The breech door was locked shut by a revolving ring on the rear end of the tube. Cut-outs on the ring matched the segments on the hinged door, and when this was closed, the segments on the ring revolved behind those on the door. This was a quick-acting arrangement, but the door could not stand extreme pressures, which posed a threat if the external shutter and muzzle door had been damaged by depth-charge explosions. Also, it meant that torpedoes could not be launched when the boat had gone deep. The Type XXI was redesigned to overcome these problems, but none fired a torpedo in anger before the end of the war.
An interlock prevented opening the breech door if the outer muzzle door was open. Before the breech door could be opened, it was also necessary to recock the firing rod in order to open the breech door-locking ring. This retracted the engine trigger latch, so a new torpedo could be inserted. Another interlock prevented the firing mechanism from being activated if the muzzle door was closed, and a third device prevented opening of the muzzle door when the tube was being drained.
With the introduction of the electric Type G7e torpedo, it was necessary to preheat the electric storage batteries in the torpedo to 30 °C shortly before launch, in order to obtain their maximum rated performance. The cells in the batteries contained thermostatically controlled heater elements, and to activate them an electrical plug had to be inserted through a hole in the torpedo tube to connect with a socket in the top of the torpedo body. This ad hoc arrangement did not lend itself to mechanical interlocks, so none were fitted: the plug had to be manually removed and the hole sealed with its waterproof cover.
The small coastal Type XIII U-boats which entered service towards the end of the Second World War were too small to carry reload torpedoes. They were armed with only two torpedoes, which had to be muzzle-loaded from the outside when in harbour, since the torpedo room was too short to allow the torpedoes to be fully withdrawn rearwards. They could only be partially withdrawn into the torpedo room if it was necessary to carry out repairs or maintenance.
US SUBMARINE TORPEDO TUBES
The earliest Holland-type boats, also used by the Royal Navy, had single bow torpedo tubes, closed at the muzzle end by a vertically hinged bow cap. A single reload torpedo could be carried inside the crew compartment, which severely limited the useable space in these small craft.
As in other navies, the torpedo batteries of US submarines steadily increased until the Fleet boats of the Second World War era reached a total of ten tubes, six forward and four aft. The following description relates to US submarine practice of this period, with all the photos taken from the official manual of 1944.
The late-war tube was designed to take the Mark 14 torpedo with the shallow flat guide stud. Earlier torpedoes with the high T-shaped guide stud could be launched after changing their guide stud for the later pattern. The internal view of the tube shows this rectangular guide, and also the recesses in the bottom of the tube for four bronze rollers on which the torpedo sits, to aid in loading. Because of electrolytic corrosion these were changed to nylon rollers later in the Second World War. The various holes in the tube are for the retractable pinions for depth, speed and gyro setting, the stop bolt and engine start latch, the compressed air inlet and the water drain outlet.
The breech end door was machined from solid bronze, slightly concave, with eight studs around its circumference. To lock the door shut, the external ring, which was screwed on to the breech end of the tube, was rotated so its segments turned behind the lugs on the door and locked it. This was the opposite of the system used on US above-water tubes, where it was the door which revolved to lock in a fixed ring. The door was fitted with a pressure gauge, and two reflex water gauges, to check the tube was empty before opening the breech door.
To load a torpedo, the propellers were first locked with the special tool, the starting lever was protected by a safety guard, and a tail piece was inserted in the propeller shaft, connected to a block and tackle. The block was attached to the eyes on top of the tube breech door ring. Now the torpedo could be eased forward on its runway, ensuring that the guide stud entered the guide slot. When it was part way into the tube, the safety guard was removed from the starting lever, the indexing dials were checked to see they were all properly set, that the torpedo stop valve was open, and that the starting gear was set to run. Finally, the torpedo entered the tube completely, and was brought gently up against the torpedo stop. After the propeller guard had been removed, the door was closed, and the screw pad moved by knurled nut in the centre of the door was brought up into contact with the tail of the torpedo. The tube was now ready to be flooded and the torpedo fired.
If for any reason it was necessary to unload the tube, the same tail piece was inserted in the propeller shaft, the block and tackle was attached at the rear end of the torpedo room, and the torpedo was gently pulled backwards onto the runway, care being taken to ensure that the various setting spindles had been retracted.
Surprisingly, the manual stated that British 21in torpedoes could also be loaded and fired from these tubes, with certain modifications. It was necessary to screw the guide lug to the top of the British torpedo, cut off the tip of the starting lever which otherwise would have fouled the inside wall of the tube on launch, set the various depth, speed and gyro controls before inserting the torpedo in the tube, as the spindle points were different and, amazingly, ‘to improvise means, such as a lanyard for starting the torpedo’. Obviously, when one was desperate and replacement torpedoes were a thousand miles away, one could resort to such make-do-and-mend measures.
Another interesting feature was a means of internally venting the compressed air used to launch the torpedo before the weapon completely exited the tube, to avoid excess air bubbles rising to the surface and giving away the position of the boat and the launch.
Since the Great War era, major surface warships had their gunnery controlled by the mechanical computer in their transmitting station. Battleship torpedo-firing in British ships was directed from the torpedo control tower, and destroyers of the Second World War had their own torpedo directors.
Submarine commanders faced the same difficulties as their surface counterparts, plus the requirement to act in a vertical plane as well. In addition, their visual tracking means were much more limited and often had to be intermittent, to avoid exposing their periscope head for too long. Otherwise they had to rely on hydrophone bearings. British submariners had experimented with multi-frequency Asdic (sonar) ever since the first hunter-killer set was installed in the experimental submarine cruiser X 1, but during the Second World War the Royal Navy only used its submarine Asdic as a listening device, not emitting pulses for fear of alerting the enemy.