U-Boat Type XXIX H (1944 unfinished design study)

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U-Boat Type XXIX H (1944 unfinished design study)

Post by kfbr392 » 06 Jan 2022 13:27

What really was needed in 1943 (actually, in 1939) instead of the Typ XXI was a fast (submerged speed) ~750t one-screw boat with a ~8000nm range that could be used in the operational area "North Cape to Gibraltar".
The XXI was in fact only built because it had already been calculated and pre-designed by May 1943 as a Walter propulsion boat, and Dönitz accepted it in June 1943 despite reservations because of its unnecessarily large size.


In 1944, the topic was revisited and resulted in the Typ XXIX designs.
Personally, my favorite is the Typ XXIX H.

This design was only proposed in the fall of 1944, and of course never built.
It used only 124 battery cells, whereas the XXI had 372. Overall, it would have be a lot faster and easier to build than the Typ XXI, and more suitable for convoy battles in the Western Approaches and around Great Britain.

http://www.hisutton.com/Uboat_XXIXH.html
https://www.secretprojects.co.uk/thread ... boat.28945
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Re: "Bastard" Electro U-boats From Early 1943?

Post by TheMarcksPlan » 06 Jan 2022 16:13

kfbr392 wrote:
06 Jan 2022 13:27
What really was needed in 1943 (actually, in 1939) instead of the Typ XXI was a fast ~750t one-screw boat with a ~8000nm range that could be used in the operational area "North Cape to Gibraltar".
The XXI was in fact only built because it had already been calculated and pre-designed by May 1943 as a Walter propulsion boat, and Dönitz accepted it in June 1943 despite reservations because of its unnecessarily large size.


In 1944, the topic was revisited and resulted in the Typ XXIX designs.
Personally, my favorite is the Typ XXIX H.

This design was only proposed in the fall of 1944, and of course never built.
It used only 124 battery cells, whereas the XXI had 372. Overall, it would have be a lot faster and easier to build than the Typ XXI, and more suitable for convoy battles in the Western Approaches and around Great Britain.

http://www.hisutton.com/Uboat_XXIXH.html
https://www.secretprojects.co.uk/thread ... boat.28945
Thanks, I hadn't heard of T29 - even uboat.net doesn't list it on its projects page.

While it certainly would have been an excellent weapon, note some drawbacks vs. T21:
  • Slower submerged: 15.5kn vs. 18kn (using design speeds for both)
  • Less submerged range: 120nm@6kn vs. 340@5kn (works out to ~240nm@6kn)
  • 12 torpedoes vs. 23
  • Slower surfaced: 13kn vs. 15.5kn
The submerged range is a big factor: T21 creates ~4x the required search area given its ~2x submerged range.

Submerged range also figures into how much of sprint into attacking position each sub could do, leaving enough battery juice for evasion/escape after attack. T21 can stay submerged an moving for ~2x as long as T29 at equal speed.

T21 could have launched 18 torpedoes in 25min, and still had 5 more for later. From your diagram, it looks like the T29's torpedo room doubles as accommodation, with reloads stored below rather than ready for reload as in T21 (that may be the drawer's choice though). T21's dedicated torpedo loading room took a lot of space, which probably isn't feasible on a smaller boat.

I lean towards believing that Doenitz' fixation on the smallest-possible ocean-going boats was an artifact of pre- and early-war resource constraints that he never overcame mentally. Germany established a 30/mo build rate (aside from bombing impacts) late in the war, which should have been sufficient boats to mount a picket line across the seaways.

That's not to say definitively that either would have been the better use of resources, just that there's tradeoffs. Thanks again for posting.

--------------------------------------

I'd actually suggest building something even larger than T21, though not as the main boat and not in Germany's actual 1943 resource constraints. Along with the bigger boat, build bigger torpedoes capable of attacking convoys from standoff range. The bigger boat will be even faster and longer-ranged underwater, assuming equal HP-weight ratios. Or keep the same speed and more torpedo tubes. This would force Allied escorts to sail farther from the convoys - opening gaps for the main type - and/or force more resources thrown into hunter/killer groups and away from the convoys.
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Typ XXIX H

Post by kfbr392 » 06 Jan 2022 18:33

TheMarcksPlan wrote:
06 Jan 2022 16:13
While it certainly would have been an excellent weapon, note some drawbacks vs. T21:
  • Slower submerged: 15.5kn vs. 18kn (using design speeds for both)
  • Less submerged range: 120nm@6kn vs. 340@5kn (works out to ~240nm@6kn)
  • 12 torpedoes vs. 23
  • Slower surfaced: 13kn vs. 15.5kn
The submerged range is a big factor: T21 creates ~4x the required search area given its ~2x submerged range.

