"Bastard" Electro U-boats From Early 1943?

[TheMarcksPlan]

Type 212
Electric power: 1,700 kW
Weight submerged: 1,830 to
Max. speed submerged: 20 kn

Although the actual type 212 has less than half the power of the WWII type XXI it has a greater underwater speed due to it's perfect streamlined hull and it's modern efficient propeller

Seems a bit of stretch to suggest a 1990's design as a feasible alternative to Type XXI.

Does anyone have stats on max submerged propeller RPM for these two designs?

Going back to this reply to make my point more explicit: Propeller RPM and HP-speed ratios can't be ignored. This is just basic physics.

Doubling RPM requires ~8x more HP for ~4x more thrust (=resistance at top speed).

If you quadruple propeller area instead of doubling RPM, you get 4x the thrust for half the HP.

Compare the propeller size of the Type 212 and Type XXI:




I still don't have the numbers (does anyone?) but it's obvious that Type 212's giant single propeller has greater area than Type XXI's two small propellers. Thus it's practically certain that Type 212's HP was more efficiently deployed (had greater propulsive efficiency).

In addition, modern propeller designs using CFD and decades of experimental data are better able to cope with cavitation. That's probably both of the cause of greater propeller area, relative to Type XXI, and an additional delta to propulsive efficiency (except at the greatest depths where cavitation recedes to higher ambient water pressure).

Finally it may be the case that - even with the above-discussed modern cavitation-amelioration - that Type 212 can only do 20kn at the greater depths it reaches beyond Type XXI.

[PunctuationHorror]

The smaller type XXIII had a bigger propeller (in relation to the whole submarine).

Besides of the 'bring engine power in the water' problem, I guess another reason for the two shaft design of the XXI is rooted in redundancy and reliability: Chances for both engines and shafts breaking down at the same time - which would lead to a almost guaranteed loss of the boat - are limited. Type XXI is a rather big boat with ~1650t compared to ~250t of the XXIII, so it is more valuable than the latter and the cost to risk assesment is different. On the other hand, good old type II had this typical double engine setup like any german boat since pre WW1 - so probably priorities had shifted when type XXIII was constructed, maybe engines had become more reliable by then or the one engine setup was regarded as 'good enough' for the purpose and expected lifetime of the new boat and more beneficial in regard of the whole design.

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

I want to point to a hull more optimized for the hydrodynamics of submerged cruising.
Concerning the outside shape of the hull (mere its cross section), XXI is still a conventional design: It is a vertical, narrow, slim ellipse. Very narrow bow. XXIII is better shaped because it is a more dulled ellipse, but its 'potato shape' is still no circle. See its cross section:
From the outside, type XXIII looks like a torpedo with conning tower whereas XXI still bears some resemblance to type IX.

However, looking at the inside, they went for an 8-shaped pressure hull for type XXIII and for XXI.
Why bother with an 8-shape when they could 'simply' enlarge a circular shape? The forces on an 8-shaped hull are much more complicated, create weak points on the 'wasp waist', were beyond their means of calculation back then and consequently yielded a lower crush depth compared to older type VII (which had a circular pressure hull). An O-shaped pressure hull has none of these problems. Geometry can't be neglected. Even today's deep diving russian submarine uses spheres for its pressure-proof hull because a sphere inherently features certain physical advantages which can otherwise only be achieved with great efforts. So does a circle.

An alternate XXI with O-shaped pressure hull and O-shaped outside hull would be broader and have better hydrodynamics because of the O-shape. It has not to be an O-shape: A bulbous, fat horizontal elliptical cross section of the external hull would do a similar good (better?) job.
Maybe here is the reason for the original narrow XXI design and shape: section construction. Externalized production of these sections required transporting them to the shipyards. An O-shaped XXI would have sections as broad as high and this would make the transport a serious challenge:

Post war german subs like type 201 - that are somehow the continuation of type XXIII design after a hiatus of fifteen years - have a circular pressure hull and a circular outside shape, e.g. type 205:
It resembles a whale.
So they left the vertical potato shape of the XXIII for a circle and returned to it with type 212 after a few decades (to an extent, it is more circular than XXIII). Has anybody a cross section of 212?

