I haven't played this game in nearly 35 years, however the other torpedos were Aft torpedos, the bit you were missing is you can fire them, but you have to turn the periscope to be facing the rear using the handles of the periscope, so facing away from the convoy using your arrows to position the sub, but then turning the periscope 180 degrees thus ensuring your aft torpedos can be fired. FYI - WGSC - Worlds Greatest Submarine Captain!
I used to be able to get WGSC rank every time using tactics that were not very close to those used in the actual war. I would sink the escorts with my torpedos then surface and sink all the merchantmen with my deck guns.
I don't think I ever got higher than maybe Lieutenant in all the times I played it as a kid. Considering how few torpedoes you have, it makes sense to be as selective about using them as possible.