Submerged range also figures into how much of sprint into attacking position each sub could do, leaving enough battery juice for evasion/escape after attack. T21 can stay submerged an moving for ~2x as long as T29 at equal speed.

T21 could have launched 18 torpedoes in 25min, and still had 5 more for later. From your diagram, it looks like the T29's torpedo room doubles as accommodation, with reloads stored below rather than ready for reload as in T21 (that may be the drawer's choice though). T21's dedicated torpedo loading room took a lot of space, which probably isn't feasible on a smaller boat.
All true!
Regarding underwater endurance at various speeds I have made these calculations: see screenshot below.
Regarding reload speed: no speed loading possible with this setup; loading time would be similar like in a Type VII, but I suspect even slower since more accommodation needs to be removed before getting to the reserve torpedoes.

On the positive side, compared to the XXI:
- only 33% of batteries (a bottleneck in 1944)
- only ~50% of engines (a bottleneck in 1944)
- only (probably) 3 hull sections instead of 8
- less sonar and radar echo
- ability to snorkel at flank speed
- more maneuverable
- shorter diving time
- greater diving depth (due to circular pressure hull)
- greater resistance to depth charges (boat will „roll with the punches“ more; this phenomenom was in fact reported by XXIII crews, where it applied to great extend)
- 27 crew instead of 57
- shorter working-up/ shakedown time
- only one type needed (XXIX H) instead of 2 (XXI and XXIII); massive economies of scale
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Re: "Bastard" Electro U-boats From Early 1943?

Post by TheMarcksPlan » 06 Jan 2022 20:06

kfbr392 wrote:
06 Jan 2022 18:33
Regarding underwater endurance at various speeds I have made these calculations: see screenshot below.
I don't quite understand the calculations. From the endurance vs. speed columns I infer you're roughly tracking endurance as correlated to [1 / speed^2]? Horsepower (therefore battery drain) moves roughly with the cube of speed (that's rough because many smaller factors are present such as battery draw efficiency, Reynolds number and therefore friction coefficient, propeller cavitation).
kfbr392 wrote:On the positive side, compared to the XXI:
- only 33% of batteries (a bottleneck in 1944)
- only ~50% of engines (a bottleneck in 1944)
Yes there's definitely positives to the design.
kfbr392 wrote:- ability to snorkel at flank speed
I didn't catch this. In the article you linked? I wonder if this really would have worked...
kfbr wrote:- greater diving depth (due to circular pressure hull)
Your linked article says depth undetermined. It's not necessarily true that circles have greater crush depth - depends on how strongly-built something is. I can blow a spherical ballon that will crush in a few feet of water, shallower than a square metal box...

I mentioned upthread that I suspected T21 used its floor beams to reinforce the structural discontinuity at the "double bubble" join. From pictures I've since seen this seems obviously true:

Image

This is a very strong structure, only marginally less efficient than a circle (weight of the floor beams but you need those anyway). Like a circle, it uses "hoop stress" on ~95% of its surface - all except the bubble-join where it's reinforced by the floor beams.
kfbr392 wrote:- only one type needed (XXIX H) instead of 2 (XXI and XXIII); massive economies of scale
What about Caribbean ops? The first T21 was on its way there when the war ended... And South Atlantic, Indian Ocean.

You might say ignore the peripheral theaters but enables the Allies to concentrate all their ASW in the North Atlantic.
kfbr392 wrote:- greater resistance to depth charges (boat will „roll with the punches“ more; this phenomenom was in fact reported by XXIII crews, where it applied to great extend)
Yes, another good point.

