Sorry to post so often so close together, but I'd been saving them up until I reached the end of the thread in case somebody else came up with the answer I was going to give.
Another question had been asked about the "grapnels", the long poles attached at bow and stern of U-boats.
At least on the water and in marine supply stores, these "grapnels" look like what we call 'boat hooks'. Most modern ones are made of lightweight aluminum tubing, semi-sealed so that if they fall in the water, they'll float for about 5-10 minutes, or enough time to hopefully retrieve them.
Older ones, (and by older I mean 30+ years) were made of wood for the same reason. They wouldn't have been made of aluminum during the war because the aircraft industry needed all the aluminum it could get, so it wouldn't have been wasted on U-boats when wood would do.
This seems to be confirmed by the length of the boathook/grapne;s on the U-boats. Modern ones telescope down to about 1-2 meters in length but stretch out to as long as you can handle. Since wooden ones can't telescope, then they'd be made as long as possible right from the start.
Boathooks of course have all sorts of uses, but commonly are used to catch things floating on the surface (U-boats used to check garbage to see if they could learn anything about the ships they were hunting), catching and holding small boats that would come alongside (like inflatable rubber dinghys used to land operatives on shore), or men who had fallen overboard.
Modern boathooks have a straight point with a blunted tip and a curved, blunted hook underneath but I've seen variations on this in drawings of older boathooks, some with semicircles for the tips, some with Y-shaped ends, etc.