[TheMarcksPlan]

From the outside, type XXIII

Thanks, hadn't seen this one before.

Why bother with an 8-shape when they could 'simply' enlarge a circular shape?

IIRC they did this on the T21 because, in the rush to get a serviceable boat, they simply adapted advanced design from the Walther boat. In the Walthers I'm guessing a segregated compartment under the main one had safety benefits for the highly volatile (in fact nonviable) fuel. I can't remember where exactly I read this but now but IIRC Doenitz and everyone knew full well that it was suboptimal (opposite of pun intended).

OTOH, the extent of suboptimality is probably less than at first glance. Why? Well a T21 still had to function as a ship: its structure needed to withstand a critical wave when running on the surface. Having a pressure hull taller than wide has serious structural benefit because the second moment of inertia/area (basically resistance to bending per amount of material resisting that bending) increases with the square of height.

I suspect this surface-running structural side benefit wasn't relevant to the Type 212 design. There you have a pressure hull intended for 2300ft depths, which probably implies a pressure hull so strong that it has more than sufficient bending resistance when the sub is surfaced. In structural terms, the ship is "pressure dominated" rather than "bending dominated." Type XXI, by contrast, dove to ~800ft and had an accordingly weaker pressure hull.

This dynamic plays out in commercial airliner fuselages, btw, which are also duel pressure/bending structures. While some commercial airliners have circular fuselages (e.g. Boeing 777 and Airbus A330), the last two large airliners developed (Boeing 787 and Airbus A350) each have "double bubble" cross sections analogous to Type XXI/XXIII.



It's sort of imperceptible in that graphic but 787 and A350 are taller than wide. They ameliorate the inefficiency by using the cabin floor beams to carry tension when the cabin is pressurized. Type XXI/XXIII appear to do similarly, using the floor of the crew compartment over the lower bubble to carry compression loads from the structural discontinuity at the bubble joins.

So all that's to say that while the pressure hull design is somewhat suboptimal it's probably a minor factor.

I'd agree they probably could have taken streamlining of the external hull even further. Doenitz seems to have envisioned significant surface running, however, and a teardrop form would have severely hurt surface performance (see Type 212's 12kn surfaced speed).

[PunctuationHorror]

Why bother with an 8-shape when they could 'simply' enlarge a circular shape?

IIRC they did this on the T21 because, in the rush to get a serviceable boat, they simply adapted advanced design from the Walther boat. In the Walthers I'm guessing a segregated compartment under the main one had safety benefits for the highly volatile (in fact nonviable) fuel. I can't remember where exactly I read this but now but IIRC Doenitz and everyone knew full well that it was suboptimal (opposite of pun intended).

OTOH, the extent of suboptimality is probably less than at first glance.

Note that the Russians never adapted the 8-shaped pressure hull on their versions of type XXI, aka project 611 'Zulu' and

pr. 613 'Whiskey' (and other descendants) where outside shape is almost identical to XXI. Neither did other powers. USS Nautilus had a circular cross section of its pressure, too. French and British
There must be a reason why 8-shape is so rarely found in pressure hulls. Even gas cylinders are ... cylindric.

However, sometimes it is used. Looking for blueprints, I stumbled upon this example where they needed a high and narrow pressure hull to fit between rocket boxes. And note how it differs from type XXI. Obviously, they used two cutted circles of the same size. Symmetry.
Since XXI had no rockets, there was no need for a high and narrow pressure hull in type XXI.
Other finding: The aft sections of type XXI with Diesels etc. were circular.

Anyways, I'm not sure if type XXIII had its design roots in the Walter boat like XXI did. If this was not the case, the double bubble diametered pressure hull was not needed and a conventional circular design of the pressure hull would have been simpler.

