Yes they do unfortunately, unless there's a way to stop it doing that? Besides, the ships I was sinking only needed a 300lb bomb to sink, so therefore I could sink 3 in one sortie instead of just one with a torpedo. With the bigger ships (destroyers+), torpedoes are probably more effective.
I did a little test of sinking different ships, and 2 torpedoes seems to be able to sink anything (not sure about carriers). It also seems to matter where abouts on the ships they hit. For example, 2 right in the middle of a BS sunk it, 1 in the middle and the other one just hitting the stern did not.