David Bushnell first thought of the concept of an underwater boat during the 18th century. Attempts were made during the American Civil War to produce submarines. The first submarine in the Royal Navy was developed by John P. Holland and launched in 1902. France led the world in the design and construction of steam-powered underwater boats. But, after 1905, Germany began to develop a submarine with fighting qualities. In 1913 Germany produced its first diesel-powered Unterseeboot (U-boat).
By the time WW1 broke out Germany had 10 diesel-powered U-boats and 17 more under construction. The German Navy also had 30 petrol-powered submarines. Britain had 55 submarines and the French had 77. Submarines were slow, fragile and able to dive for only a couple of hours at a time. Submarines had four torpedo tubes in the bow (front), one or two torpedo tubes in the stern (back), 16cm guns, and mine laying equipment. Torpedoes posed a serious threat to other ships. Minefields provided a way to blockade ships without risking ships.
Submarines could dive 30 to 75 meters. On the surface a submarine could travel 18 knots and only 7.5 to 8.5 when submerged.