Too many ships had to be abandoned or scuttled after they lost power to lighting and pumps. Organising the machinery on the unit basis, where instead of boilers and engines being grouped together in two large compartments, the propulsive machinery was divided up into separate units of boiler/engine combinations, reduce the risk of losing all power to a single torpedo hit. Similarly, shock effect from explosions led to the need to cushion machinery from its effects, again to ensure power to lighting and pumps, to aid maximum survivability.
DEFENSIVE ASDIC
Asdic (the British designation for sonar) was originally designed for offensive purposes, to hunt down and destroy enemy submarines, and was extensively fitted to destroyers, frigates, sloops and smaller anti-submarine vessels. In the late 1930s the Royal Navy gave thought to providing its larger ships with Asdic sets, to be used not for attacking U-boats, but for listening out for the approach of a torpedo. In order to give sufficient time for avoiding action to be taken, a torpedo had to be detected at a range of at least 2000yds (1800m).
The prototype Type 132 set was tested in the cruiser HMS Southampton in May 1938, in a large retractable dome. It could also be used for echo-sounding. On the successful completion of trials, the Type 132 and its wartime successor the Type 149 were fitted to many cruisers and even several battleships and aircraft carriers. A final development was the introduction, beginning in 1943, of the simplified Type 136 defensive Asdic, intended for medium-sized merchant ships capable of a maximum speed of 15–18 knots.
CHAPTER 18
Modern Developments
Active and passive anti-torpedo measures today fall into the two classifications of ‘hard-kill’, meaning an incoming torpedo is counter-attacked and destroyed, either by direct impact or by proximity fuse, and ‘soft-kill’, meaning that the torpedo is lured away from the target ship and pursues the decoy until it runs out of fuel at the end of its run.
SOFT-KILL MEASURES
U-boats commenced the countermeasures battle when they began launching BOLD pellets in early 1943. These were small metal canisters some 4in (100mm) in diameter containing calcium hydride. A U-boat commander under attack would order the release of these canisters using a ‘Pillenwerfer’ ejector (literally, a ‘pill thrower’). On contact with seawater the chemical produced hydrogen bubbles which created a false Asdic target, allowing the U-boat to escape. The Royal Navy referred to them as SBTs (submarine bubble targets), and an experienced Asdic operator could recognise them by the fact that they were designed to remain stationary at a set depth, which a U-boat under attack was unlikely to do. A modified device known as ‘Sieglinde’ was meant to overcome this problem, by being able to move at up to 6 knots.
During the Second World War, the introduction of GNAT homing torpedoes by U-boats was a serious development, especially as they began by targeting the escort vessels which had been so successful in driving them away from the convoys. The first time they were deployed, against convoys ONS18 and ON22, three escorts were sunk with heavy loss of life. Captain Walker’s escort group had on more than one occasion resorted to the desperate anti-torpedo hard-kill measure of dropping a shallow pattern of depth charges astern of their ships, as a defence against homing torpedoes. But doing this at slow speed, even if it succeeded in detonating the approaching torpedo, posed the very real risk of blowing the stern off one’s own ship, if not inflicting considerable damage to her hull and systems.
A soft-kill rapid response to the GNAT was needed, and it arrived in the shape of the Foxer, a noise-making device towed behind the escort vessel, with the aim of attracting the GNAT to attack the device and not the towing ship. Foxer was constructed of two lengths of piping, with holes cut in them, chained together, the whole assembly weighing some 3000lbs (1360kg). When towed through the water 200yds (180m) behind the ship they made a noise estimated by U-boat hydrophone operators as ten to a hundred times louder than that from a ship’s propellers. They referred to it as the ‘Kreissäge’ (‘circular saw’) or ‘Rattalelboje’ (‘rattle buoy’). Foxer was usually deployed as a pair, streamed astern of the ship and kept around 100yds (91m) apart by means of paravanes. It was cumbersome to stream and recover, could not be towed at more than 15 knots, and wore out quickly. One problem was that in some cases the Foxer interfered with the escort’s own Asdic (sonar), but experienced operators could compensate. A more serious drawback was that it could also be picked up on hydrophones from a great distance away, revealing the convoy’s position.
The Canadian version, the PNM or ‘pipe noise maker’ (codenamed ‘Cat’), was a much simpler device consisting of two lengths of metal fastened to a frame, which was towed sideways through the water. One of the two bars was free to move and bang against the other and the frame. Cat could be deployed by one man throwing it overboard, it could be wound in by a simple hand winch, it allowed for speeds of up to 18 knots, and it lasted several times longer than Foxer. Cat was the device chosen for employment by the US Navy.
After the Second World War the US Navy introduced Fanfare, a towed acoustic device which mimicked the sounds made by the towing ship’s propellers. Modern towed devices include the Nixie. When launched and towed behind the ship its signal generator acts as a decoy to homing torpedoes using passive sonar. They can also receive an incoming active sonar emission from the torpedo and send it back at an intensified level, making the torpedo think it is coming from a large target such as a ship’s hull.
Modern countermeasures devices launched from a submarine or surface ship under torpedo attack work in the same way as the original Pillenwerfer/SBT, creating gas bubbles which mimic a propeller to an acoustic homer and a large target to an active sonar homer. Its launching from a surface ship or a submarine depends on being able to detect an incoming torpedo in time to activate the countermeasure, as it is not a continuous decoy like the Foxer and Fanfare. Today, many firms offer expendable countersmeasure devices in diameters from 75mm up to 200mm, such as American Ultra Electronics Ocean Systems, who claim to have sold ten thousand devices since 1986.