damage by ramming?

[flakmonkey]

Something wierd happened on my last patrol, i was submerged at periscope depth while in the middle of a convoy, i managed to get rammed by a large merchant (c3 in the stock game) and despite a nasty metallic grinding sound and madly swinging sausages i suffered no damage whatsoever.
An impact like that would have cut a uboat in half, what gives? ive been rammed by destroyers before and that caused massive damage so why not this time??