a high-speed attack run while the others tracked the u-boat at low speed, making escape very difficult.
For anyone who has read this far, realize that the odds have shifted dramatically in favor of the sub with modern submarines and torpedoes. Submarines certainly do not creep around passively hoping to escape the destroyers.
I have participated in exercises where we repeatedly “sunk” destroyers and cruisers actively searching for us. Modern passive sonar deployed by subs and modern homing torpedoes make most surface ships sitting ducks. Nuclear-powered subs also have a significant speed advantage over surface ships, and of course, never run out of fuel or have to surface.
Aircraft do throw a wrinkle into the mix, but they are usually more of a nuisance than a real threat. Besides, in the exercises, all combatants are limited to a prescribed area for a prescribed length of time. The aircraft are usually already in the air when the exercise begins. In an actual combat situation, a modern sub would sneak in, attack, and get out. Any ship-based aircraft would never even have a chance to take off.
A modern nuclear-powered submarine does not have to lie in wait for a good angle to attack as a convoy goes by—she can maneuver into any position she wants. Modern subs can even attack over the horizon with subsurface-launched over-the-horizon anti-ship missiles.
As the saying goes, “How do you know if there is a hostile sub in the area?” Answer: “The big explosion.” 
Training does definitely make a difference, though. One cruiser, in particular, always seemed to zig at the right time to screw up our firing solution. It took a lot of effort to finally “sink” her.
Nitpick: Destroyers and smaller escort vessels weren’t armored. The nickname “tin can” was a reference to the lack of armor plate.
I’ve seen footage of Flying Tiger P-40 aircraft shooting .50 cal machine gun rounds through the hull of a Japanese destroyer and out the other side.
Saying they weren’t armored, of course, adds, rather than subtracts, from the reputation for courage many destroyer and escort crews justifiably earned.
On-topic: I’ve read that it was actually pretty hard to outright destroy a submarine throuh depth-charging unless you caught it near the surface, and thus had a good idea of depth and position. However, prolonged depth charging was very effective psychologically at causing panic and bad decisionmaking in submarine captains/crews. Many sub captains were forced into a mistake that led to their demise, cowed into limping home with damage, or driven to the surface to abandon ship, even without being directly sunk.
Sailboat
A WWII-era U-boat bears about as much resemblance to modern nuclear-powered fast attack submarine as a common housecat does to a Bengal tiger. Most modern attack submarines–Russian ones, too, despite what you read in Tom Clancy novels–can cruise with an accoustic signature that is quieter than the ambient sound level. (Two exceptions to this, the Projekt-705 (NATO: ‘Alfa’) and Projekt-685 Komsomolets (NATO: ‘Mike’) classes were designed as high speed hunter-killer subs that could strike and then dive deeply or move out of range faster than the torpedos of their day could follow.) The Soviets lagged in passive accoustic detection capability, primarily due to more primitive computer systems to process and filter data, but the subs were never as loud as many made them out to be, and were only modestly edged out contemporary American attack subs at the high end of the range where the more sophsticated anti-cavitation screws gave the Los Angeles class subs an edge.
Per NATO exercises (and supporting robby’s anecdotes) submarine squadrons, especially those equipped with standoff cruise or short range ballistic missiles, would have wrecked havoc with Atlantic shipping lines in the case of a European theatre ground war. A modern sub, nuclear or diesel, would move just at steerage underneath a shipping route near some source of noise or scatter, wait for a merchant fleet to float over, and loose four or six torpedos in the water before anybody had a clue as to what happened, then use the resulting bedlam to move off and pop off a few missiles at distance, before sinking back into the background. The SOSUS net was an attempt to keep the Soviet submarine fleet penned up by detecting their movements and alerting trailing US and RN boats to the presence of Red Banner Northern Fleet subs, but the Soviets were likely to keep SSBNs in the Barents and Greenland Seas and Arctic Ocean (being very experienced in submarine operations under icepack), and attack subs would zip across the net at high speed and then disappear in littoral or deep sea clutter, and proceed to do the same thing to NATO shipping as the Germans did to Allied shipping in WWII, only with greater effect. This was a serious strategic flaw in the NATO Plan of Operations, and has been aptly demonstrated by the recent Dustup in the Desert, all the supplies you brought with you that you think are adequte for a 4 week campaign will generally run out on about Day 10.
Stranger
Another one speaking up to support robby’s position about modern nuclear subs, and the fact that they decisively have the initiative and, I think, the advantage. During my own time in the Navy, the accepted thinking was that the only way to deal with enemy subs was to have them attacked by either another sub (the preferred choice) or with a helo.
FTM modern diesel electrics are nothing to sneeze at either. After the Bonefish fire the US decommed the last three in it’s fleet, but those boats could do things that WWII fleet boats could only dreamed of.
For the most part, without using active sonar, surface ships just made too much noise to be able to find modern subs on their own. And if you can’t locate the sub enough to send out a seeking fish, you can’t fire back.
