Dude it'a a dud

During the first year and a half of the America's involvment in the war - a large percent of the skippers of U-boats just sat in their subs under the water fleeing from engagment with the Japanese. They were weeded out slowly. They were mainly officers who had joined the navy in the 20s who had wanted a career in a peace time navy - attending white uniform officer parties and seeking for peace time methods for advancment.

SO screw the torpedoes - if you want a real realistic dud, just cower in your chair and take your sub down deep and find excuses why it would be a bad idea to engage the enemy.