The smaller 400mm tubes were incorporated in training mounts, and were even carried in mixed calibre training mounts, with two outside 21in tubes and two inside 400mm tubes. The 400mm torpedoes were reportedly unsuccessful in service. Some submarines operating out of Britain during the Second World War had their mixed calibre (two 21in and two 400mm) tubes rearmed with three 21in tubes instead, but this was probably because of the difficulty of obtaining stocks of the 400mm torpedoes.
Another navy that designed external training tube mounts into its boats was the Royal Netherlands navy. Dutch boats continued to be built with them right up until the outbreak of the Second World War, but it is notable that when the Kriegsmarine took over and completed the unfinished O 27, they deleted the training tube mount. Dutch boats normally had covers which enclosed the opening on either side of the hull for the traversing mount, preserving the well-streamlined shape of their boats. In a similar manner to the French yards, when the Dutch built two submarines for Poland, the Sep and the Orzel, they included the training tubes.
U-BOAT TORPEDO TUBE SYSTEMS
German U-boats of the Great War did not usually display their torpedo tubes, even when out of water, but a photograph of U 110 in dry dock provides an opportunity to see the bow tubes. Having been rammed and sunk, then salvaged by the British, U 110 was undergoing repair with a view to being returned to service in order to examine her characteristics. The quite different upper and lower bow doors are shaped to fit the hull contours. Despite the damage to the bow which was the cause of U 110 sinking, the torpedo tube section appears intact. The Admiralty plans to study U-boats by re-conditioning U 110 came to an end with the Armistice, and she was unceremoniously scrapped. Surrendered U-boats in better condition such as the U 126 and U 141 were made available to the Royal Navy to dissect and analyse.
The torpedo tubes used on all the operational U-boats in the Second World War were similar in design to those found in the boats of most other navies, so the description here can be taken as representative of contemporary submarine tubes — apart from two specific German innovations.
The muzzle doors were opened by the rod seen in the photo, and the vertical rod on the right of the door opened the outer hull shutter at the same time. When the boats were submerged, the water pressure pushing against the outer door made it difficult, if not impossible, to open the muzzle door to launch a torpedo, so it was first necessary to flood the tube. This could be done by using water from outside the boat, which would tend to upset the trim, or else by a complicated method of pumping water from an internal tank, which did not have such a marked effect on trim. German U-boats did not have an automatic device for trimming the boat to compensate for the torpedo having been fired, so the retrimming had to be done manually.
The tubes were made of bronze, although late-war boats had tubes made of steel because of a shortage of copper, 55.36cm (21.8in) internal diameter and 7.552m (24ft 9in) long. They were built in three sections, bolted together, one section being inside the pressure hull and the other two outside. The tubes had retracting connections which entered the torpedo body and set the depth, speed and gyro angle. One major German innovation was that, from February 1943 onward, an additional device was added, to set the FAT torpedo, which from July 1944 also set the LUT. These connections were automatically withdrawn from the torpedo body when the firing rod was operated. Depth and speed were set manually, but the gyro angle and the settings for the FAT and LUT torpedoes were transmitted by servo motors. The gyro setting control could also be used to set each gyro for firing a spread.
A torpedo was manually loaded into its tube (although the late war Type XXI U-boats had power reloading), and pushed forward until it engaged the front and rear stop bolts. These positioned it correctly in the tube so that the depth and gyro angling connections could be inserted at the correct positions in the torpedo body.
It was too dangerous to start the torpedo engine inside the tube before launching it, because of the hot exhaust gases and the lack of flow of cooling water to the engine. It was started automatically as it was ejected. On activating the firing rod, the torpedo stop bolt was withdrawn and the trigger latch descended into the tube. Compressed air forced the torpedo forward at a speed of 10m/sec (33ft/sec) and the trigger latch started its engine.
Here the German engineers had added an innovative feature, the launch piston. On launching a torpedo, the compressed air would exit the tube and rise to the surface, threatening to give away the position of the boat. To avoid this, the Germans inserted a piston behind the loaded torpedo. The piston was made of steel, weighed 35kg (77lbs) and was machined to fit closely inside the tube. It had two projecting studs which fitted in guide slots inside the tube. These slots ran for almost the whole length of the tube, ending 90cm (2ft 11in) behind the muzzle door. At this point the outside of the tube was strengthened by external steel ribs, which enabled the bronze tube to stand the shock of the piston reaching the end of its travel.
The system had two operating methods. For submerged launches, the compressed air was injected behind the piston, and did not escape when the torpedo was ejected. The piston was arrested in its forward motion at the end of the guide slots, and the seawater pushed the piston back inside the tube, recompressing the expanded air, which was vented to a holding tank. When the muzzle door had been closed and the tube pumped dry, the piston could be removed ready for reloading the next torpedo.
When launching the torpedo on the surface, it was not necessary to effect this operation, and also the water pressure would be insufficient to push the piston back in place. So the piston was locked in position at the rear of the tube and the compressed air was injected in front of the piston to launch the torpedo.
This system worked well, but the precision sliding fit of the piston caused problems when the tubes were subject to pressure from exploding depth charges or aircraft bombs, and instructions were issued stressing the need to inspect the tubes after each heavy attack.