During the American Civil War, there were further attempts at submarine warfare using semi-submersibles. On 5 October 1863, the Confederate craft David, a cigar-shaped, steam-driven vessel, caused severe damage to the USS New Ironsides by exploding against its hull a 27 kg (60 lb) copper-cased torpedo, manipulated on a long spar.

The coming of the U-boat
However, significant submarine development had to wait for the simultaneous development of the petrol engine and the electrical storage battery. The first Royal Navy submarine was launched in 1902, having been developed by John P Holland, a British emigrant to the United States. In 1905, the German armaments firm of Krupp introduced the Karp-class diving boat, which could act as both a defensive and an offensive weapon. Then, in 1913, the Germans produced the first diesel-powered Unterseeboot - better known as the U-boat.

By 1914, the German U-boat fleet included 10 of these diesel warships, with 17 more under construction. In addition, the Germans fielded about 30 petrol-powered vessels. The British submarine fleet numbered some 55 vessels, the French 77 and the US Navy 38. On 8 August 1914, the German U-15 fired a torpedo at the battleship HMS Monarch, without success, but nevertheless this marked the first occasion that an automotive torpedo was fired against an enemy from a submarine. The submarine war on commerce - the first Battle of the Atlantic - began in October 1914, and for the next four years, the U-boats inflicted heavy losses on Allied shipping.

Increasing vulnerability
After the armistice in November 1918, all major nations continued to develop submarines; by the outbreak of World War II, the British fleet numbered some 70 vessels and the German 98. As a defence against the latter, in the 1930s the British had developed ASDIC, or 'sonar' as the Americans called it - an underwater echo-ranging device that initially required two or three convoy escorts, working in concert, to operate effectively against U-boats.

Throughout the war, there were two basic types of U-boat: coastal (mainly Type VIIs) and ocean-going Type IXs. Both were diesel-powered, achieving underwater propulsion through auxiliary electric engines. However, their speeds hovered at only around 4 knots, slower than any merchant vessel, and their underwater range was limited to about 80 nautical miles, because of the need to resurface and recharge batteries. This meant that the U-boat in the Atlantic had to spend most of its time on the surface, and as Allied detection techniques and anti-submarine weapons improved, it became increasingly vulnerable.