I think trying one hit kills is a bad idea if you can't use the deck gun afterwards.
1. The chance that you actually hit the target is smaller, because:
a. you may have miscalculated the target course or speed causing the torpedo to be off target.
b. in case of magnetic torpedoes there is a chance that it will explode prematurely
2. If you hit the target it will become very hard for you to hit it again with a torpedo, because the target will:
a. change speed (either loose speed because of the damage or speed up to get away from you)
b. start zizagging.
3. You never know if it's going to sink from that first torpedo. This takes time in which enemy aircraft or warships may be on the way.
My advice if you can't use the deck gun (properly): Always fire two torpedoes at a small ship and three at a big ship.
The first torpedo at the bow, and the last torpedoe at the stern, or vice versa. Simple and effective.
