What if the Type 21 U-Boats became operational in 1940?

[T. A. Gardner]

One counter the Allies could have done was bomb the battery factories that made those for U-boats.

https://uboat.net/technical/batteries.htm

There weren't very many, and the equipment and skill set to make batteries for U-boats is rather specialized. So, dropping bombs all over the factories would have had serious repercussions of production.

[TheMarcksPlan]

Subs are always relatively restricted if they cannot surface and use a deck gun to attack shipping, and it was where this was possible subs managed to make the biggest impact, the Type XXI doesnt even have such a weapon, making it great for attacking high value naval targets but a lot less useful for attacking merchant ships.

I know you're a WW1 expert - are you maybe importing the 1914-18 Uboat tactics into WW2? Most Uboat kills were via torpedo, the KM started removing deck guns by 1943. https://uboat.net/technical/guns.htm

They dont need to be magic bullets, they fire forwards, thus allowing even relatively slower escorts to fire them to ensure the sub keeps its distance from a convoy.

I don't get this point. A sub close enough to be hit with Squid/Hedghog is already within torpedo range of the convoy unless you're talking about hunter/killer groups.

The Germans didnt even have the ability to make 500 Type XXI's by the wars end

Per Speer:

We would have been able to keep our promise of deliver-
ing forty boats a month by early in 1945, however badly the war was
going otherwise, if air raids had not destroyed a third of the submarines
at the dockyards.

https://archive.org/stream/Inside_the_T ... r_djvu.txt

So a 1945 Germany that had lost all its empire and had mobilized all its men could still produce almost exactly 500 Type XXI's per year.
A 1943 Germany with far greater resources could have done much better than 500/year.

[TheMarcksPlan]

One counter the Allies could have done was bomb the battery factories that made those for U-boats.

https://uboat.net/technical/batteries.htm

There weren't very many, and the equipment and skill set to make batteries for U-boats is rather specialized. So, dropping bombs all over the factories would have had serious repercussions of production.

True enough, late-war Germany was F'd no matter what.
But the Allies couldn't reliably bomb point-targets until well into 1944 so an earlier-war or ATL Germany could have pumped out hundreds of Type XXI's.

[Terry Duncan]

I know you're a WW1 expert - are you maybe importing the 1914-18 Uboat tactics into WW2? Most Uboat kills were via torpedo, the KM started removing deck guns by 1943. https://uboat.net/technical/guns.htm

The most effective periods of the sinkings were also prior to 1943, by 1943 it was too risky for subs to stay on the surface at all really. A submarine is not overly efficient when reduced to torpedoes only. WWI proved exactly the same lessons.

I don't get this point. A sub close enough to be hit with Squid/Hedghog is already within torpedo range of the convoy unless you're talking about hunter/killer groups.

The outer escorts would be sufficiently far out as to make it a very long shot at any ship and then allows a pretty good chance of running down the sub by the rest of the escorts. The advantage of forward firing is that it allows an engagement forty five to sixty degrees to either side of the escorts course, removing the need to pass over or close to the sub.

Per Speer:

We would have been able to keep our promise of deliver-
ing forty boats a month by early in 1945, however badly the war was
going otherwise, if air raids had not destroyed a third of the submarines
at the dockyards.

https://archive.org/stream/Inside_the_T ... r_djvu.txt

Yes, a shame most of the modular parts didnt fit together at all and meant the Type XXI was restricted in dive depth due to hull flaws built in by the modular construction.

https://en.wikipedia.org/wiki/Type_XXI_ ... nstruction

118 built and only 4 fit for service. That is a pretty poor use of resources.

So a 1945 Germany that had lost all its empire and had mobilized all its men could still produce almost exactly 500 Type XXI's per year.
A 1943 Germany with far greater resources could have done much better than 500/year.

If only they worked...

[Russ3Z]

Assuming a 1943 construction program for something like a Type XXI design (being an earlier design it's probably fairest to assume its performance is a bit lower than the Type XXI as well), Germany may not have felt the same sense of desperation late-war that caused it to attempt a modular design method. Perhaps it might be more accurate to extrapolate production numbers based on traditional assembly methods, taking into account both the greater size and complexity of these boats along with the inevitable teething problems and ramp-up time for a new design.

The numbers produced in this case will doubtless be lower, and as noted above they will probably be less potent than the later Type XXI, but more of that number will be fit for service.

