Hi Simon,
Outstanding. I had to use a magnifying glass to see all the little details. You have even got the rivets at the top of the Askania, I didn't even think about rivets being there. I think you are going to 3D print your U 1308 as an actual working boat. I hate to think how many objects your have or how long they take to render.
The deck railing could be black on U 1308. Usually the deck railings (and the railings on the tower) were the same grey as the upper colour. But there were exceptions. U 371, for example, did have black railings. So maybe U 1308 had black railings too.
Cheers,
Dougie