Torpedo Firing

[PaulH513]

I am trying to understand the sequence of what actually starts the props on the torp once it is ejected from the tubes.

I see in this diagram http://www.maritime.org/torpfire.htm that compressed air forces the torp out of the tubes and wonder if it is the forward motion of the torp that somehow begins its propulsion.

My thought is that there is some sort of mercury switch but I can not find anything online that shows or describes what starts the propulsion system.

Thanks in advance for any info.