U-BOAT HUNTING

Otago Daily Times, Issue 23970, 20 November 1939, Page 12

U-BOAT HUNTING

TIME AND PATIENCE HOW NAVY’S ATTACK IS LAUNCHED DETECTORS AND DEPTH CHARGES In the last week of two comparatively few ships have been sunk by the enemy, and the opinion is growing that the menace of the U-boat ; has been scotched. If this be the case (states a writer in the Dominion), it says a great deal for the combined operations of the navy, the naval air arm and the coastal command of the Royal Air Force. The hunting down of submarines is a task of considerable difficulty and one calling for an expenditure of endless time and patience. At this stage it is impossible to say which of the several new methods of attack on submarines is proving most effective. It is known, however, that in recent years the navy has developed to a high state of efficiency many devices first used in the Great War —and has acquired others. If a raiding U-boat can be caught on the surface, or its periscope sighted, the task of attacking it is a more or less simple one. At least it is a straight-forward fight in which the opponent of the submarine is not, as a recent writer put it. “ like a blind man groping for a frog in a pond.” Like an Aquatic Mole But anti-submarine forces seldom have this advantage. When a ship has been torpedoed the submarine commander, if he knows that a destroyer or motor torpedo boat is hurrying to the scene, will lose no time in diving—and diving deep. Not even his periscope shows. He creeps away like an aquatic mole, steering by compass and hoping for the best. When the surface ships arrive their first task is to try to hear the submarine; their second to get a “ fix ” of its approximate position. In listening for it they use hydrophones—or the modern equivalent of these wartime devices, which picked up sound impulses on a rubber diaphragm immersed in the sea and transmitted them electrically to earphones. Hydrophones are also capable of finding direction, so it is possible for the operator of one instrument to get a general bearing on the sound of the submarine’s propellers—a rhythmic beat, described by one Great War operator as “ a sort of ticking.” If, then, three surface vessels are able to form a triangle in the area suspected to contain a submarine, their three bearings should cross at the point where the sound originates. Listening for 30 Miles There is a great deal of hush-hush about the latest hydrophone, but it is oebeved that it has been developed to a point at which one instrument is capable of - fixing the position of a submarine unaided. In 1930 it was asserted that the United States Navy had a hydrophone—the “ M.V. tube ’’—capable of picking up a propeller beat 20 or 30 miles away and also determining the bearing within one degree. This in itself woyld not provide a true fix, but it is claimed that the British Navy’s jealously-guarded “Asdig” (anti-sub-marine detection indicator gear) can nlot the position, course and depth of a U-boat—in other words, can get a twodimensional “fix.”

Haying located the enemy the next job is to attack him. Let us suppose a destroyer is to make the attack. While Asdig has been in operation the ship has been quiet; now she vibrates and hums with the power of her engines as the slim hull cuts a furrow in the sea. The Attack Is Made At perhaps 35 knots the “ x ” of the “fix”, is approached, the destroyer swinging in a wide arc so as to run on to the course the U-boat is believed to be taking. Then, as the spot is reached the ship slows down to make the attack. _ At the stern, and amidships on both sides, depth-charge crews are standing by. Depth-charges are known as “ashcans,” because the big tin containers of the bombs rather resemble these homely receptacles. Amidships they are usually shot into the water, well clear of the hull, by little cannon; at the stern they are simply slid into the wake. A depth-charge operates very simply. It contains a quantity of explosive, which is ignited by the depressing of a plunger attached to a pressure valve. The valve is set according, to the depth at which the explosion is required to take place, and as the bomb sinks the water pressure on the exposed valve does the rest. The destroyer (so the navigator hopes) is now over the submarine. There follows a “ salvo ” of ashcans, one from the stern, then (after a care-fully-timed interval) two from amidships—one to port and one to starboard. Another interval, and the fourth can drops from the stern. A “ Diamond ” Salvo All eyes look out along the wake. There are four concussions—dull, monstrous thuds that shake the ship. Four patches of water boil, forming a diamond pattern. Perhaps the submarine has escaped. If no air-bubbles or oil patches appear, the destroyer begins the performance all over again, perhaps making more elaborate patterns, or,, in conjunction with other surface vessels, riddling the sea with explosions. If the U-boat is lucky, it creeps away through the explosions, its brave crew ashen-faced and sweating as the frail hull vibrates to every concussion. If the water is shallow, maybe it lies “doggo,” with engines off, on the sea bottom. If Lady Luck has her back turned on the submarine, oil and air bubbles will send up a mute message from a steel coffin, sagging into the depths.