Just entering Jan 43 so FaTs are now available.
Are they any use?
For me the impact on ships is a little all or nothing (non modded) with torpedo hits. Either it blows up and sinks (too often), or gets hit and lists a little but doesn't drop in speed (too little effect). Doesn't seem to be enough non explosions but with crippled slow ships.
Only hits to the engine (which usually makes it blow up anyway) or hits to the props (rare)that cause the drastic slowdowns which enable you to pick it off later as it falls behind the convoy. In a random FaT hit I would expect these hits to be rare.
So I'm thinking a FaT probably a waste as if it does get a ship during it's ladder pattern.. it's most likely that ship carries merrily along and no credit to you.