What if the OKM had approved the Walter Submarine when he first mentioned the concept?

Ok, I don't really know all that much about the technical details of these boats, so anyone that does, please list sources so I and everyone else can learn more.

As the thread title says, what if Walter type U Boats had been approved by the OKM in 1934?

Here is my source info:
History of the Walter boats - German U-boat Types of WWII - uboat.net

History and Development

Professor Hellmuth Walter

Walter was a brilliant engineer that in 1933 worked on a new gas turbine at the Germaniawerft shipyard in Kiel and there his idea of a closed circuit propulsion system was born. He dreamed of making a U-boat that would by driven by hydrogen peroxide (H2O2) in a stabilized form called Perhydrol. The combustion of that fuel and oil was a complex one but held the promise of a smaller and lighter engines that then normal U-boat diesel/electric engines of that time. Also boat of that design would not have to spend long hours on the surface to recharge batteries which was a major plus.
The Proposals
In October 1934 he proposed incredible proposals to OKM (Kriegsmarine high command) namely of a U-boat of some 300 tons (size similar to IIA) with the maximum surface speed of 26 knots and amazing underwater speed of 30 knots! That boat was to have endurance of some 2,500 miles at 15 knots or 500 miles submerged at that speed. The current U-boat of that time (IIA) had a maximum surface speed of 11 knots and could make about 7 knots underwater.
His ideas were rejected promptly by the Kriegsmarine in 1934 as too unconventional and fanciful but Walter did not give up and in 1937 he showed his revised plans to Kapitän zur See Karl Dönitz who was at that time commander of a U-boat training flotilla. Dönitz was so impressed with the proposal that he got it into the proper channels and in 1939 a design contract was made for a small research vessel, called V-80, based on that design.
The first real submarine
The V-80 boat was designed by Walter and Germaniawerft in Kiel and built in the greatest secrecy on a slipway surrounded by a large fence. The boat was launched on April 14, 1940 and the test results carried out during the following spring with Walter himself at the controls were sensational. The boat reached more than 23 knots submerged which was more than double the underwater speed of any submarine in the world at that time (modern nuclear-powered fast-attack submarines like the Soviet Alfa-class are said to reach some 42 knots).
After the prototype
The OKM was so impressed by the test results from V-80 that they suggested immediate construction of six coastal boats based on that design but many in the U-boat department questioned that a real operational boat could be built on this system and also that the shipyards were badly needed to continue delivering the conventional VII and IX U-boats that seemed to be doing what was required of them at that time. A compromise design of a 600-ton boat called V-300 (XVII, later called U-791) that would reach some 19 knots submerged was proposed and construction started at Germaniawerft but was cancelled when Walter found the design to be too slow and proposed his own version of smaller vessels of some 220 tons.
The next generation is born
After a meeting with Dönitz in January 1942 contracts were awarded for 4 boats to be built at Germaniawerft in Kiel (Wk202 - U-792 and U-793) and Blohm & Voss in Hamburg (Wa201 - U-794 and U-795). The keels were laid down in December 1942 for all 4 boats and the first one, U-792 entered water on 28 September 1943 with U-794 on 7 October. These boats were used extensively for training and reached up to 25 knots submerged in 1944, at one such run in March 1944 carried out by U-794 in in the Bay of Danzig Dönitz and 4 other admirals witnessed themselves the boat reach 24 knots.
In 1942 Walter proposed the huge 1,475ton XVIII that would carry 23 torpedoes and two contracts were awarded on Jan 4, 1943 to Deutsche Werke in Kiel (U-796 and U-797) but they were cancelled on 28 March, 1944 in favour of the XXI which was really a spin-off of the Walter design but with massive battery capacity instead of the Walter turbine that gave the boat 17 knots maximum submerged speed.
The Down side
The Walter boats were extremely complex to build and maintain, also they relied on uncertain supply of Perhydrol. The fuel was highly flammable and the British abandoned the idea after the war as too dangerous for combat vessels.
The Walter boats and the outcome of the war
No Walter boats ever saw enemy action and no such boats even entered frontal service as only 3 boats that were intended for combat were launched before the war was ended and none of them (U-1405 - U-1407) had been taken through trials and training. Thus the Walter boats did not have any real effect on the war but they were a very interesting development and according to admiral Dönitz with a little courage and vision they could have been in service 2 years earlier and then they certainly would have had enormous impact on the war.
The fate of the Walter boats
All 10 Walter boats (3 not in commission) and the V-80 prototype (which was never commissioned into the Kriegsmarine officially) were scuttled in May 1945.
Professor Hellmuth Walter passed away in 1980, then 80 years old.

