There would probably be a valve inside the pressure hull. A simple cap with a seal would probably be sufficient to keep water out of the tube under most circumstances, as the deeper the boat went the more pressure would be exerted on the cap, holding it closed. You wouldn\'t need that much thickness of metal for the tube itself, since the reduced diameter would make it stronger. (A 30mm tube with walls 2mm thick will probably tolerate about as much pressure as a 10 metre tube with 2cm walls. An engineer could work out the exact relationship. A thinner cylinder is inherently stronger than a thicker one, since there is less surface area for the pressure to act against.)

The principle advantage of a voice pipe is, of course, that it still works if the power is cut off. The photo of U-505\'s control room on p. 43 of the U-Boat Commander\'s Handbook appears to show several voice pipes in the upper left corner. Can\'t really tell where they go, of course.

J.T. McDaniel