Most photos of torpedos loaded on U-Boats show grease as they are loaded into the boat and more grease as they go into the tubes. The grooves in the tubes are to keep the torpedos from spinning (like a football or artillery shell) that would cause the gyros to topple. THe grooves also guide the piston pushing the torpedo out of the tube and stop the piston from leaving the boat and taking with it a large air bubble that would give away the position of the boat. The piston stops because the grooves do not go all the way to the end of the torpedo tube. The torpedoes need the grease because they are supported by the tube and they weigh a ton or more and they have to be removed from the tubes every four days or so for maintenance