-------------------------------------------------

On the other hand, many things cost just as much for T29 as T21:
  • All advanced sensor systems, submerged and surfaced
  • Communications suite
  • Schnorkel
  • Periscope
  • Torpedoes
A final judgment might come down to how much cheaper is the T29 and how many more ships would a T21 kill?

T21's ability to throw out 18 torpedoes in 20 minutes, then evade at higher speed for longer, suggests to me that it might have ~3x the killing power of a T29, which probably will cost at least half as much.

Then again there's other factors (bottlenecks, construction/training time, number of boats at sea and therefore reconnaissance). I'd still probably pick the T21 if given only one choice but I can see the other side of the argument.
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U-Boat Type XXIX H (1944 unfinished design study)

Post by kfbr392 » 06 Jan 2022 20:40

TheMarcksPlan wrote:
06 Jan 2022 20:06
kfbr392 wrote:
06 Jan 2022 18:33
Regarding underwater endurance at various speeds I have made these calculations: see screenshot below.
I don't quite understand the calculations. From the endurance vs. speed columns I infer you're roughly tracking endurance as correlated to [1 / speed^2]? Horsepower (therefore battery drain) moves roughly with the cube of speed (that's rough because many smaller factors are present such as battery draw efficiency, Reynolds number and therefore friction coefficient, propeller cavitation).
I merely did a linear extrapolation based on the following:
- the measured performance of U 3507 in its final configuration as stated on de.wikipedia.org/wiki/U-Boot-Klasse_XXI
- the fact/assumption that at submerged speed of 6kn a XXI could go 256nm while a XXIX H would go 120nm.
In short: I multiplied the performance (submerged range@speed) of a XXI by the factor 0.469 to arrive at the performance of the XXIX H.
TheMarcksPlan wrote:
kfbr392 wrote:- ability to snorkel at flank speed
I didn't catch this. In the article you linked? I wonder if this really would have worked...
Please use deepL.com to machine translate the German language wikipedia article on Typ XXI. It is very good and comprehensive, because it is mainly based on the books by Eberhard Rössler, the German expert author on Uboats.
You will see that the XXI telescoping snorkel had some serious design flaws. It had too small diameter and was not rigid enough, leading to vibrations at certain snorkel speeds. Plus the periscopes could not be used at underwater speeds of >8kn due to vibrations.
This would not have been the case in XXIX H, which would have used a folding snorkel which stabilized the periscope (a so called "Sehrohrschnorchel" - periscope snorkel). In fact, it probably could have been used very much like a weaving, alternatingly snorkeling and deep-diving Schnellboot to break through the convoy screen at night!
TheMarcksPlan wrote:
kfbr392 wrote:- greater diving depth (due to circular pressure hull)
Your linked article says depth undetermined. It's not necessarily true that circles have greater crush depth - depends on how strongly-built something is.
Please read the German wiki page on this issue also.
The strength of the figure-8 hull of the XXI could not be calculated reliably beforehand and it turned out later that it could not dive deeper than a VII because of that.

TheMarcksPlan wrote:
kfbr392 wrote:- only one type needed (XXIX H) instead of 2 (XXI and XXIII); massive economies of scale
What about Caribbean ops? The first T21 was on its way there when the war ended... And South Atlantic, Indian Ocean.
You might say ignore the peripheral theaters but enables the Allies to concentrate all their ASW in the North Atlantic.
True.
But that is where traffic is concentrated and where the war is won or lost. And successful patrols there mean more Allied tonnage sunk/day/boat.
The boats need to march undetected to north of Ireland, find a convoy, break through the screen, fire the 6 ready torpedoes, disengage, then return to port; use the 4 reloads to target DEs, targets of opportunity, and damaged stragglers. Short patrols of less than 20 days, like in the fall of 1940, close to England. With substantial numbers of XXIX's.
I would argue type XXIX H would have been more effective and survivable than XXI. It could have re-enabled convoy battles in the Western Approaches close to Ireland, an area where uboats have been driven away from since May 1941. According to Rössler, it also featured noise dampening to enable quieter snorkeling, which would have somewhat compensated for its reduced battery capacity.

One XXIX could also shadow an outgoing convoy for days, report its position, evade HUFFDUFF attackers below the surface, and re-enable large scale wolfpack attacks further out in the Atlantic where fewer escorts could be expected.

