[SandyCaesar]

I've heard of destroyers built with specially reinforced bows just for the purpose. Destroyers may be small ships, but they're still two and a half times the tonnage of your U-Boat, and plus they have way more reserve buoyancy. What would send a destroyer to the dockside could easily keep a submarine on the surface or beneath it permanently. On the surface, it was sometimes more feasible than gunfire (you know how small your silhouette is!) to just run a U-Boat over. Especially at night, when visual detection range might have been only a few hundred meters.