Interesting. I can confirm nomad findings, and yes, I also had prematures (until 1943 or something) even if set to contact only.
Pepsi is right too. I´ve read Submarine! (E. Beach), mk14 were doomed for the first years of war. Skippers were usually (and sadly) blamed for "missing" their targets (bad solution, incorrect settings, and so on...).Even the japanese high command knew it

.
Some captains, as Mush Morton, ordered the supply and tech guys to double check every torp before loading them to his sub, after apparently "missing" around 10 of 14 shots.
So, as other skippers did, Im sticking to old but reliables mk10s for now.