Additionally, a few XXIX could be sent out further - to Freetown or the US - using older large conventional uboats as milk cows.
But with long patrols, a lot of time is wasted in transit, since these boats (XXI and XXIX) would have marched always submerged at ca. 5-6kn average.
TheMarcksPlan wrote: A final judgment might come down to how much cheaper is the T29 and how many more ships would a T21 kill?

T21's ability to throw out 18 torpedoes in 20 minutes, then evade at higher speed for longer, suggests to me that it might have ~3x the killing power of a T29, which probably will cost at least half as much.

Then again there's other factors (bottlenecks, construction/training time, number of boats at sea and therefore reconnaissance). I'd still probably pick the T21 if given only one choice but I can see the other side of the argument.
I can see that point also and agree with you that both concepts had their merit.

The XXIX was conceptualized 15-24 months later than the XXI and thus incorporated some lessons learned from the XXI detailed design and construction.

Also, in parallel to the XXIX, the one-screw Typ XXXI was drawn up in 1944 as a possible successor to the XXI; it had half the number of engines, 75% of the batteries, but still almost the same performance as the XXI. The XXXI too was never built, of course.
This makes it clear that the German officials did not consider the XXI an ideal solution, not even as a boat for long range operations. And definitely not for operations in the North Atlantic.

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Re: "Bastard" Electro U-boats From Early 1943?

Post by thaddeus_c » 07 Jan 2022 17:08

TheMarcksPlan wrote:
06 Jan 2022 09:04
thaddeus_c wrote:
05 Jan 2022 17:59
I used the Med for a quick reference to the size/type of boat I was speculating upon, but more broadly I meant something that could have been transportable overland, even if in sections. (well suited to the Black Sea)

certainly when they "inherited" so much coastline to defend, some "different" if not smaller boats would have been warranted?
They didn't really rely on the Uboats for coastal defense, however - not to any great degree. Rather the (IMO correct) strategy was to use Uboats to sink shipping, which would reduce Allied strategic mobility and protect Europe in general.

This strategy worked to a greater degree than most commentators realize. Allied shipping shortages were THE constraint on strategic options during 1942-43. Absent the U-boats, Allies probably could have invaded Europe in 1942 - they certainly could have done ROUNDUP in 1943. For roughly 5% of the armaments budget and a small manpower commitment, Uboats bought Germany roughly two years before facing the full might of the Wallies.
-------------------------------------------

Re transporting Uboats overland in sections, not sure that's really possible. Unlike an Eboat or Siebel Ferry, a Uboat's structural requirements are such that you'd need, basically, to have a final assembly point on the Med/Black seas.

...which isn't a bad idea if Italy, Romania, Bulgaria, or occupied SU had the capability/willingness to host such facilities and had Germany invented modular submarine construction earlier. There was some German shipbuilding happening in Bulgaria, Italy, and occupied SU (Nikolaev), not sure whether they could have been brought up to snuff for Uboats though, absent a crash investment and workforce-relocation/training program. Doenitz and Raeder did discuss moving U-boat production to the Baltic before Ostheer started retreating. I wonder whether the archives contain any detailed analysis of such plans or if they were just notional.


For a Med-optimized sub, what would you imagine? T7 was ~800t, T2 ~250t. I could see a 500t sub being useful and less expensive than T7 for non-coastal Med ops. You don't need 8,500nm range to operate effectively in the Med.

The per-boat savings seems perhaps unjustified, however, by the need to design a whole new boat, invest in new construction facilities, and loss of scale economies for T7/9 production (assuming resources are shifted Med-ward from T7/9).
my posting may have been unclear, IDK about the modular construction, the requirement or "desire" was to have a "transportable" smaller u-boat with Type XXIII, think by the time it was launched they didn't have anywhere to transport it to!? (so I don't know if it was easier or similar to the Type II to move overland)

Hitler, Doenitz, and the Baltic Sea is an interesting account of why they clung to some areas late war.

historically they used S-boats for harassing shipping and laying mines, just IMO, they should have continued development of smaller boats, rather than find in 1944 or 1945 they need a crash program (manned torpedoes and Seehund) of smaller vessels?

the whole premise of this thread is an earlier Elektroboote or otherwise enhanced oceangoing submarine, that could provide the savings in resources to build a number of smaller boats?

