OK found something
Quote:
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It takes great care to make torpedoes that will explode only when intended. One safety device the Mark 6 exploder relied on was a small propeller on the underside of the warhead that spun as it traveled through the water. After 450 yards, when the torpedo had leveled off at its predetermined depth and on its set course, the spinning propeller would move the detonator into the booster cavity, arming the torpedo.
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(450 yards = 411.48 meters)
Sounds like a lot maybe...but.
Source:
http://www.americanheritage.com/arti...998_4_56.shtml