The following summation is by J. David Perkins (author of Canada's Submariners 1914-1923), and was originally posted on the sub-list mailing list 31 January, 1997:
Torpedo tubes must be flooded in order to equalize the pressure inside the tube with the surrounding sea pressure so that the bowcap (muzzle door) can be opened. In most cases the tubes were flooded and equalized on going to action stations. The bowcaps (or stern-caps for rearward facing T-tubes) were opened only on command from the control room. Torpedoes that were not fired had to be "drawn back" afterwards, washed down with fresh water, dried off (or blown dry using compressed air) and regreased before reloading. The interior fittings on the tubes also had to be regreased.
In British service in WW1, torpedoes were ejected using compressed air. There was no automatic venting of the discharge air back into the boat so the weapon was followed by a big bubble of air. A certain degree of compensation of the lost weight was possible by manually opening the torpedo tube drains and allowing seawater to pour into the Torpedo Operating Tanks for a fixed length of time, but this was neither very safe nor accurate and over-compensation sometimes resulted.
The disadvantages of this method are pretty obvious and between the wars the problems were solved by the instalation of Automatic Inboard Venting (AIV) gear. This system vented the firing air back into the boat before it could escape out the T-tube muzzle and allowed compensating seawater to follow for a timed interval and then the valve shut automatically. Firing, however, was still confined to periscope depth. This was not a big problem, however, as the periscope was essential to the attack.
With the advent of more accurate sonar detecting and locating equipment after WW2, it became desireable to fire at all depths. Larger firing resevoirs (compressed air bottles) and a depth regulated system of firing air release and inboard venting and compensating was perfected called Dual Pressure Firing Gear during the 1950's that allowed boats to fire down to 500+ feet.
Some models of electrically propelled torpedoes did not have to be ejected and could "swim" out on their own. The old Brit Mk XXI and the USN Mk 37s did this.
With the nuclear SMs the deep discharge problem was beyond the capabilities of high pressure air systems and a method of discharging the torpedo using the output of a high-pressure seawater pump was perfected by the Americans. This could be used at any depth that the boat was capable of. And that's how it's done today.
In WW1 boats settings were applied to the weapon before it was loaded. Essentially this was only running depth.
In WW2 the only fire control settings to be applied to a torpedo in the tube were gyro angle and possibly (but not in all boats) running depth. These settings were applied mechanically by hand from information passed over the intercom from the control room. Speed (high/low) was pre-set and a nominal depth (depending on the season and area of operation) was put on before loading.
During the attack estimated taget speed, course and range were fed into a mechanical angle solver ("fruit machine") which was also provided with the submarine's gyro heading. One of the solutions provided by this calculator was the Torpedo Track Angle, the gyro angle to be applied to the torpedoes. This angle was relative to the direction the bow would be facing when the fish was fired. Torpedo spread was achieved by maintaining a steady course (of the submarine) and varying the gyro angle and firing interval (time between firing) for each fish. The advantage of this was that the bow didn't necessarily have to be pointed towards the target.
The primary firing method was usually electrical from the control room. Other methods also existed but the electrical system was the most common. There was an electrical firing lever for each tube in the control room and a bank of elctro-mechanical firing levers between the tubes in the tube space. When the control room lever was pulled the electric current activated a solenoid that knocked the mechanical firing lever into the fired position to initiate the compressed-air operated firing system. At the same time the order to "Fire #" was passed to the "communications and firing number" over the intercom and an illuminated "Order Instrument" with a display for each tube indicated "FIRE". If the appropriate lever did not operate immediately the "communications and firing number", who was stationed at the firing panel between the tubes, yanked it back "right smart like". Any appreciable delay in launching the fish would upset the carefully calculated spread.
The relative merits of British, German and American WW2 submarine weaponry and fire control arrangements are explained in layman's terminology in THE UNDERWATER WAR, 1939-45 by (Commander) Richard Compton-Hall, a one-time submarine CO who used to be the Director of the Royal navy Submarine museum. He makes some interesting observations in his book that might spark some debate amongst us.
According to Compton-Hall's book, British submarines achieved the highest "hit to torpedo fired" ratio among the Allied submarine forces. RN captains were well trained in torpedo attack techniques and had an excellent record.
Swinging the ship's head to achieve a spread was an older method of firing developed before gyro angling was available. It was a viable method as long as the submarine did not have to compromise itself in doing it.
Electrically appllying the settings to torpedoes using a water-tight "umbilical" was introduced in a limited way on USN boats during WW2 and perfected after the war. Controlling torpedoes by sending electrical impulses down a fine wire connected between the FireControl System and the torpedo was introduced in many navies during the Cold War. Modern ship-borne sonars made torpedo detection and evasion relatively easy so means had to be found to run the fish in deep and then to bring them up to acoustic homing range when in the vicinity of the target.
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