Nice, I am trying to fit torpedoes into my sub as well, you can check the thread I've started in the submarine forum (Pen Tube Torpedoes? Possible!). I doubt you'll be able to sink a ship with a model torpedo, you can develop a detonating system that will allow it to explode under the ship's hull, it might cause damage to thinner hulled ships. You can always try ramming, the u-boat has a nice net cutter in the bow, or you can try Captain Nemo's method, the Nautilus!