I was a surface fleet sailor, and enjoyed using all the ribald nicknames and jokes that the surface fleet keeps to insult those people crazy enough to serve on ships that sink on purpose. They had the much better insult term for us, because it was so apt: “targets.”
Some ASW destroyers and frigates are equipped with towed sonar arrays to make detecting and locating enemy subs easier (or possible at all), and you can deploy sonobuoys and MADs to give you a wider berth of coverage (and most are also equipped with systems that, although they can’t actually “hide” the ship, make it more difficult to get a firm solution on it), but yeah, there’s a lot of scatter and noise on the surface in addition to what the ship itself is making, and if it’s escorting a bunch of helpless merchant marine ships, picking anything up in the background is straining for gnats. I believe doctrine was to send attack subs and/or ASW cans ahead to “plow the road” for waiting submarines, but a sub sitting quietly adrift or on the ocean floor (as several Russian subs were designed to do in littoral spaces) is essentially undetectable without an active sonar search, something no submariner ever wants to hear, even (or perhaps especially) coming from his own boat. I think RBNF would have taken significant losses in an Atlantic conflict, but they also would have cleaned the clocks of merchant groups and their escorts, and could have put serious hurt on any carrier group that ventured too far beyond 40°N.
Stranger
Stranger, I agree with your conclusions.
My ship had a towed array. But the Virginia class CGNs were not primary ASW platforms. So, AIUI the towed arrays on some other surface vessels are actually effective.
The story I heard from more than one ST aboard ship was that ours was… well… let’s just say, I’m glad my life never depended upon it. And I’m skepitcal of the efficacy of the surface towed sonar arrays, because of that. FTM, the vast majority of the dedicated ASW ships in the US Navy seemed to be switching to LAMPS capable helo platforms. Sometimes even if they had to sacrifice a towed array. (I believe some of the first flight Oliver Hazard Perry class ships did this.)
As for the masker systems - it could help, but my understanding was that after the screws it was the distinctive sounds of reduction gear sets that really carried underwater. Our ship had at least 10 reduction gear sets. In each engineroom. Not all would be used at once, all the time, but…
Have I mentioned the term “target.”
Well, part of the reason for using the Seahawk is that it gives you much longer striking range and a better chance of spotting sign of a submarine that is at periscope depth. But yeah, what I’ve heard of surface towed arrays is questionable, and of course once you start performing maneuvers you might as well wear earmuffs until you clear the tail back in line.
It wouldn’t surprise me if the reduction gears created some kind of harmonic that was distinctly artifical. In an ocean conflict, I’d definitely want to be…at about 40k feet, flying comfortably above the ocean surface, with a Black Bush in one hand and a le Carre novel in the other, far away from any noisy business involving exploding fish and machines that go “Ping!”
Stranger
I recently read Submarine by Edward L. Beach which is partially an autobiography of his WWII U.S. submarine service interspersed (chronologically) with important historical events on other U.S. subs - all focusing on the Pacific war. He shows that it was standard practice to try to avoid fighting a destroyer if if can be helped. Subs of the period had to fire their torpedoes from pretty close to the surface to be able to aim by visual sighting of the target which also made them a target for the deck guns and torpedoes of the enemy destroyers. Besides destroyers were not considered targets. It was considered better to ride out a heavy depth charging than to waste a precious torpedo that can be used to sink a major warship or a large freighter. As for the American use of homing torpedoes, they came pretty late in the war. The Tirante (also here , which seems to be the source for most of the Wikipedia article) didn’t have any until the Spring of 1944 when they were given… one. “Cutie” was used only when they felt they had no choice (they were being attacked by three Japanese destroyers) but when it worked they wished they had more. It was the only ordinance they had that could be used for defense while they were at depth. (The nickname of the homing torpedo is because “Cutie is an affectionate little fellow, always wanting to nuzzle up to fellows bigger than he is”).
There are at least one Allied sub and one Allied warship who managed to torpedo themselves during WWII. I have a book on the warship incident (I think it was a destroyer) at home, I don’t recall the details here at work but in essence it launched one in the North Atlantic and the cold did something to the torpedo’s guidance system; it curved around in a perfect arc and struck the ship that launched it. I’m not sure if that counted as a kill to their tally.
Just an anecdote regarding depth charging that happened on my first patrol. We were a couple or three days out of Holy Loch and got close to an area where they were doing seismic exploration for oil. The ships were using explosives of some sort and were actually quite a ways away. No idea exactly how far.
The explosions sounded exactly like an old WW II movie and freaked out the newbies. (Me emphatically included!) The CO rigged for silence and by the end of the watch I expected to see John Wayne come in and take the con. 
I still don’t know if the CO rigged for silent because of the passive sonar the exploration crews were using or just to freak out the FNGs or maybe it was just a good excuse for a drill.
Regards
Testy
Here are some interesting links to WWII torpedo technology.