[Cptn_Enth]

I would also think that alot would have to do with the angle of impact between the two ships, and at what point the prow of the destroyer makes contact with the hull of the U-Boat. I would think that if a DD catches a U-Boat on the surface and rams her dead center, such that the prow of the DD and the Conning tower collide first, I would think that the U-Boat would either be split in half/rolled along the Longitudinal axis of the sub like a barrel, causing all manner of damage to men and arms/ordnance/equipment. The destroyer, on the other hand would probably go up and over the sub, probably causing the boat to broach along at least a good part of her hull, and depending on how much buoyancy the sub had left to resist against the Destroyer's displacement, might end up compromising her keel and ripping the bottom out of her/splitting her in half.

If the sub were rammed in the bow or stern, I would think that the sub would be pushed along and out of the way, with the destroyer either deflecting off but still sustaining damage...but it would depend on how far far forward/aft of the conning tower she was hit. Kind of the same principal as in an auto accident, where differing degrees of energy transferrence between the colliding vehicles would depend on point of impact, speed, vectors at time of collision, etc...

This is just me thinking out loud...feel free to correct me if I am wrong...