Regarding structure and 'pressure dominated' vs 'bending dominated': How comes that type VII, IX, ... , with circular pressure hulls and obviously would fall in the category of 'bending dominated' do not have the problems with structural integrity?


After looking at all this, I'd say: The pressure hull design of XXI and XXIII might be an example of pointless engineering, It created more problems and complexity than it solved.

/Edit for spelling

[thaddeus_c]

The smaller type XXIII had a bigger propeller (in relation to the whole submarine).

Besides of the 'bring engine power in the water' problem, I guess another reason for the two shaft design of the XXI is rooted in redundancy and reliability: Chances for both engines and shafts breaking down at the same time - which would lead to a almost guaranteed loss of the boat - are limited. Type XXI is a rather big boat with ~1650t compared to ~250t of the XXIII, so it is more valuable than the latter and the cost to risk assesment is different. On the other hand, good old type II had this typical double engine setup like any german boat since pre WW1 - so probably priorities had shifted when type XXIII was constructed, maybe engines had become more reliable by then or the one engine setup was regarded as 'good enough' for the purpose and expected lifetime of the new boat and more beneficial in regard of the whole design.

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

I want to point to a hull more optimized for the hydrodynamics of submerged cruising.
Concerning the outside shape of the hull (mere its cross section), XXI is still a conventional design: It is a vertical, narrow, slim ellipse. Very narrow bow. XXIII is better shaped because it is a more dulled ellipse, but its 'potato shape' is still no circle. See its cross section:
From the outside, type XXIII looks like a torpedo with conning tower whereas XXI still bears some resemblance to type IX.

However, looking at the inside, they went for an 8-shaped pressure hull for type XXIII and for XXI.
Why bother with an 8-shape when they could 'simply' enlarge a circular shape? The forces on an 8-shaped hull are much more complicated, create weak points on the 'wasp waist', were beyond their means of calculation back then and consequently yielded a lower crush depth compared to older type VII (which had a circular pressure hull). An O-shaped pressure hull has none of these problems. Geometry can't be neglected. Even today's deep diving russian submarine uses spheres for its pressure-proof hull because a sphere inherently features certain physical advantages which can otherwise only be achieved with great efforts. So does a circle.

Post war german subs like type 201 - that are somehow the continuation of type XXIII design after a hiatus of fifteen years - have a circular pressure hull and a circular outside shape, e.g. type 205:
It resembles a whale.
So they left the vertical potato shape of the XXIII for a circle and returned to it with type 212 after a few decades (to an extent, it is more circular than XXIII). Has anybody a cross section of 212?

thanks for all the information!

I've always speculated the KM should have built the Type XXIII first and the Type XXI later, if at all, but that speculation was based more around the industrial capacity and the pressing need for a coastal boat.

guess the "bastard" u-boat Type VII could have come into play as a stop gap and/or complement to the smaller Type XXIII?

[PunctuationHorror]

guess the "bastard" u-boat Type VII could have come into play as a stop gap and/or complement to the smaller Type XXIII?

A better type VII ? Indeed:

Instead of a reconstruction of type IX, as it was proposed here, why not take type VII D, the minelayer (see picture below), or type VII F, the torpedo supplier?

Both types had an additional section of ca 10 meters aft the conning tower. A conversion would be much easier.
Remove the installations for the mines (or installations for torpedo transport in type VII F) and fill it with batteries. It would give a third 'Akku-Raum' and would easily add 50% more batteries. Or even more batteries, as there is more space available than in the two other 'Akku-Raum' rooms.
The most important quality of the VII D/ VII F: the section is already in the middle of the boat, so the center of mass would not be much shifted with the additional weight of the batteries. Balancing problems would be smaller because the design is already adjusted for the extra section. This would not be the case with a yet to be newly constructed type IX lookalike.

If the 'Munitionskammer' in the front is removed, 'Akku-Raum 2' could be increased to house a few more cells.