[Terry Duncan]

I dont disagree, a more conventional construction method would have been better but the numbers are then very limited from what I understand, hence the decision to mass produce modules.

[Carl Schwamberger]

...
The main technological counter to a high-speed submerged vessel is a high-speed escort, as improved radar and sonar are of limited effectiveness if the ships employing them can't catch their prey in the first place (think Flower-class difficulties). The trade-off for the British will be to make a relatively smaller number of high-speed escorts vs the larger number of low-speed escorts historically employed, at least in the early years.

Of course this British disadvantage in escort numbers and longer build times is offset by essentially the same trade-off for the Germans, who will likewise have a smaller number of possibly-better U-boats.

This touches on the under considered portion of the discussion. The Brits reacted according to their perception of how the Battle of the Atlantic was going. Change the submarine type or performance & Brit perception & decisions change.

... This really drives home that the late-war Schnorkel equipped Uboats weren't so much blown out of the seas as suppressed by aircraft, rendering them too slow and blind to cause great damage. ...

Most of the discussion has been on the details of the convoy escorts and their weapons. The under equipment of Coastal Command and the RN air arm in general does touch on this subject. In 1941 the Brits thought they might be coping with the submarines, if not winning the BoB. The realization there is a far faster and longer ranged sub out their, rapidly replacing the older short ranged types, means a change in priorities and decisions from strategic to tactical.

[TheMarcksPlan]

The most effective periods of the sinkings were also prior to 1943, by 1943 it was too risky for subs to stay on the surface at all really. A submarine is not overly efficient when reduced to torpedoes only. WWI proved exactly the same lessons.

I did a bit of googling on this but I'm gonna ask for help: Does anyone have stats on the proportion of WW2 Uboat kills - especially against convoys - that deck guns accounted for? My sense from reading about the operations is that, except when using the deck gun to save torpedoes in cases where the Uboat felt totally secure (e.g. when attacking an isolated ship in far-flung water), the vast majority of WW2 Uboat kills were by torpedo. I don't have the goods at the moment so I'm happy to concede this point if the data show me wrong.

The advantage of forward firing is that it allows an engagement forty five to sixty degrees to either side of the escorts course, removing the need to pass over or close to the sub.

Yeah I get the basic tactical concept, what I don't get is how this keeps the vast majority of Type XXI's away from convoys. Two factors:

• Not every sub approaching a convoy is picked up by the escorts, which would have a functional frontage of 30-50 miles to cover with half a dozen or so vessels.
• Only 20-25% of subs engaged by the forward-throwing weapons were killed.

Given these facts, I don't get how forward-throwing weapons can be argued to insulate the convoys. They'd increase Uboat losses relative to depth charges of course.

I dont disagree, a more conventional construction method would have been better but the numbers are then very limited from what I understand, hence the decision to mass produce modules.

Given existing shipyards, traditional construction practices would have allowed for 20 Type XXI's per month IIRC.

"being an earlier design it's probably fairest to assume its performance is a bit lower than the Type XXI as well

This isn't a valid assumption for anything but sonar quality. The critical thing about Type XXI ATL's is that the Type XXI wasn't new technology, it was a simple application of existing battery and engine technology at greater scale. As mentioned upthread, the Japanese built a submarine even faster than Type XXI nearly a decade before the first T21's cruise.

A 1943 Type XXIA with 1943 sonar would still be a formidable opponent. It would be relatively easy to modify it to 1944/45 Type XXI's B/C/D as sonar quality improved - just as with Types VII/IX.

The Brits reacted according to their perception of how the Battle of the Atlantic was going. Change the submarine type or performance & Brit perception & decisions change.

There's always a basic science limit on the scope of technological adaptation.
For example one really effective counter to Type XXI would be to plug all escorts into a GPS-driven global matrix linked to satellites using ground-penetrating radar to continuously monitor the exact worldwide location/speed/heading of all seaborne submarines.
Obviously there's a basic science constraint to the USN/RN's adoption of that approach.

IMO the only other way to reliably dominate the Type XXI is to have sonar that works on a very fast escort and on very fast homing torpedoes. There seems to be a basic science constraint on development of that technology as well, as the incentive for better sonar existed throughout the war and yet even 1945 sonar was largely useless on vessels exceeding 20 knots.

