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British Upholder. The first three "U" Class were ordered in 1936 to serve as unarmed targets for antisubmarine vessels, but they were completed with torpedo-tubes to allow them to carry out normal submarine training as well. In 1940 it was realised that their small size suited them for the North Sea and Mediterranean, and so they were put into quantity production. HMS Upholder was commanded by Lt Cdr Wanklyn VC, and she sank over 90,000 tons of German and Italian shipping in the Mediterranean before she was herself sunk in April 1942. Displacement: 648 tons (full load, surfaced). Armament: Four 21-in torpedo-tubes; one 3-in AA gun. Speed: 11¾ knots (surfaced), 9 knots (submerged)

The increased German naval activity in the Norwegian campaign gave Allied submarines much greater opportunities for attacking. The Polish Orzel sank a large troopship, the Spearfish damaged the pocket battleship Lützow severely, and the Sunfish sank four ships, among others. Later the Clyde inflicted heavy damage on the battle cruiser Gneisenau. These casualties, when combined with the depredations caused by surface action, were sufficient to reduce the Kriegsmarine's strength below the level needed to support the invasion of England planned after Dunkirk. Once more the submarine had intervened decisively in the conduct of war at sea; and had exerted an influence beyond all proportion to her size and cost.

German Type VII (U236). The Type VII U-Boat was the standard design for the U-Boat in the Second World War. It was developed from the Finnish Vetehinen design before the expiry of the Versailles Treaty, and many improvements were effected as a result of war experience. U236 was one of the Type VIIC, the third version, and she came into service in January 1943. She was scuttled in 1945 after suffering damage by air attack. Displacement: 769 tons (surfaced). Armament: Five 21-in torpedo-tubes; 14 torpedoes carried; plus a variety of light AA guns. Speed: 17 knots (surfaced), 7,5 knots (submerged)

In one respect the British had been extremely lucky throughout this first phase of submarine warfare; whereas their own torpedoes were reliable the Germans' magnetic pistols had proved to be uncertain.

An estimated 30 attacks by U-Boats against British ships during the Spring of 1940 yielded only the submarine Thistle to U4 off Skudesnes. Both the British and the Germans had been experimenting with magnetic pistols for torpedo warheads before 1939, the advantage being that a torpedo exploding beneath the keel of a large warship would do far more damage than it could by exploding against its side.

British Telemachus. Like the German Type VII the British "T" Class went through many wartime modifications. This example shows how many changes had been made in the original design: a changed bow shape, external tubes now facing aft, and a platform on the conning tower for a 20-mm AA gun. She was completed in 1943 and scrapped in 1961

New weapons often create their own antidote, and so it happened in this case that a process called "degaussing" was immediately introduced to reduce a ship's magnetism. This could be met by increasing the sensitivity of the magnetic pistol, but the pistol was then liable to interference from the Earth's magnetic field, with the result that a deep-running torpedo might explode either prematurely or not at all.

Conforming to the well-known tendency for military and naval tactics to move in circles, the Royal Air Force matched the failure of German torpedoes by dropping bombs which did not sink U-Boats. Pre-war practice had made airmen over-optimistic about the accuracy of bombing runs against submarines, and also about the value of the conventional bomb. A bomb needs a direct hit, which is hard to achieve against the small, slender target presented by a submarine, whereas a near miss from a depth-charge can inflict vital damage to the hull. The answer to the problem was simple; the Mark VII naval pattern depth-charge was modified to make it more suitable for dropping from aircraft and came into service before the end of 1940. Later, however, properly designed airborne depth-charges were produced.

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