A quarter of a century ago (Omigawd, izzit really that long?) I was peripherally involved, more or less on a hobby basis, in the rectification of a misidentification of the identity of a North Atlantic wreck which was known/suspected to have been twatted by a u-boat.
The misidentification had arisen from a (then) modern misunderstanding of the German u-boat grid system.
I spent many hours/days trying, ultimately successfully, to identify exactly where that particular attack had been recorded as having taken place. It was a bear of a job which I could not have surmounted without the very able assistance of Prof/Dr Jürgen Rohwer who is (I hope he's still alive and still as sharp as a tack) a serious expert on all things u-boat.
I'd like to add to this thread a website/tool which I'd love to have existed when I did those geodetic gymnastics in the early 1990s.
It's a lovely converter for finding the Lat/Longs of the u-boat (and I think most of the rest of the German Navy) grid system position.
Naval Grid Calculator