Uboot type XVIII: thoughts on its underwater endurance

[msxyz]

Warning: long and boring technical discussion ahead !

Subject: Typ-XVIII uboot underwater range while running on batteries

Recently, I picked up off the bargain bin SH4 and its expansion. While I'll leave outside this topic the many issues I've found with this game, I'll concentrate on an historical aspect which, ironically, has been modelled correctly but that -for me- is a matter of controversy. Another little warning is necessary, though: keep in mind that no actual typ-XVIII uboot has been ever built. Some were laid down but construction was halted to free resources for the "Elektroboot" crash program. The unfinished hulks were either broken up or reused to build type XXI submarines. For this reason, everything that follows should be considered as pure speulation although based on historic data and research.

After I noticed that in SH4 the underwater range of type XVIII seemed too short, I decided to look into the matter. To my surprise, though, some ww2 submarine authorities such as E. Rossler and E. Bagnasco credited the typ-XVIII uboot as being able to travel submerged for 45Nm at a speed of 4kts using its batteries. This is, indeed, consistent with what has been portrayed in SH4 ! Not satisfied with the answer, I decided to investigate a little further. Rossler's book "Die grosse Walter Uboote" not only has a nice layout plan of a typ-XVIII submarine (a copy of a document made by Deutsche Werke Kiel in 1943) but it also contains detailed tecnical data.

Typ-XVIII was supposed to rely on conventional electric motors and an innovative closed cycle steam turbine while running underwater. The first were used for cruise while the latter were used in short burst to obtain high speed and place the boat in a favourable attack position against protected convoys. For this purpose, the boat carried 204t of high concentration hydorgen peroxide (a cheap, liquid monopropellant which releases energy and oxygen - Walter engines actually use the heat and the oxygen of the reaction to burn another combustible and increase the energy output) and a conventional array of batteries.

Rossler's book gives a further information regarding the battery array: it should have been composed of 124 cells of the type "33MAL800W". Ring any bell? This cell was used in typ-VIIC boats. The submarine layout plan in the same book shows two large battery rooms under the forward crew compartment, arranged in the same manner as in typ-IX boats. So, thers's no doubt that the german engineers gave this submarine a decent battery. After reading this, the reported figure of 45Nm @ 4kts seemed to me more and more suspicious!

Let's do some simple math. While we don't know for sure how typ-XVIII submarine would have performed (projections were based on scale models tested in tunnels) we've actual data from the near identical (in size and shape) typ-XXI uboot. Let's taker a look at the underwater endurance of a typ-XXI uboot:

• 420Nm at 4Kts
• 340Nm at 5Kts
• 280Nm at 6kts

Typ XXI uboot carried 372 lead-acid cells of the type "44MAL740". Energy stored per cell was 11300Ah. Thus the uboat battery was able to supply a total of 33900Ah with a typical voltage of 240 volt. (each lead acid cell has a voltage of 2.1-1.9 volt)

If Rossler's data about Typ-XVIII uboot battery is correct, this submarine should have 124 lead acid cells of the type "33 MAL 800", just like a type VII submarine. Energy stored per cell is 9100Ah, thus the complete battery array could supply 9100Ah at 240 volt.

If typ-XXI and typ-XVIII are near identical in every aspect, then it should take the same amount of power to move them! Since we know the energy stored in the battery array of each boat, calculating the underwater range of typ-XVIII from typ-XXI data is easy. This is the result:

Speculated "real" underwater range of typ-XVIII uboot while running on batteries:

• 112Nm at 4Kts
• 90Nm at 5Kts
• 75Nm at 6Kts

It's noteworthy to mention at this point that a typ-VIIC submarine, using the same array of 124 cells, could travel underwater for 80Nm at 4Kts. Typ-VIIC submerisbles were much smaller but had a higher drag due to their shape which favoured high speed and good traversal stability while surfaced.

One last question remains to be given an answer: "where does the credited figure of '45Nm at 4kts' comes from?" I can only speculate:

* It's a typo: both Bagnasco and Rossler might have read some post war documents which reported an incorrect figure. With no submarine of this type ever built, this data cannot be verified. It's interesting to point out that, if we substitute 4Kts with 7kts (the actual maximum projected speed of a typ-XVIII boat while using its electric motors) an endurance of 45Nm is a very likely figure!

* Rossler is wrong about 33 MAL 800 batteries being used aboard typ XVIII. All my speculation is based on THAT aspect. If the submarine used, for example, a smaller battery type such as 26 MAL 570 (used in the experimental Walter submarines built up to that date), then 45Nm are a credible figure. Note that the batteries depicted in the 1943 layout plan found in Rossler's book are, however, too big to be some kind of low capacity battery used on coastal or midget submarines. I consider this explanation less likely.

That's all. Apologies for the lenght of the post. I dunno if it has been covered before. I've been on an almost two years long shore leave from Subsim.

[Frame57]

You can get the info you are asking about from Horst Bredow of the U-Boat archive in Cuxhaven-Altenbruch. I think he has a lot of technical info on these boats.

[msxyz]

Thanks for the tip. My library on the subject is quite vast but it seems that most of the authors took the most "exotic" informations about u-boote from the same source. Hence why errors spreads.

Accessing 1:1 copies of original or early postware documents on the subject would be ideal; a shame that most of them probably lie neglected in some US Navy old storage facility along with all the "prizes" taken after the war. Maybe they've even been already destroyed.

[Frame57]

Yeah, i do know what you mean about that. I has a lot of misinfo. especially on the Walther boats. I am reading a book now about the Golf class Soviet boats and it seems they took the technology of the XXI and expanded it when they produced the Golfs and Foxtrots. Very similar stuff.