Milchkühe, why so small?

[Gudestein]

Guess this is more of an amateurs technical question on the atlantic part of the WW2, so hope its in the right forum.

The germans had anticipated to fight cruiser wars against french and later British ships from 1938, and had seen the ability to refuel their raider at see as a critical issue. They must have known they would be vulnerable.
What I am wondering is, do we see any technical reasons why submarines could not have been massively enlarged as fuel carrying vessels. The fuels could have been carried outside the pressure hull. Sure, it would be clumpsy, but its defense would be concealment.
There was a purpose build merchant sub (Deutschland) of about 1500 tons that could carry a cargo of 700 tons of solid materials. I guess it could be 50/50 or more in favor of fuel load.
WHat was build was a scaled up type IX submarine with a 25% of total weight as its load for refuelling type IIV's.

Why wasn't a big clumpsy tanker not not seen as the ideal solution to keep Bismarck and Tirpitz in the Atlantic?

[Aktungbby]

Gudestein! https://en.wikipedia.org/wiki/German_tanker_Altmark!

[Betonov]

If I throw my amatuer 2 cents in:

the fuel needed for such large battleships would far outweigh the capactity for a sub, I wouldn't want to command or serve on a submerged fuel drum that is being dept charged by the RN and I'm havng doubts how a refuel would go if a RAF plane squadron spots them.

[Oberon]

I imagine dive speed would be a big problem and you'd need an impractically big pair of engines to drive such a large vessel at any decent speed, and the underwater speed would be pretty bad too since this is the era before tear-drop hull shapes.

A design like the I-400s might work, but you'd need to build a lot of them and that would take precious German manufacturing away from other projects. Remember that Doenitz wanted 300 uboats (100 on patrol in combat areas, 100 in dock, 100 in transit), but by wars start he only had 26 at sea and didn't get 100 at sea until 1942.

[GoldenRivet]

for larger scale operations in the Atlantic, the kriegsmarine needed Atlantic bases ultimately... not just supply ships and subs. as was pointed out earlier... even very large supply subs just wouldnt hold enough fuel to support surface ops.

France was a start, gaining ports for their warships and u-boats in the Biscay bay.

I think the running and gunning blitz the Germans put forth that captured large territories in a short time gave them a bit of overconfidence.

I seriously think that German high command felt that Britain would have been forced out of the war in somewhat short order, as happens in military circumstances and operations - the enemy, and the whole strategic picture really - were underestimated. Had they not overestimated their enemy, Germany may have sought to secure the Azores, or form an alliance of sorts with Iceland as part of their long term operational strategy, but this didnt happen

this underestimation forced the Germans into a bit of a constant "catch up" game in the Atlantic. The allies, for the most part, stayed a step ahead of the Germans in that particular theater of operations. not only in production of the sheer number of merchant craft, but ultimately in technology and tactics as well

they obviously never recovered the upper hand

[Gudestein]

Originally Posted by GoldenRivet

for larger scale operations in the Atlantic, the kriegsmarine needed Atlantic bases ultimately... not just supply ships and subs. as was pointed out earlier... even very large supply subs just wouldnt hold enough fuel to support surface ops.

France was a start, gaining ports for their warships and u-boats in the Biscay bay.

I think the running and gunning blitz the Germans put forth that captured large territories in a short time gave them a bit of overconfidence.

I seriously think that German high command felt that Britain would have been forced out of the war in somewhat short order, as happens in military circumstances and operations - the enemy, and the whole strategic picture really - were underestimated. Had they not overestimated their enemy, Germany may have sought to secure the Azores, or form an alliance of sorts with Iceland as part of their long term operational strategy, but this didnt happen

this underestimation forced the Germans into a bit of a constant "catch up" game in the Atlantic. The allies, for the most part, stayed a step ahead of the Germans in that particular theater of operations. not only in production of the sheer number of merchant craft, but ultimately in technology and tactics as well

they obviously never recovered the upper hand

I think I'll agree to this and it might explain why there does not seem to be even a design plan available. However, does anyone see the technical problems as insurmountable/difficult. That is provided we accept a low speed of course.

