Hi Harvey,

There was a plate that was inserted behind the torpedo before the inner torpedo door was closed. If you look closely at the lower left torpedo tube in the photo at: [www.uboat.net]? you will see the plate in place. There is a bar across the plate. I am not sure what metal the plate was made of but it weighted about 77 pounds. It had two "ears" that protruded from it that fit into corresponding groves in the torpedo tube walls - one grove at about 2 o'clock and the other grove at about 8 o'clock for the two tubes on the right and at about 10 o’clock & 4 o’clock on the left hand tubes. The bar in the photo shows their location. These groves were tapered near the outer end of the tube such that the plate slowed down before it stopped (perhaps to keep the noise down when it reached the end of travel but this is just a guess). When the compressed air entered the torpedo tube it did so between the plate and the inner torpedo tube door. As it expanded it pushed the torpedo out of the tube at a rate of about 10 meters a second. The plate stopped when it got to the end of the groves thus expelling the torpedo and sealing the tube so that none of the compressed air escaped into the sea and therefore no bubbles which would identify the U-boat's location were released. Water pressure after the torpedo was expelled forced the plate back down the tube venting the compressed air into the boat in the process until the plate stopped at the inner torpedo tube door. It did raise the pressure in the boat. There were also other things inside a U-boat (compressors etc.) that raised the pressure inside the boat but I suspect that firing torpedoes was one of the worst. Sometimes when the conning tower hatch was opened after the boat had been submerged for a while the hatch almost flew up as the pressure outside the boat was so much less than the pressure inside the boat. I don’t know if the pressure differential in the boat ever actually caused any problems more serious than ear problems but under the right circumstances it could have. Opening the conning tower hatch is the only way I know that they equalized the pressure inside the boat.

The plate (I don’t know it’s proper name however it is sometimes referred to as a “piston”) had to be removed before the tube could be reloaded and replaced again before the new torpedo could be fired.

I don’t know how soon the outer torpedo tube door had to be closed but at a minimum it had to be closed so the water that forced the plate back could be drained and the tube reloaded. An open outer tube door would have added some drag to the boat too so I assume they closed it as soon as possible.

I am not sure when the torpedo propellers started running normally however I have read of “hot runs” in the torpedo tubes of a U-boat before. In one case a “mine ejector” was used to eject the already running torpedo however I have no idea what a mine ejector looked like or how it worked.

The G7e torpedoes (electric) had counter-rotating propellers. I know that the early G7a torpedoes (used compressed air and a chemical to form steam which turned the torpedo's props and left a bubble trail in the process) didn’t have counter-rotating propellers (and there are many photos of them around) but am not sure about the later ones.

Hope this helps.

Regards,

Ken Dunn