I did a little research in the Walther boats and there where a total of 4 versions produced starting with the V80, only one was built and used for research. The first versions of the type XVIII where the XVIIA and XVIIB as more research vessels. A total of 7 of these where made, 4 of the XVIIA and 3 of the XVIIB, but they where only commissioned. The last was the XVIII and where never built. Had they been built they would have greatly changed the U-Boat war in Germanys favor. If more funding and more research had been made the sciencetists would have found a better fuel than H2O2. Check it out here
http://www.uboat.net/types/walter.htm .