Just to post the details of the 'private' design study with the closed cycle deisel
powerplant:

The design was developed by Deschimag in early 1943 as a direct competitor to the 'Walter' boats.The hull was along cylinder with tapering stem and stern,a bit like a torpedo,but with a 'finer' bow.The bow looked like the bow of a modern US sub,only a bit more tapered.The design was powered by 2 E-Boat diesel engines,each giving 1500 shp and a 50 shp electric motor for running on batteries.THe diesels were supplied with gaseous oxygen stored at 400 atmospheres in 10 pressure bottles.There were 2 torpedo tubes with 5 reloads.
The sub was 37 meters long and had a beam of 4 meters.Displacement was approx 350 tons and maximum speed on the closed-cycle plant was 22.5 knots.
What is interesting is that this design proved very attractive,with the whole diesel closed-cycle program receiving the highest 'SS' priority in May 1943 by the Reichs Minister for Armament and War Production.The reason for this was that it was seen as an easier technical option to high underwater speed than the 'Walter' process and that the other sub yards (i.e not Bolm und Voss) who were not involved in the 'Walter' development felt that they needed a competitor to the 'Walter' process.The advantages of the gaseous oxygen closed-cycle were numerous:

1. No special fuel and turbines needed...oxidant was commonly available gaseous oxygenn and ordinary diesels could be converted (though the Germans did concentrate on developing closed-cycle versions of their E-Boat diesels because they offered higher power densities than the convential sub diesels.)

2. Cheaper.Oxygen was several times cheaper than H2O2.

3. Availability.The German Navy reckoned that if they brought the 'big' Walter boats of the type XVIII into service they would need to massively increase H2O2 production,probably needing 20,000 tons a month of H2O2.While the plans were in place for several new H2O2 plants none were eventually built.


Yet despite this the and nunmerous design studies no German diesel close-cycle sub saw service duringthe war.One of the 'Walter' boats was converted to closed-cycle propulsion the so called type XVIIk U798,but she was lying un-completed at GermaniaWerft at the wars end.

The American after the war investigated this technology calling it 'Kreislauft' but did not adopt it.The Russians on the other hand adopted this technology and produced a series of coastal subs,the 'Quebec' class.Though sucessful,with graeter underwater endurance than any contempory convential submarine (the Russian design emphasised underwater endurance rather than high speed),the design gained something of a evil reputation among Russian sailors.They nicknamed the boats 'cigarette lighters' for the frequent fires they suffered in service.

Rgrds

Sniper.