Every ship has some amount of armored plating set to a certain depth below the waterline. Some more than others both in thickness and depth. Your fish with impact fuses need to be below that armored jacket to be truly effective. The SH4 user manual comes with all that information, and how many evenly spaced torps it should take to sink various types of ships. It is effective and usually works to specs if they're deep enough and you've got enough hits according to tonnage and type. One thing I don't agree with is a 4"/50 Cal. Deck gun taking 90 seconds to reload
specifications put the test fire rate at 8-9 shells per minute, or a shot every 5-6 seconds or so. I can understand bad weather increasing that time somewhat, but if it's calm enough to man the guns, then it's calm enough to keep up with those fire rates. Sure I can see ships taking multiple hits from AP shells at the waterline and still staying above water, but it shouldn't take no 100 shells to put down a fishing trawler
IMO there is some tweaking that needs to be done with these issues in RFB 1.4