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Re: "Bastard" Electro U-boats From Early 1943?

Post by kfbr392 » 07 Jan 2022 19:39

IMHO, a German Med optimized uboat for 1943 onwards would have had to have the following characteristics:
- assembly in the Med (straits of Gibraltar had become too dangerous) from prefab sections
- ability to march and attack submerged more or less permanently (too many Allied ASV aircraft, escorts, and also shore based radar systems)
- small size in order to not be spotted from the air while at periscope depth (the water was very clear)
- quick diving time
- endurance >2500nm
- at least 2 torpedo tubes and no less than 4 torpedoes

Basically, the XXIII would have almost perfectly fit the bill, albeit on the low end, if it had been lengthened by about 1300mm and been given two torpedo reloads in the bow room. This actually was a consideration in February 1944, but because it would have resulted in delays in XXIII deliveries the proposal was rejected. The wrong decision, IMO.

The XXIX would have been an exellent fit also, on the high end.

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Re: "Bastard" Electro U-boats From Early 1943?

Post by TheMarcksPlan » 07 Jan 2022 21:07

kfbr392 wrote:
06 Jan 2022 20:40
Please use deepL.com to machine translate the German language wikipedia article on Typ XXI. It is very good and comprehensive
Somebody added a ton of great info to this article since I last looked at it a year or so ago.
kfbr392 wrote:the XXI telescoping snorkel had some serious design flaws. It had too small diameter and was not rigid enough, leading to vibrations at certain snorkel speeds. Plus the periscopes could not be used at underwater speeds of >8kn due to vibrations.
This would not have been the case in XXIX H, which would have used a folding snorkel which stabilized the periscope (a so called "Sehrohrschnorchel" - periscope snorkel).
I was aware of T21's schnorkel problems but not of the proposed fix on T29. Are we confident it would have worked?

It also raises the question of just putting a better schnorkel on T21. In any circumstance in which Germany can build T29, it can modify T21 as well.
kfbr392 wrote:But that [North Atlantic] is where traffic is concentrated and where the war is won or lost. And successful patrols there mean more Allied tonnage sunk/day/boat.
THe most successful patrols IIRC - aside from 39-40 - were in the Caribbean.
kfbr392 wrote:The boats need to march undetected to north of Ireland, find a convoy, break through the screen, fire the 6 ready torpedoes, disengage, then return to port; use the 4 reloads to target DEs, targets of opportunity, and damaged stragglers. Short patrols of less than 20 days, like in the fall of 1940, close to England. With substantial numbers of XXIX's.
Sounds like a good strategy to try - probably first priority effort whether it's T29 or T21.

The one concern I'd have is that Schnorkels could be detected by Allied aircraft, albeit at reduced radar range. If you concentrate solely on the Western Approaches then the Allies have enough planes that - if they're focused there - they can blanket the area, find the subs, and attack with air/surface cooperation.

If, however, that main effort is supplemented by serious attacks throughout the Atlantic, the Allies can't possibly cover the entire hemisphere with enough planes. That requires at least some long range boats. Even for attacks around Greenland, it's maybe better to send a T21 with twice the torpedoes given the weeks of deployment time required.
kfbr392 wrote:I would argue type XXIX H would have been more effective and survivable than XXI. It could have re-enabled convoy battles in the Western Approaches close to Ireland, an area where uboats have been driven away from since May 1941. According to Rössler, it also featured noise dampening to enable quieter snorkeling, which would have somewhat compensated for its reduced battery capacity.
You may be right on the actual designs but I'm more interested in how size selection drives the decision. As I said, anything "small" like a schnorkel that is put into a T29 seeing service maybe in 1946 could also be put into T21.
kfbr392 wrote:One XXIX could also shadow an outgoing convoy for days, report its position, evade HUFFDUFF attackers below the surface, and re-enable large scale wolfpack attacks further out in the Atlantic where fewer escorts could be expected.
Assuming the schorkel is really superb. With any kind of swell/waves in the North Atlantic, it becomes difficult to see long-term and high-speed schnorkelling viable. Even if it worked exactly as designed, wouldn't the schnorkel have to be so big/tall that it would give away the boat?
kfbr392 wrote:Also, in parallel to the XXIX, the one-screw Typ XXXI was drawn up in 1944 as a possible successor to the XXI; it had half the number of engines, 75% of the batteries, but still almost the same performance as the XXI.
Is this from Rossler? I should probably get a copy...
kfbr392 wrote:This makes it clear that the German officials did not consider the XXI an ideal solution, not even as a boat for long range operations.
It definitely wasn't ideal. Upthread I promised a citation to Germans acknowledging that the Walther-boat-derived hull form wasn't ideal. I haven't been able to track down that cite. Do you recall them saying so?