In all, these measures could give 60%-75% additional batteries which translates roughly in the same amount of additional submerged endurance. So instead of a range of 70 nautical miles it would have 115 nmi @ 4knots (that's over 28 hours submerged cruising endurance instead of 17). Above the batteries would be space for additional pressured air cylinders and crew accomondations as well as better hygienic facilities.

Italy (allied) and Netherlands (occupied) were farer advanced in snorkel technology. By simple 'inspiration', not to say copying, some german subs could have had basic snorkel installments to gain experience as early as 1940. By 1943 they could have had more reliable snorkels than in OTL and they could have been implemented in the turret design regularly instead of being retrofitted in a sort of tinkered provisional.

Then streamlining of the hull for submerged cruise. Besides of the removal of the deck gun and streamlining the turret, it appears to be beneficient to create a more closed surface of the upper deck via reduction of the grated deck space on the upper deck. In late war they did such convertions (sadly, I can't find a picture at the moment) to create a more closed, more rounded surface. Type VII is mainly a cylindrical pressure hull with some casing on its upper part, so removing/changing some of it could reduce drag in a very simple manner.

Although these measures would increase the submerged speed a bit, without new stronger electric motors - which probably would not fit in the hull - it would remain limited to 10-12 knots (at utmost best) for a duration of perhaps 30 minutes. However, a uboat with a reliable (!) snorkel that could cruise submerged at a speed of 8-10 knots would be much less endangered by aircraft.

Anyway, a new boat type would become necessary. For a start I would keep the conception of the XXI design, only with a circular pressure hull.

Kriegsmarine could have come up with simpler 'emergency' designs that resemble a compressed torpedo: Cylindrical hull. Conning tower. Diesel-electric propulsion. Instead of XXI and XXIII for ocean going and coastal role. Particularly, if they had started to look for succeeding types earlier than in OTL.

Even more advanced designs would be possible in theory:
Look at the Walter V80 boat. Its hull is a ~ 1970/80s design.
Having something like this with diesel-electric propulsion instead of hydrogen peroxide and in XXI scale by 1943 would be a entirely different thing.
With a disregard for u-boat design paradigms that were around since ww1 (unlikely to happen in Kriegsmarine structures) and real support of innovative men like Walter (likewise unlikely) a real submarine could have been made. We 'only' need a different Kriegsmarine for that.

[TheMarcksPlan]

Instead of a reconstruction of type IX, as it was proposed here, why not take type VII D, the minelayer (see picture below), or type VII F, the torpedo supplier?
...
Although these measures would increase the submerged speed a bit, without new stronger electric motors - which probably would not fit in the hull - it would remain limited to 10-12 knots (at utmost best) for a duration of perhaps 30 minutes

You seem to have answered your own question re T7.

One reason the T9 is the preferable bastard platform - apart from the square/cube dynamic - is that its hull is more likely to accommodate larger e-motors. Worst case scenario they are staggered on the length axis, with a shaft passing abreast of the rear motor.

I'm still unconvinced that this design is worthwhile, however.

The pressure hull design of XXI and XXIII might be an example of pointless engineering, It created more problems and complexity than it solved.

I'm still meaning to get you a citation for T21 cross section being a Walther boat legacy. Discussion in GSWW v.5-2, IIRC, which I don't have at hand atm.

Note that the Russians never adapted the 8-shaped pressure hull on their versions of type XXI, aka project 611 'Zulu'

The T21 cross section is suboptimal, that's clear. I'm only questioning the extent. I'd guess the difference is no more than a few tenths of a knot. For a clean sheet design you'd never consciously choose even a minor suboptimality but Germany didn't have a clean sheet option in 1943.

[PunctuationHorror]

Instead of a reconstruction of type IX, as it was proposed here, why not take type VII D, the minelayer (see picture below), or type VII F, the torpedo supplier?
...
Although these measures would increase the submerged speed a bit, without new stronger electric motors - which probably would not fit in the hull - it would remain limited to 10-12 knots (at utmost best) for a duration of perhaps 30 minutes

You seem to have answered your own question re T7.

