I read that the Germans built two thousand mini subs training young men almost boys to crew them. Launched 600 to devastate English shipping. Only sinking one merchantman and damaged two and only a small handful returning. Seems a high price for one ship. Always makes me wonder if the channel floor and the southern coast is littered with hundreds of derelict subs. Were they actually that ineffective. The italians had some success. The British tried to use minis against the Tripitz but.failed. Japan built a bunch but I don't think they did well.