And here is a table I have edited (correctly I hope) to make a good comparison between the Walter XVIII and their replacements, the type XXI.



I hope this table makes clear the reason these submarines were not completed, but I cannot help but wonder what the effect of this research and development, if started in 1934, might have led to.

I cannot be to specific unfortunately, but lets explore the idea of a merger of these two technologies. I'm thinking that perhaps these two different approaches to an ocean going u boat might have born fruit in unexpected areas. First though, lets think about what would likely happen if the concept had actually been built, starting in 1934 with the V80, and what might have happened after it proved itself?

Personally, I think that we can have an interesting time exploring this idea, and then develop it into a more explicit 'what if'.

 

So as a way of guiding the discussion, lets look at these facts:

1933, the invention occurs.
1934, the OKM (in typical fashion), tells the poor guy to take a hike!
1939, after a needless 6 year delay, they start building the thing.
1940, the first proof of concept is constructed.

OK, by 1940, the war is in full swing, and the course is basically set. But what if, the guy didn't take a year to be able to present his idea (just to get laughed at), but actually gets the thing laid down in 1933, and it's in the water by April 14, 1934!

The war isn't even close to starting yet, and this leaves quite a bit of room for an entirely different U Boat fleet to be built. Would the type VII even be built? Or would German submarine designers, in order to compete with the Walter design, be forced to build 'electro boats' right from the start?

What then would the U Boat fleet consist of at the time the war started?

Any thoughts?

 

Just a real guess on my part here, our resident published author on all things "naval" (Tiornu), Richard Worth may have more information at hand than I.

Perhaps the danger of the system and its limited range had much to do with the Walter-type turbines being largely ignored/underdeveloped? "Walter's primary design goal was high underwater speed, rather than long endurance, and indeed, his first submarine prototype, the experimental V80, reached 28.1 knots submerged in its 1940 trials - at a time when conventional submarines were limited to 10 knots or less. Thus, V80, only 76 tons and 22 meters long, also served as an early test bed for studying the dynamics and control of high-speed underwater vehicles.

Later in the war, the Kriegsmarine attempted to scale Walter's prototype up to a useful operational size, but although seven Type XVIIB H2O2 coastal boats were completed before Germany's final defeat, none saw combat. These Type XVIIs displaced 300 tons and were powered by two 2,500 horsepower turbines, in addition to a conventional diesel-electric plant. More ambitious plans to build larger Walter-designed ocean-going submarines, such as the 800-ton Type XXVI and the 1,600-ton Type XVIII were thwarted by the unsuccessful course of the war and the realization that the industrial capacity needed to supply sufficient quantities of hydrogen peroxide could never be achieved. However, the Type XVIII was modified into the highly successful Type XXI "electro-boat," in which larger batteries provided a submerged speed of 17 knots, which could be maintained for 90 minutes. That innovation, and the adoption of the snorkel, yielded a potent combination that strongly influenced the postwar design of conventionally-powered submarines on both sides of the Iron Curtain."

