I only use low speed torps when the weather is bad or at night. I'll trail my target till night if necessary. In clear weather they'll see the slow fish most every time. You just gotta attack on the best terms available even if it means spending a lot of time setting up a good shot.
If you do have to shoot in clear weather and low speed try putting one fish in front of the bow and at least you will than know where she is going to go and shoot the next torps on that information. She will slow and turn and you can figure out where to shoot to get hits. Usually one in front and two or three at the stern. Gonna waste a few fish but that's better than wasting a lot of fish.
Magic