[JAG13]

Assuming a 1943 construction program for something like a Type XXI design (being an earlier design it's probably fairest to assume its performance is a bit lower than the Type XXI as well), Germany may not have felt the same sense of desperation late-war that caused it to attempt a modular design method. Perhaps it might be more accurate to extrapolate production numbers based on traditional assembly methods, taking into account both the greater size and complexity of these boats along with the inevitable teething problems and ramp-up time for a new design.

The numbers produced in this case will doubtless be lower, and as noted above they will probably be less potent than the later Type XXI, but more of that number will be fit for service.

There is nothing wrong with modular construction for submarines, that is how they are usually built today, the German problem was due to the conscription of inexperienced builders, companies that had no experience on submarine construction nor the ability to properly build the modules to spec, hence the problems.

A postal of the remains of the Argetinian sub shipyard experiment with the modules for their last two subs:



Had they thought of it earlier, they could have designed for and set up for modular construction, gradually ramping up production.

[Carl Schwamberger]

Any comparisons to the US submarine construction 1939-1945?

[TheMarcksPlan]

Modular construction was running fine by the end of the war. The bigger reason for failure to deploy is that training on the new tactics took a long time and, because the Baltic training ground were heavily mined by the Allies, it took even longer. Had the Germans thought of Type XXI earlier, the ~6 month delay to fix the modular issues would have been well worth the doubling of production in the long run. It would have taken more than six months to double the capacity of shipyards and more resources to boot.

[Russ3Z]

While it is certainly likely that Germany could have perfected modular construction earlier, the fact remains that such a system was new to the German shipbuilding industry, or at least to its submarine building industry (I don't know enough to say if other German shipbuilding efforts had utilized such techniques previously). The introduction of any new methodology is potentially prone to delays, errors, and other unforeseen difficulties, which I'm sure they were at least generally aware of. Hence, my comments are only meant to illustrate that an earlier time frame may have caused them to evaluate the potential risks and rewards differently than they did later in the war. They might indeed have made the same decision as in OTL, but it seems worthwhile to also examine a scenario where they do not.

[Takao]

Any comparisons to the US submarine construction 1939-1945?

Not really.

The US went with prefabrication of parts rather than modular construction(that would not come along until the nuclear boats). Portsmouth Navy Yard was the best, and did much of their prefabrication on site and could launch a submarine in about 5 months. The average time for others was about 8 months from keel laying to launch. With Cramp being the worst, having some of her boats made watertight and towed to Boston for completion.

The Germanns, IIRC, were building their modules at three different sites, then sending them to an assembly yard. This, of course, led to problems when the section were fitted together.

[Takao]

XXI modular construction did not have structural issues...the US postwar, collapsed U-2315 at 900 feet. Th British collapsed on of theirs at 800 feet, the number escapes me ATM, but this points to quality control issues, rather than defects in the design. Clay Blair, in his criticism of the design, seems to think that this advanced UBoat should have been able to dive much deeper than its predecessors.

The major defect in design, was the hydraulic piping, which the XXIs relied on instead off electric motors, for control. Some of this piping ran outside the pressure hull, and could be easier damaged by depth charges - nor could that damage be repaired submerged. This was done to speed the design process, with the hydraulic system being taken straight from the XVIII design. Also, it was overly complicated, which led to further problems.

The XXI design would have benefited from a longer gestation period to iron out defects, but the Germans were desperate.

[TheMarcksPlan]

my comments are only meant to illustrate that an earlier time frame may have caused them to evaluate the potential risks and rewards differently than they did later in the war. They might indeed have made the same decision as in OTL, but it seems worthwhile to also examine a scenario where they do not.

Absolutely. The prudent course for an earlier T21 would have been to use, say, half of the ways/docks for old-style construction from keel up and the other half for assembling modules. Then as you work out the predictable bugs in the system you shift to all-modular. That at least guarantees operational experience with the new type and live training of crews on it.

I was just making the point that even if Germany made the same choice earlier as it did OTL as a desperation shot, they'd still have produced a potentially war-winning weapon.

I'd also think T21 would have been supplemented by 1945 with further enhanced eBoats. Like a 3-4x larger one that can do 25 knots and therefore chase down fleet assets (30kn+ max speed for most fleet task forces but they dont steam at those speeds because only the capital ships have meaningful range at the highest speeds).