From:

Air Independent Propulsion - Historical Speed and Modern Stealth | Sub-Log.com

Oh, another "safety" thing, that concentration of peroxide is similar to the peroxide fuel of the little Me-163, which would dissolve human flesh on contact. Not a good thing to have as a fuel source in a manned vehicle to my mind. Isn't it suspected that a peroxide powered torpedo was the cause of the submarine Kursk disaster a few years ago?

Post war the Royal Navy and the USN experimented with Walter-type propulsion systems, and with far more advanced alloys, and safety precautions than the Kriegsmarine had at its disposal in the thirties. As an example; "HMS Excalibur was built at a cost of £1,142,000. Both Explorer and Excalibur were fitted with the latest submarine escape arrangements including the one-man escape chamber and equipped with the most modem escape breathing apparatus for use by the ship's company in the event of an emergency."

"Explorer and Excalibur were, not unnaturally, known as the 'blonde' submarines, because of their peroxide fuel and they served a useful purpose inasmuch as they gave the Royal Navy's anti-submarine forces some valuable practice against fast targets. Their main use, however, was to prove finally that the HTP system was only a stopgap. HTP proved difficult to the point of being dangerous, and there was more than one contemporary report of explosions in the two submarines, and at least one instance when the entire crew was forced to stand on the casing to avoid the noxious fumes, which had suddenly filled the boat. 'I think the best thing we can do with peroxide is to try to get it adopted by potential enemies', said one RN submariner." (emphasis mine)

From:

Post War

If I am not mistaken, the British boats were designed with the aid of Professor Helmuth Walter and his staff, as they came from Germany into the employ of Vickers, and the RN immediately after the war.

And let’s not forget that the Soviets (not known for their crew safety concerns) also abandoned the Walter type drive (the S-99), after many "accidents". They had captured the plans, a few working turbines, and another group of German engineers who had worked on the Type XVIIB boats for the KM. The Soviet engineers are not "dullards" either, and proved themselves expert at retro-engineering weapon-sytems, so it must be really a non-starter. I say that because if the Germans, the Soviets, the British, and the Americans couldn’t make the dang things work in many peace-time (but cold war) years, what chance did the Kriegsmarine have in the few years of "relative" peace before the shooting war?

As mentioned, just a guess on my part and only my opinion.

 

For some background information:



This table shows the totals for each of the main types of U Boats constructed in WWII, the years they were in production, and the basics of their characteristics.

As can be seen from the table, the U Boats (as a general trend) got bigger as the war went on, with range increasing markedly. My what if supposes that for some bizarre, unknown reason, the German leadership actually did something right , and puts the V-80 together straight away, and thus it is available 6 full years sooner than historically. Given that (and the impossibility of continued rational behavior on the part of the German government), the Walter submarines proceed as historically, but 6 years earlier.

I'll whip up a table to show what this would mean, in a bit...

Cheers.

 

The design remains dangerous, unreliable, and short-ranged no matter how well designed and produced. If they cannot be made in peace time, with post-war improvements in metalurgy and safety precautions, why should it be assumed they could be done years before?

I just don't see the possibility for the development myself. Sorry, doesn't fly in even my imagination.

 

Ah, a reply! Sorry I missed your post, I was busy with other stuff.

Yep, I would say that the speed (at the expense of range), and underwater duration were the strengths of this design, and the danger and complicated aspects of construction and maintenance, along with the short range, would be the big detractors.

As far as the cold war years, I think that the competition from 'electro-boats' & 'nuke boats' would play a big part, along with the fact that neither side was faced with the situation Germany was in. Basically, I think that in the late 1940's and 1950's, better techs were available, and there was less of a need. To me, this seems to be more of a reason not to try to hard to make a H2O2 boat a reality.

For the Germans, without the nuke boats, the Walter type would seem to have a very nice appeal indeed, so they would naturally try very hard to make it work. If the Germans did make a workable attack boat on this design (and from the info I have, the only reason they cancelled them was to build the type XXI instead), wouldn't this have forced the modification of competing designs?