One reason the T9 is the preferable bastard platform - apart from the square/cube dynamic - is that its hull is more likely to accommodate larger e-motors. Worst case scenario they are staggered on the length axis, with a shaft passing abreast of the rear motor.

Out of curiousity, I checked the dimensions of the pressure hull of VII and IX.
Turns out type VII has a bigger diameter/beam of the pressure hull than type IX. 4.70 m vs 4.40 m. Seems a bit odd, but blueprints seem to confirm this - at first glance.
Pressure hulls of VII D, VII F and IX (except for IX Ds) have a length of ~ 60 m.

So both types have surprisingly similar pressure hull dimensions and Emotors that would not fit in a VII D wouldn't fit in a IX, either.

I'm still unconvinced that this design is worthwhile, however.

They would get more survivable boats out of already existent obsolete types. Instead of mass producing type VII C in 1941-43, ramp up production of an 'electro' VII D - and/or an 'electro' type IX with longer range due bigger fuel tanks - and have lower loss rates. Of course, new, not obsolete types would be preferable.

[TheMarcksPlan]

Turns out type VII has a bigger diameter/beam of the pressure hull than type IX. 4.70 m vs 4.40 m. Seems a bit odd, but blueprints seem to confirm this - at first glance.

Well that's surprising and would certainly negate my "square-cube" comments. Can anyone confirm wiki's data?

They would get more survivable boats out of already existent obsolete types. Instead of mass producing type VII C in 1941-43, ramp up production of an 'electro' VII D - and/or an 'electro' type IX with longer range due bigger fuel tanks - and have lower loss rates.

True. I was thinking the OP addressed an early 1943 PoD but there's no reason to limit the discussion that way. As you say, it requires a different KM.

[ThatZenoGuy]

You can't really fit in any more batteries in the Type 7/9 without ripping something out. The 21's were pretty big girls with an optimized hull design.

The 7 and 9's could be optimized further than historically sure, but after a certain point the changes would be so large that you run into the problem of 'make changes and make less subs, or keep it the same and make more'.

Is 2 'better' subs more effective than 3 'not better' subs? Who knows!

I think Germany could've maybe had closer ties with the Japanese to learn how they did their modular submarine designs. As far as I can tell they had less problems than the 21's production.

[TheMarcksPlan]

You can't really fit in any more batteries in the Type 7/9 without ripping something out.

That's why Punctuation and I have focused on the "D" models of T7/9 as bastard candidates. Each was an enlarged version of the baseline T7/9, with extra space used for things other than batteries (mines for T7D, extra diesels and generators for surface cruising for T9D).

The 21's were pretty big girls with an optimized hull design.

As I've estimated upthread, T9D was about the same overall size as T21. Narrower but longer, especially within its pressure hull.

Not being streamlined is a drawback, obviously, but given a square or cubic relationship between speed and horsepower it's feasible to get a bastard T7/9 to at least 12kn. Consider that the USN's GUPPY program turned its old WW2 submersibles into T21-performing submarines by adding batteries and doing superficial streamlining to existing boats.

The 7 and 9's could be optimized further than historically sure, but after a certain point the changes would be so large that you run into the problem of 'make changes and make less subs, or keep it the same and make more'.

Is 2 'better' subs more effective than 3 'not better' subs? Who knows!

Yeah this is the point where there's perhaps too much hindsight. You're right that a bastard E-boat would have required rejiggering of production and, for the same resources, fewer boats. Unless you know that the submersible is about to become useless, it's not obvious that fewer but more effective subs is preferrable. That was definitely Doenitz's viewpoint - even after the theoretical design of T21 was completed, he intended them only to replace T9 until May '43 when T7's total defeat was obvious.

What's interesting to me is whether the Germans would have come up with T21 earlier, had the KM possessed more resources overall (if Russia was clearly on its last legs as 1941 closed, for example, or if Germany had mobilized its total resources earlier). In that hypothetical, I can see Raeder and Doenitz putting more attention to the Walther boats in '41 and to incremental improvement during early 1942 - when the airplane threat was well recognized and with it the need for higher underwater speed/endurance.

