You never see this when watching films, or even when looking at old war time photos, but when a torpedo is loaded, a large piston appears to be placed behind the torpedo. This I assume, is then forced down the tube by compressed air, pushing the torpedo out.

This obviously happens after the tube is flooded. The clever bit being, that no compressed air escapes to reveal the subs position, as the piston stops just short of the end of the tube.

Anybody got more info on this? By the way, a good example of this piston can be seen on the U534 recovery photos on Uboat.net

http://uboat.net/gallery/index.html?gallery=U534E&im g=12

Hello !

I really don't know much about the operation of this. I will ask my father on this one. I do know that because of the speed of the piston and the narrow diameter of the tube. The tropedos had to be greased up insure their sliding thru the tubes with no troubles of binding or being held up in the tubes.  The tropedos did leave a trail of bubbles as they went thru the water however.  So a U-Boot could not afford to just sit in one location and fire it's tropedos.  You recall during the surface firing in the movie Das Boot, that they were moving.  They only set still when they knew they had a target that was "safe" to hit.