--------------------------------------------------

To return to the ideal size issue... My notion is that if you're choosing between 200 large subs and 300 smaller subs, you go for 300. I.e. the choice Doenitz advocated.

If, however, you're choosing between, say, 700 large subs and 1,050 smaller subs, I think the answer changes. With a 700-sub fleet, you probably have enough coverage already that the marginal reconnaissance value of each additional sub is lower. You want to maximize killing power more.

So T29 is probably a better answer for IRL Germany than T21, but stopping all submarine production in early '43 is the best possible answer.

For a Germany that has an actual chance of a sustained elektroboot campaign - say a Germany that's defeat Russia - a larger sub seems justified because Germany will have the resources to field perhaps a couple thousand uboats.

Finally, in the latter scenario, I could see the Allies trying to adapt by using faster merchants. A 16kn Type C2 cargo ship cost "only" twice as much, per ton, as a Liberty Ship. The Allies could have switched to that kind of shipping (at massive expense), which reduces their losses by a lot. Against those kinds of ships, each additional submerged knot would really count in taking opportunistic shots.

--------------------------------------------------

On the torpedo reload issue, my intuition is that being able to fire 18 torpedoes in 20-25min makes a small wolfpack ("wolfpod") the equivalent of a much larger wolfpack. That 20-25min timeframe fits nicely within the T21's ability - on a full battery - to sprint, maneuver, attack, and evade. The timeline for normal reloads means that each attack comes with another convoy/escort encounter, which doubles the sub's risk of destruction (or viewing it another way, cuts its usefulness by 2/3 per each time it's risked).

To be clear, this is the decisive issue IMO. The longer range isn't sufficient to justify the larger sub as the primary type and maybe not even as a supplemental type. Were it possible to have a dedicated ready-reload chamber on a smaller sub, then definitely go with the smaller sub.
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Re: "Bastard" Electro U-boats From Early 1943?

Post by TheMarcksPlan » 07 Jan 2022 21:47

@kfbr392 somewhat off-topic but do you know whether the KM considered using T7/9's as "scouts" for the elektroboots? They still had hundreds of the hulls lying around. I'm thinking they could have left off some torpedoes and associated crew, packed in more provisions. Then send out the older boats to form a recon picket line, not expecting them to attack convoys (just a few torpedoes to defend themselves). As they'd be mostly loitering instead of cruising, they don't need more fuel for longer endurance - just more crew supplies. With schnorkel, the T7/9's were pretty safe at sea, so long as they didn't try to attack convoys.

I could see this not being considered because the crews were all transitioning to elektroboots or being pulled into land formations. Or because the cost of maintaining the boats wasn't worth it.
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Re: "Bastard" Electro U-boats From Early 1943?

Post by kfbr392 » 08 Jan 2022 07:40

TheMarcksPlan wrote:
07 Jan 2022 21:47
@kfbr392 somewhat off-topic but do you know whether the KM considered using T7/9's as "scouts" for the elektroboots?
Well possible that someone contemplated this, but in reality it took 22 months from build decision for the first XXI to go on a war patrol and in the meantime, the VII and IX boats were asked - and were willing - to sacrifice themselves to tie down Allied anti submarine resources to save civilian German lives and cities. And then the war was over.

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Re: "Bastard" Electro U-boats From Early 1943?

Post by kfbr392 » 08 Jan 2022 08:03

TheMarcksPlan wrote:
07 Jan 2022 21:07
I was aware of T21's schnorkel problems but not of the proposed fix on T29. Are we confident it would have worked?
Pretty much. This concept was about to be tested on 2 Typ VII C in May 1945, and snorkel speeds up to 11kn were envisioned (on the modified VII C!). It was also called the Oelfkenschnorchel after its inventor. It should have been mature by the end of 1945.

