If it was a torpedo from a sub then your method would work just by powering the torpedo out of the tube, but in this case its a 'fast launch' which the torpedo is thrown from the deck, which means my method would do the job of launching it of the deck, powering the torp up I'm leaving to the builder to figure out, (bit of self help goes a long way) although i can't really see a point in making the torpedo's work, you will most likely loose most you fire, which is a costly feature, unless its being used for display purposes it becomes a redundant feature.
There are to many things to consider on a torpedo, planes to help it plane right in or above the water, and if there fixed then you have issues at scale with a poor launch can result in just making a splash and the item hitting the lake bed with some force, which is not the effect you were going with, that's just one of the list of things that you have to over come.
Trial and error through bench testing it best before fitting to the model. I have my doubts if you can achieve working launchers but i would like to be proven wrong, so crack on boy!