In December 1941, the US submarine skippers were unsure of what they would encounter. Their flotilla commanders had told them that the Japanese might have unknown ASW weapons never before encountered. In fact, one of the first suppositions of the dud torpedo rate was that the Japanese had invented a device that rendered the magnetic exploders useless...
- Another problem was the US pre-war attack doctrine, where a sub attacked via sonar at approximately a 100 foot depth... misses were rather common.
The IJN also did not have a concerted ASW doctrine, leaving it to local authorities to cover (or not) ships as they left. Convoying was not required. Early in the war, there was basically minimal escort protection for anything other than troop transports and task forces. Also, we might have to look at the dud rate further if you didn't have enough duds that five of the seven you encountered didn't get away with a dud or two sticking in their hulls...
