Why was the submarine such a deadly weapon

The submarine became a potentially viable weapon with the development of the Whitehead torpedo, designed in 1866 by British engineer Robert Whitehead, the first practical self-propelled or 'locomotive' torpedo.[20] The spar torpedo that had been developed earlier by the Confederate States Navy was considered to be impracticable, as it was believed to have sunk both its intended target, and probably H. L. Hunley, the submarine that deployed it. In 1878, John Philip Holland demonstrated the Holland I prototype.

Discussions between the English clergyman and inventor George Garrett and the Swedish industrialist Thorsten Nordenfelt led to the first practical steam-powered submarines, armed with torpedoes and ready for military use. The first was Nordenfelt I, a 56-tonne, 19.5-metre (64 ft) vessel similar to Garrett's ill-fated Resurgam (1879), with a range of 240 kilometres (130 nmi; 150 mi), armed with a single torpedo, in 1885.

A reliable means of propulsion for the submerged vessel was only made possible in the 1880s with the advent of the necessary electric battery technology. The first electrically powered boats were built by Isaac Peral y Caballero in Spain (who built Peral), Dupuy de Lôme (who built Gymnote) and Gustave Zédé (who built Sirène) in France, and James Franklin Waddington (who built Porpoise) in England.[21] Peral's design featured torpedoes and other systems that later became standard in submarines.[22][23]