A torpedo boat is a relatively small and fast naval ship designed to launch torpedoes at larger surface ships. They were created to counter battleships and other large, slow and heavily armed ship by speed and agility.
During the late 1800s, the development of metal-hulled ships of large size, and the use of gyroscopes to even out the motion of waves, allowed for the rapid development of the very large gunship, which soon became known as battleships, later dreadnoughts. These were fiendishly expensive, so only the largest and richest nations could afford to continue in the race to build such ships.
But at the same time, the new weight of armor slowed them, and the huge guns needed to penetrate that armor fired at very slow rates. This allowed for the possibility of a small and fast ship that could attack the battleships en-mass, at a much lower cost. The introduction of the torpedo provided a weapon that could cripple, and sink, any battleship.
In the late 19th century, many Navies started to build torpedo boats - relatively small ships, about 30 to 50 m in length, armed with up to three torpedo launchers and small guns. They were powered with steam engines and developed speed of 20 to 30 knots (37 to 56 km/h).
They were relatively inexpensive and could be purchased in quantity, allowing for mass attacks on larger fleets. While some of them would undoubtedly be lost to the guns of the larger ships while they ran into firing range, their cost was so low that sinking even one battleship in return would be a victory.
It is commonly acknowledged today that the very first torpedo boat was the Royal Norwegian Navy's HNoMS Rap—the name meaning 'fast'—ordered from Cheswick, England in 1873. The first recorded launch of torpedoes from a torpedo boat (which itself was launched from a battleship) in an actual battle was by Russian admiral Stepan Makarov on January 16, 1877, who used self-propelled Whitehead's torpedoes against a Turkish battleship during the Russo-Turkish War of 1877-78.
The introduction of the torpedo boat resulted in a flurry of activity in the fleets around the world, as smaller and faster guns were added to existing ships to ward off the new threat. Eventually an entirely new class of ships, the torpedo boat destroyer, was invented to counter them. These ships, today known simply as destroyers, were just enlarged torpedo boats, with speed equal to the torpedo boats, but including heavier guns that could attack them before they were able to close on the main fleet. Destroyers were also armed with torpedoes. Considerable effort was put into designing fleet actions that would allow the destroyers to operate far enough from the main "van" to keep the torpedo boats away, while still remaining close enough that they couldn't be "picked off" by an opposing fleet.
Destroyers appeared so useful and universal ships, having better seaworthiness, than torpedo boats, that they eventually replaced most torpedo boats. Until World War II, classic torpedo boats remained only in a small number in some navies, e.g. German and French. By that time, they were ships 70 to 100 m long, armed with a 2 to 3 guns typically 100 mm (4 in) and torpedo launchers. After the war they eventually disappeared.
Before the World War I, as torpedo boats were growing larger, armed with heavier guns, there appeared different class of torpedo boats, going back to their roots, being small and fast ships again. The introduction of the internal combustion engine resulted in a power source that could offer much higher output from a small source.
The result was a small torpedo boat, perhaps 50 to 100 feet (15 to 30 m) in length with high speed 30 to 50 knots (56 to 93 km/h), carrying 2 to 4 torpedoes, fired from simple fixed launchers, and several machine guns. Such torpedo boats remained useful until World War II, in particular the Royal Navy (RN) Motor torpedo boats (MTBs), Kriegsmarine 'S-Booten' (Schnell-boot or fast-boat: British termed them E-boats) and US PT boats (standing for Patrol Torpedo) served their users well.
A classic fast torpedo boat action was the Channel Dash in February 1942 when German E-boats and destroyer defended the flotilla of Scharnhorst, Gneisenau, Prinz Eugen and several smaller ships against RN MTBs.
By World War II torpedo boats were seriously hampered by the higher fleet speeds, although they still had a speed advantage, they could only catch the larger ships by running at very high speeds over very short distances as demonstrated in the Channel Dash. An even greater change was the widespread arrival of patrol aircraft, which could hunt them down long before they could even see their targets.
The class has not entirely disappeared, due to the arrival of the guided missile. Today a number of navies operate boats of the same general size and concept as the older torpedo boats, but armed with long-range anti-shipping missiles that can be used at ranges between 30 and 70 km. This reduces the need for high speed chases to a degree, and gives them much more room to operate in while approaching their targets. Aircraft remain a major threat, and any fleet combining air elements makes their use almost suicidal.
They are still used by many navies and cost guards to police their territorial waters against smugglers, particularly those smuggling narcotics and weapons to insurgents. The interdiction and boarding of potentially armed hostile fast boats, which often are indistinguishable from legitimate coastal craft, is something which has to be done from a heavily armed fast boat, often with the assistance of maritime patrol aircraft.