If the Germans did build a number of 'fast attack' Walter subs, they would next have had to turn their heads to at sea replenishment in a big way, or the Walter boats would be relegated to coastal work only. This is what I meant by the combination of these tech bearing unexpected fruit.

 

Hmmm. The only reason that the Germans didn't finish construction of the XVIII's, wasn't that they didn't work, but that the XXI's worked better. Again, the allies efforts were done in a time frame where they already had better, so they would have been looking not for a boat with capabilities like the XVIII's, but for boats that could be more than a match for the modern post war subs. This I think, would be the truth behind the lack of a 1945-1955 era Walter sub, rather than a 1933-1945 Walter sub.

 

It is one of those fascinating 'what if's' in relation to German submarine design to ponder the earlier availability of Walter boats...However looking at it objectively you have to conclude that it not only would have been highly unlikely that the Kreigsmarne would have adopted the system in 1934, and that even if they had of, it was an unstable and dangerous system.

Firstly the system itself; high test peroxide is an appalingy nasty stuff, dangerous to use under service conditions. The aforementioned HMS Explorer and HMS Excaliber where nicknamed HMS Exploder and HMS Excruciator by their crews, even with post war technology and Walter himself as a design consultant the RN could not make the system either reliable nor safe.

Second the system was an open cycle gas turbine design, as the submarine went deeper the back pressure against the turbine incresed, theryby decreasing turbine efficiency, resulting in a loss of speed; essentially the high speed of the Walter Turbine was only available to a submarine at shallow depths. High speed operation at shallow depths with the German penchant of removing bow planes from their high speed designs invites a broach and running at high speed shallow causes your propellors to cavitate.

Thirdly, the control dynamics of high speed submarines had not been adequately investigated by the Germans, post war trials by all the major powers showed that conventional shaped submarines suffered from numerous dangerous handling quirks at high speed, only with the development of USS Albacore and her 'body of revolution' hull could submarines safely achieve high speed.

Fourthly the decision to abandon Walter boats and use its hull design as the basis of the Type XXI came about as the Germans realised that A. the system was still 2-3 years away from being ready for service B. the system competed with other more critical parts of the German war economy for high temp alloys/specialist steels etc. and C. The production capcity to produce H2O2 was tiny compared to the projected operational needs of a fleet of Walter boats, it would have taken 2 years to build plants that would have been able to supply enough peroxide.

Germany was running out of time and resources when the decision was taken to convert the big Walter boat into the Type XXI, the decision was in the main made so that the U-Boat arm would have a new submarine in service with the least possible delay.

So to re-cap the Walter system was a technical dead end for a whole host of reasons, now if the Germans had instead taken the advice of Korv.Kapt Heino von Heimburg (writing in 1927!) who recomended that future U-Boat development be concentrated on fast torpedoes that leave no wake, fast deep-diving submarines with quiet propulsion systems and no deck armament and that in any future war the patrol aircraft would be the U-Boats main enemy......

Sbiper

 

wise words biper, range was the all important factor the XXI would of been the answer. Interesting the amount of success though very short range small Boot ~ XXIII for such late war ops.

 

Thats not technically 100% true either, the OKM did not like the Type XXI, they felt it was too big and used too many resources. They designed a series of successor boats to itsthat roughly mirrored the range of the 'main' U-Boat types, i.e. they designed 'electroboot' equivilents of the 'diving' types II, VII and IX. These were characterised by all having a single propellor, single main diesel engine, very large battery complement and large numbers of torpedeo tubes vs reloads (a reversal of previous U-Boat practice).

The Type XXIII scored the last merchant kills of the U-Boat waffe, sinking 2 merchants with a single torp each, fired solely on the basis of passive sonar contacts.

Sbiper.

 

I realize that for your reasons given the Boot is huge

scanning through slowly E. Rösslers work of which I assume you own a copy, one can tell of all the experimentations to come to the total conclusion of the all one and only perfect boot......... which obviously never happened

 

Well the KM was not in the same situation as the USN with regard to submarine design, it needed various sized and ranged types to fulfill its missions, the USn on the other hand just needed large long ranged boats to fight a pacific war.