[ThatZenoGuy]

You can't really fit in any more batteries in the Type 7/9 without ripping something out.

That's why Punctuation and I have focused on the "D" models of T7/9 as bastard candidates. Each was an enlarged version of the baseline T7/9, with extra space used for things other than batteries (mines for T7D, extra diesels and generators for surface cruising for T9D).

The 21's were pretty big girls with an optimized hull design.

As I've estimated upthread, T9D was about the same overall size as T21. Narrower but longer, especially within its pressure hull.

Not being streamlined is a drawback, obviously, but given a square or cubic relationship between speed and horsepower it's feasible to get a bastard T7/9 to at least 12kn. Consider that the USN's GUPPY program turned its old WW2 submersibles into T21-performing submarines by adding batteries and doing superficial streamlining to existing boats.

The 7 and 9's could be optimized further than historically sure, but after a certain point the changes would be so large that you run into the problem of 'make changes and make less subs, or keep it the same and make more'.

Is 2 'better' subs more effective than 3 'not better' subs? Who knows!

Yeah this is the point where there's perhaps too much hindsight. You're right that a bastard E-boat would have required rejiggering of production and, for the same resources, fewer boats. Unless you know that the submersible is about to become useless, it's not obvious that fewer but more effective subs is preferrable. That was definitely Doenitz's viewpoint - even after the theoretical design of T21 was completed, he intended them only to replace T9 until May '43 when T7's total defeat was obvious.

What's interesting to me is whether the Germans would have come up with T21 earlier, had the KM possessed more resources overall (if Russia was clearly on its last legs as 1941 closed, for example, or if Germany had mobilized its total resources earlier). In that hypothetical, I can see Raeder and Doenitz putting more attention to the Walther boats in '41 and to incremental improvement during early 1942 - when the airplane threat was well recognized and with it the need for higher underwater speed/endurance.

I think that while the additional electric-engine time of these 'bastard electro boats' could be of use, it lacks a lot of the benefits of the 21's.

The 21's weren't scary just because of their speed underwater, but rather how silent they were for their size. Sonar would still pick them up if they got hit by the ping but passive hydrophones would have more difficulty.

So we have to slap on the 21's weird supercharged diesels (which apparently had issues?) years before they were even experimented with, the creep motors which also didn't exist, etc.

And that's not even getting into the weirder stuff like the 21's sonar/hydrophone array and it's mechanical loading system.

[TheMarcksPlan]

The 21's weren't scary just because of their speed underwater

Underwater speed and endurance were the main reasons though. British tests against their own subs, amped up to only 12kn, showed their existing escorts would have been basically helpless. The quietness, quick reload, and better sonar are very threatening aspects but not necessary to change the game. Combining all factors, Allies couldn't have handled hundreds of T21's even in 1950. There would have been a shipping massacre.

Schnorkel is an under-rated factor. Most don't realize that, with schnorkel, conventional Uboats were about as safe at sea in 1944 as they were in 1941. So why no revived Uboat threat absent T21?

Because a schnorkelling or submerged Uboat was akin to a manned mine. They were so slow and short-ranged that a ship/convoy had nearly to run them over to come within torpedo range.

Higher underwater speed/endurance changes the game because it enables the sub to sprint into attacking position while submerged, rather than needing to surface (suicide by mid-43) or else just watch ships steam by through the periscope.

[thaddeus_c]

in my view the French Atlantic bases affected u-boat development, if they had operated from Norway or the Channel some of the issues would have to be dealt with sooner?

after June 1941 the life or death struggle for Germany was in the East, and an open wound was in the Med?

it is almost as if the KM was waging a whole separate war, albeit they were almost forced to send u-boats into the Med, which should have been a time to develop smaller boats?

[Carl Schwamberger]

What size would you recommend ?