Fact of the matter is that the sweepers draft is shallower than the torpedoes can be set for.
Even at the shallowest setting possible the torps will still slip beneath them.
Evasion is the best tactic in this case, however in your case it may be too late. Still if you can get some distance at some point surface and hit flank. I believe you can out run them. Just keep a skinny profile to make yourself hard to hit.
Good luck