 

I think the variety would stem for the KM in the basic fact it was hemmed in from all sides, reduced to a nut shell with only certain waters it could travel in, seems the ever present mining to the end of English coastal waters / or the Merchant hunt, and of course this went even beyond it's U-Boot needs

 

The two were not mutually exclusive, the KM did carry out patrols in UK coastal waters with snort VII's and a handfull of XXIII's late on in the war. From the reports they were highly satisfied with both types in and around the UK's coasts (and thus with their ability to operate under allied air cover and in area's that were both heavily mined and swarming with escorts).

For the KM the best strtegy would have been to develop 2 types of electroboots, one that approximated the Type VII and one the Type IX. The Type VII electroboot is 800 - 1000 tons in size and optimised to operate in the waters west of Scotland and North West of Ireland, where the Atlantic ocean convoys (homewards and outbound) converge. the second design is equivilent to the Type IX and designed to work in distant waters (US east coast, Caribeen and south Atlantic). Throw in a few submarine tankers (with ability to refuel underwater, something which the Germans had tried sucessfully in tests) and thats all the KM needs U-Boat wise, forget dedicated minelayers etc. 2 main types and 1 special type, sectional construction a la XXI.

OFC hindsight is 20/20.

Sbiper.

 

Wow, lots of good posts and information here all of a sudden (after the thread sat for so long, I was thinking it was a total flop), so I have to say thanks for the posts guys. If I wanted to read up on the whole hydrogen peroxide/turbine propulsion system, can anyone post a link to online content, or a book title or two?

I suspect that there is more to this story than has survived in the histories, as there seems to be a logical step not taken here.

Perhaps someone can tell me (the site I posted a link to is virtually everything I ever heard of the Walter submarines), what were the main reliability problems form the perspective of front-line service, and what maintenance challenges did the XVIII's pose v. the XXI's?

I understand the Walter invented the H2O2 turbine (or was credited with the major work) in 1933, but didn't even present his initial plans to OKM until Oct of 1934. And then, it took till 1939 to get the V-80 laid down and not in the water till April of 1940. My thoughts were that this was due to lack of belief in the proposed craft, rather than technical problems. Is this in fact the case, or were the plans submitted in 1939 all that much better than the initial ones in 1933?

I have to concur with the concept of a submerged replenishment system, as my understanding is that the Germans didn't build enough 'milch cows', and with their codes broken, the few they did have were soon picked off by the allies on the surface. It seems incredible that the Germans could have failed to realise that their codes were compromised, especially when they changed their codes and started immediately enjoying greater successes (till the new codes were in turn broken shortly thereafter).

My own take on the Walter boats is that their construction would serve as a proverbial 'good swift kick in the @ss', and force the improvements that were later made anyway. Had the V-80 been in the water in 1934 instead of 1940, and the subsequent development (during peacetime, before the needs of the war imposed restrictions on non-essential construction) of the larger and more advanced Walter boats, don't you think that this would have led the Germans to come to the conclusions they historically
did in 1943, back in 1937? With the type XVIII's being built in 1937 instead of 1943, they would probably have been scraped in favor of a cruder type XXI, wouldn't they?

Also, correct me if I am mistaken, wasn't the biggest advantage of the Walter boats their ability to stay submerged far longer than the conventional U boats, rather than their high submerged speed? Anyway, thanks for the input and looking forward to more of the same.

 

First off if you want information on the German U-Boat arm you cannot go much wrong than by ordering yourself a copy of Rosslers The U-Boat, its pretty much the bible when it comes to U-Boats.

Maintenance difficulties with the walter boats stemed mainly from the difficulties in keeping the entire sysyem clean and oil free. The turbine end was also subject to extreme corrosion and pitting of the turbine baldes and consequently had a short service life.

