THREE WAR SECRETS

Greymouth Evening Star, 10 August 1920, Page 6

THREE WAR SECRETS

AN ODD PROBLEM

Although the depth charge was the most powerful of the weapons that cheeked the operations ot the U-boat, thus leading indirectly to the collapse of the Central Rowers, I imagine that not more than ten people in every thousand would be able to explain just what a depth charge l is. Jack Tar himself, who knows most things, could not tell you how and where it came to he constructed, writes G. B. Dewhurst, M.Sc., Lecturer in Engineering at Manchester University, in an exchange. Manchester University did, in fact, make the first depth charge. It would be inaccurate to say it was invented by ns; like the tank, the depth charge is nobody’s child. But we can honest]v claim that it was our engineering department which experimentally developed the depth charge to the commercial stage. It was one of the many odd problems set us by the Admiralty. They asked us for a 15-ineh bombthrower which would project a weio'ht of lewt a distance of 500 yards by compressed air. Wo sup-, plied it. They asked us for a shackle which would stand a null of one ton and melt after heiug immersed in the water for a certain length of time. It was a novel request, and probably the Admiralty thought they bad set its a poser. The object was to discover a contrivance which would shackle a mine and its sinker together on the bed of tile sea until a mine-laying submarine bad dropped all its c esg,’ when, with the melting of the shackle, each mine would rise to the determi’ied height. "We solved that particular problem with a sugar shackle—by pouring melted toffee into a cavity, dovetailed in a metal shackle! It was so devised that it could stand a pull of one ton and a-third during- the forty-five minutes that the sugar took to melt. After the first year of war several men from our laboratory took up responsible positions in 11.M.5. Yemen, the Navy’s research department at Portsmouth, and it was through H.M.S. Vernon that the investigations into the question of how to conquer the U-boat first came back to the engineering department at the university.

Two alternative designs of median ism causing a heavy charge to explode at a fixed depth under water were sent to us for criticism*, modification, and experiment, and, after some months of patient experimenting, we were able to band over to the university instrument-maker the final design from which, under our supervision, the first 50 depth charges were made. - Our aim was to produce a simple mechanism, thoroughly fool-proof, which would go off at iho pressure at which it was set. The depth charge consisted of a case containing- 3001 b of T.N.T. (ti-j-nitro-tclucl) with an attached mechanism provided with a flexible diaphragm and a piston which depressed a spring in such a way that the spring was compressed in pioporiion to the depth below the surface of the sea. By a simple arrangement worked bv a lever on the outside of the depth charge the amount of compression required to fire the charge could he adjusted to suit any depth—in practice one spring was released •0 40, a second at 80ft.

The firing gear was so arranged tint a trigger was released when the spring was compressed ly lie amount corresponding to the de‘■Ued depth. The trigger fired a detonator; the detonator exploded the- T.N.T.; and—that was the end of the U-boat.