[Commander Wallace]

Originally Posted by Gudestein

Guess this is more of an amateurs technical question on the atlantic part of the WW2, so hope its in the right forum.

The germans had anticipated to fight cruiser wars against french and later British ships from 1938, and had seen the ability to refuel their raider at see as a critical issue. They must have known they would be vulnerable.
What I am wondering is, do we see any technical reasons why submarines could not have been massively enlarged as fuel carrying vessels. The fuels could have been carried outside the pressure hull. Sure, it would be clumpsy, but its defense would be concealment.
There was a purpose build merchant sub (Deutschland) of about 1500 tons that could carry a cargo of 700 tons of solid materials. I guess it could be 50/50 or more in favor of fuel load.
WHat was build was a scaled up type IX submarine with a 25% of total weight as its load for refuelling type IIV's.

Why wasn't a big clumpsy tanker not not seen as the ideal solution to keep Bismarck and Tirpitz in the Atlantic?

Welcome to Subsim.

There were resupply subs otherwise known as milk cows

https://en.wikipedia.org/wiki/German_Type_XIV_submarine

These resupply subs as you already guessed were not able to carry all that much but they helped. Being poorly armed, they were vigorously hunted . There wasn't a lot of them built either from what I read.

[August]

I thought the German raiders like Atlantis were coal burners? However even if they did run on oil I question whether a U-boat, even a big one could carry enough fuel to fill her bunkers.

[CCIP]

Don't forget that the Germans had quite a network of surface tankers and supply ships out there as well. They were gradually hunted down - as were the milk cows. A look at the real "small" milk cows' record reveals the problem: all of them were sunk, half of them on the first patrol. Basically they were more or less doomed from the start - especially since the Allies knew about them and targeted them specifically.

[Gudestein]

Originally Posted by CCIP

Don't forget that the Germans had quite a network of surface tankers and supply ships out there as well. They were gradually hunted down - as were the milk cows. A look at the real "small" milk cows' record reveals the problem: all of them were sunk, half of them on the first patrol. Basically they were more or less doomed from the start - especially since the Allies knew about them and targeted them specifically.

Its the hunting down of the tankers I see as a predictable problem. The milch cows were hunted down as well, but only because of Ultra. That could not have influenced German decisions and it does not address if there were technical problems otherwise.
I do agree they would not have solved much - given Ultra actually being a fact.

[Gudestein]

Originally Posted by August

I thought the German raiders like Atlantis were coal burners? However even if they did run on oil I question whether a U-boat, even a big one could carry enough fuel to fill her bunkers.

They were in WW1, I am referring to WW2, but used a WW1 example where a transport sub was designed to carry +50% of its own weight as cargo.

[GoldenRivet]

Given the fact that the massive typhoon class submarine is little more than twice the length of a German type IXB u-boat... It would not be and would not have been *technically impossible* to build such a Massive supply sub which could have fueled large surface ships etc.

From a strategic standpoint, given the nature of warfare during the battle of the Atlantic as it were in WW2, such a huge supply submarine would have been likely viewed as impractical.

The German failure in the battle of the Atlantic was not in lack of front line supplies alone. Even if the Germans had such a U-boat I don't think it would've made any difference

[ikalugin]

Typhoon had both the proper shape hull and a nuclear power plant. For a forties sub you are looking into some serious issues with electric motors even if someone does get a smart idea to make the sub look like a torpedo and introduces shnorkel early.

[Torplexed]

Originally Posted by Gudestein

Why wasn't a big clumpsy tanker not not seen as the ideal solution to keep Bismarck and Tirpitz in the Atlantic?

It's always been my understanding that surface raiding was never the intended role of the Bismarck or the Tirpitz. But given the cancellation of "Plan Z", it was the only offensive purpose they were left with.

[Gudestein]

Originally Posted by Torplexed

It's always been my understanding that surface raiding was never the intended role of the Bismarck or the Tirpitz. But given the cancellation of "Plan Z", it was the only offensive purpose they were left with.

OK, here you might be nailing the real issue.
With the other replies, I guess it explains why there was no need for such plans.
I am still left with the technical issue: Would it be such a big problem to add a lot of liquid filled volume (hence pressure resistant) to a scaled up sub design?