Walter's initial submission on the Walter drive used the H2O2 decomposition products to drive a diesel system, it was only later that he designed the turbine system. The plans he submitted in the late 30's were for the experimental craft V.80, which had a 'cold' Walter system i.e. no combustion of the decomposition products. When it demonstrated its performance the KM took over design work on the U-Boat side of the equation and Walter designed the 'hot' Walter drive (decomposition products combusted with diesel fuel to provide steam/exhaust mix for the turbine). When the design moved from the experimental V.80 to test U-Boats the problems began, mainly that the test U-Boats were built to KM standards and as combat units, and they outgrew the ability of the turbines then designed to power them at an adequate speed.

The main advantage of the Walter drive was not submerged range, it was submerged speed. The Germans envisioned their Walter Boats operating like standard diesel-electric boats for the vast majority of their patrol; the Walter drive was only used to force or decline an engagement.

Also I really hate to rain on your parade here but the Walter drive is just not a practical proposition, it won't work full stop. Germany would have been better off not pursuing it, or concentrating its Walter research on torpedoes (even they are dangerous.....BUT both the USN and the Swedish navy operated H2O2 torps for years post war sucessfully).

A more optimal solution would have been to take on board Korv.Kapt. Heino von Heimburg's learnings (he was a contempory of Doenitz and a decorated WW1 U-Boat commander) and concentrated their research on getting faster torps (with reliable depth keeping/exploders) and on developing subs that were faster and deeper diving and capable of sustained underwater operation, not pursuing some fanciful silver bullet in the Walter system.

Sbiper.

 

I'll get a copy of this soon, and thanks for the info!

Is this info in "The U-boat", or another work? Thanks.

Ah. Ok then, they did do a mixed engine layout on the type XVIII's then? I wasn't sure if that was the case or not, even though the table I edited had some info I didn't understand. The table originally had a pair of entries, and I see now that it was meant to convey the specs for use of either system, rather than (as I had assumed), a comparison of the design, one fitted out with a Walter propulsion system and the other with a standard diesel/electric system.

My fault I guess.

Are my posts such that I didn't make my WI clear in that I didn't see Germany trying to field a fleet of Walter type boats? I tried to make that clear but I seem to have failed again, lol.

This WI (by intent, anyway) is supposed to be about what benefits the U-boat program might have gotten if the Walter design (and here I have to admit that I never realised there were indeed two different designs under discussion until I read your latest post) had been given a proof of concept right off, and when the Germans realize that the Walter design isn't going to give them a fleet of super U-boats, but does give them ideas on how much more they can get out of a submarine, where would they go from there? My thoughts were that they would follow the historical path, and replace the type XVIII's with an earlier, poor man's version of the type XXI's that historically replaced them. I have no doubt that the type XXI's were better boats for more then simply having a bigger battery capacity, but if their experience with the Walter system had made them try this (the bigger battery capacity) out earlier, what effects would that have on the U-boat program.

So I guess I need to restate the original question I failed to make clear (I'll go one better, and ask the WW2f community members to suggest how I could have asked this question better in the first place. Just PM me your suggested improvements to how to ask this question, or go ahead and post it in the thread):

(I'll edit this out if and when somebody finds a better wording)
What benefits would the U-boat program have gained by an earlier experimentation based upon the Walter propulsion system?

I definitely enjoy learning new things, and have greatly appreciated your input thus far, and I do apologise for the mix-up. To be honest, I guess I should have looked for more info before posting this thread, and been clearer on what I was looking for.

On a side note, were the tables I made for this tread of any help at all?
Were they a complete waste of time?
Or were they, like the thread theme potentially interesting, but not well and clearly presented?

I ask this because I intend to post more actively in the coming months, and would prefer to be able to make my threads easier to understand, so people could enjoy them more.

Any thoughts?

 

Most of the info regarding the Walter boats is from Rossler, some from varied other works on subs in general.

