I did/am currently doing the same thing with both surface forces and U boats. At least half of the capital ship kills were from sudden hull breach syndrome, curtesy of the Uboot arm. They also accounted for most of the merchant tonnage. I find 4 type IX uboots to be devastating hit and run weapons. I do wish the Royal Navy would escort their ships better but that might be because I sank most of the Destroyers and light cruisers.

Edit Forgot to mention this but the Ubooten sank the HMS Nelson as well. I'd love to be a fly on the wall of 10 Downing Street right now...