@Couger
Even the Walther does not have unlimited fuel
What the Walther turbine is/was, was an air independent propulsion system quite like the Sterling engine and the fuel cells of today only it burned hydroperoxid instead of hydrogen. The advantage was a) an enormous effect which gave a lot of power and b) extended submerged range. Once the hydrogenperoxid was burnt, it was over and the sub would be a plain old diesel electric boat again.
What you can do, I'm not sure, but I think it will work, is to open the submarine files with S3D and go to the section called hydrogenperoxid propulsion and then increase the range value there