All the Walter boast (with the exception of the V.80) were 'mixed engine' layouts, the Walter system simply did not have the range to allow it to be the sole/primary drive of any U-Boat.

To be honest Walter was an innovator when it came to certain details of U-Boat design; he pioneered high pressure hydraulic control systems and two man depth/stearing control (like USN/RN nukes have) and dynamic depth control, but he (like other German U-Boat designers) failed to do basic research into optimised underwater shapes and the underlying dynamics of high speed control.

Having the Germans deploy the Walter system in the mid 30's and then developing the Type XXI (or equivilent) from it is possible, BUT the tactical and technological conditions that lead to the Type XXI only came about post May 1943 (Black May); prior to this the German OKM were content with their conventional designs. There would have been no impetus to develop the Type XXI prior to 1943, and attempts to develop and bring to production such a design would have faced a massive uphill battle in both the OKM and the German shipbuilding industry.

TBH the only way you are going to get a Type XXI equivilent into service prior to 1943 is if you have foreknowledge of what happens in WW2, and that opens up a whole other can of worms.

The only other avenue that the Germans could have pursued is to have promoted Heinburg instead of Doenitz to head the U-Boat waffe and allow his ideas to be pursued. But even then you have to remember that Doenitz had 0 control over what submarines/submarine types he was allocated pre war; only post 1941 did he become able to direct the design staff to fulfil his operational needs.

Sbiper

Note: The IJN had several interesting high speed designs roughly comparable to the Type XXI and XXIII, some of which were pre war as far as I can remember. They used them extensively as test boats but were hampered by poor detail design and numerous mechanical defects, which negated much of the testing and research they hoped to carry out.

 

Is there any alternative to Rossler ?, I have that book but I usually hate to rely on single sources however authoritative. I'm also looking for information on the japanese No 71 and I-201 high underwater speed designs.
AFAIK the need for research on hull design only started when propulsion system that could achieve high underwater speeds became available, researching hulls shapes for fast subs made little sense until a propulsion system could be built.
Even the early nuclear boats like USS Nautilus had conventional hulls not much better than the GUPPY conversions despite being new constructions. The type XXI hulls represent an intermediate step between the "divers" and the "whales".
The Heinburg / Doenitz alternative is interesting, I believe many wolpack attacks were performed at night by surfaced boats, something the Type VII and Type IX were good at, more research in Walter design may also have led to developing tactics for submerged attacks which the Germans would need for the electric boats. A 15+ knots submerged speed sub would greatly reduce the usefullness of the allied corvettes and slower subchasers though the may be the hunters in hunter-killer tactics where one platform feeds targetting information to another that is moving too fast for it's sonar to operate effectively.

 

Hmmm.

I hate answers that refer to the blind leading the blind, lol. Even though that seems more and more to be what the Nazi's were to Germany.

I actually prefer to steer clear form this part of the war, as I am primarily interested in whether a thing could be done, from the standpoint of science and technological capabilities, not the vagaries of politics, personalities, or egotistical incompetents and all the problems they caused. Not that these were insignificante, far from it, but it's not something I have much interest in.

If we look at the typical (rather than ideal) situation, you are certainly correct, as there would be little other than a desire to keep the fleet 'state of the art' to offset the factors that you pointed out. While more fitting with historical truth, it does tend to limit the entertaining speculation of what could have been.

Just for the sake of fun, lets say that the German's had (for whatever reason) actually pursued a course of action that made putting newer and better u boats into service each year a priority, rather than the standard "if it ain't broke, don't fit it" mentality.

They would have had fewer boats in service, I think it is safe to say.
And if that is so, then they would also be forced to plan for an at sea replenishment system, to cut the time (and numbers) of boats not being in position to attack allied shipping down to a minimum. This would most likely have been done as historically, but if we are speculating on them doing something smart...Why not the submerged replenishment system you mentioned?