TheMarcksPlan wrote: THe most successful patrols IIRC - aside from 39-40 - were in the Caribbean.
Yeah, the most sinkings per single patrol were done by IX boats doing very long cruises in 1941/42. But the ONLY important metric was tons/boat/day of the whole U-Bootwaffe, and that metric was dropping since 1941 as sinkings in the NA were down so much because uboats could not operate close to England anymore, and other factors.

TheMarcksPlan wrote: The one concern I'd have is that Schnorkels could be detected by Allied aircraft, albeit at reduced radar range. If you concentrate solely on the Western Approaches then the Allies have enough planes that - if they're focused there - they can blanket the area, find the subs, and attack with air/surface cooperation.
Germany worked on ways to mask the snorkel with radar absorbing materials. Germany in 1944 was aware that Allies were using 9cm, 3cm and 1.5cm radars. Detection range was also contingent on sea state. Germany put passive radar detectors on snorkel heads and/or on masts so the following scenarii are possible:
- the uboat only snorkels at night and goes on a silent dive as soon as the detector reports a radar emitting plane approaching.
- in good weather, the uboat snorkels during daytime and uses the periscope to visually spot approaching planes before they get close
- for ~18h/day, the boat runs deep on silent engines at 5-6kn
The Allies could not be sure they had really picked up a snorkel radar echo, it could also have been flotsam.
Also consider the actual patrols done by XXIII boats around England in 1945. Not a single boat was attacked from the air during war patrol, not a single boat was lost to any anti-submarine weapon during war patrol.

TheMarcksPlan wrote: Assuming the schorkel is really superb. With any kind of swell/waves in the North Atlantic, it becomes difficult to see long-term and high-speed schnorkelling viable. Even if it worked exactly as designed, wouldn't the schnorkel have to be so big/tall that it would give away the boat?
The combustion air for the diesel engines was taken from the engine room, the snorkel fed air into the engine room. This enabled the diesel engines to remain running even if the snorkel got submerged for up to 30 seconds (IIRC). The snorkel head had a shut off valve to prevent water ingress, and water that ingressed was seperated down in the boat.

TheMarcksPlan wrote:
kfbr392 wrote:Also, in parallel to the XXIX, the one-screw Typ XXXI was drawn up in 1944 as a possible successor to the XXI; it had half the number of engines, 75% of the batteries, but still almost the same performance as the XXI.
Is this from Rossler? I should probably get a copy...
Yes, nearly all of the info I give here is from Eberhard Rössler, the German expert author on uboats.

TheMarcksPlan wrote: On the torpedo reload issue, my intuition is that being able to fire 18 torpedoes in 20-25min makes a small wolfpack ("wolfpod") the equivalent of a much larger wolfpack. That 20-25min timeframe fits nicely within the T21's ability - on a full battery - to sprint, maneuver, attack, and evade. The timeline for normal reloads means that each attack comes with another convoy/escort encounter, which doubles the sub's risk of destruction (or viewing it another way, cuts its usefulness by 2/3 per each time it's risked).
This is very true and the reason why most 1944 German uboat concepts foresaw at least 8 torpedo tubes. Type XXVI W had 10, Type XXXI had 12. It was realized that probably only a single breakthrough into the convoy would ever be possible, and so the ready torpedoes had to be maximized.

The XXIX H was on the low end in this regard. I believe it was intentionally tuned down to be low on resource consumption and complexity.


And yes, pure Victory ship convoys would really have given XXI a very hard time. This was realized in Germany in early 1944 and one of the reasons the Walter drive Typ XXVI W was ordered into limited production (only Hamburg shipyards). It was half the size of the XXI, which was all that was needed for the North Atlantic, another admission that the XXI was too big.
The XXIX was basically a Elektroboat evolution of the XXVI W.
Some XXIX concepts also had Kreislaufdiesel (closed circuit diesel engines, feeding bottled oxygen into the combustion air) propulsion to save on batteries or Walter fuel and still have high underwater speeds independent of outside air.

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