A torpedo is a long cylindrical device, with a warhead at one end and propellers and a rudder at the other. Most torpedoes nowadays are electrically propelled and are powered by a battery. They vary in length from three metres for the small, air launched type to about 6.5 metres for the long range submarine-launched weapons.
Torpedoes can be designed for specific use against surface ships, or for use against submarines; alternatively they can be designed for use against either target and this is rapidly becoming the most common type. They can be fitted with a homing device, which is either an active sonar, which emits its own ‘ping’, or a passive sonar which merely listens for the noise made by a ship or submarine passing through the water and, on detecting it, the sonar homes the torpedo on to the target. Some torpedoes can be wire guided, trailing a long wire behind them by which an operator on the launching ship can steer the weapon to its desired target (this being particularly useful where friendly ships are in the vicinity of the target or when the target may alter course before the torpedo arrives).
The advantage of a torpedo over a surface-to-surface missile is in the greater warhead (typically about 270 kg) which can be carried. Torpedo ranges are never officially disclosed although the German Tigerfish is reputed to have a range of over 32 km. Most NATO navies today use the American built Mark.44 or Mark 46.
Source: Modern Naval Armament (War Monthly Publications Vol.8 No.77, June 1980)