A fire ship is a ship which is weaponized by setting it on fire and sailing it into an enemy fleet. Fire ships were used in a number of ancient societies, but their use began to wane in the 20th century as a result of changes to ship design which made fire ships less effective. The related explosion ship is designed much like a fire ship, except that it explodes, acting as an incendiary device which causes numerous small fires to start in the target fleet.
Historically, fire ships were made either from old vessels which were about to be decommissioned, or very cheaply made vessels built specifically for the purpose. Flammable materials and accelerants were loaded on board and stationed strategically around the ship. Then the ship was launched in the direction of the enemy and set on fire. The crew abandoned the ship once it was set on its way.
Fire ships were most effective when a fleet was trapped. This might occur because ships were anchored, because there were too many ships to move out of the way easily, or when the navy using the fire ship had enough boats to keep the enemy fleet pinned in position. Otherwise, boats could simply move out of the way of the fire ship and allow it to burn out, or they could target it with munitions with the goal of forcing it to sink before it reached the fleet. Handling fire ships was also dangerous for crews, because if the ship caught fire too soon or detonated, the crew could be killed.
The Ancient Greeks were notable users of fire ships, with some of the earliest surviving descriptions of such ships coming from the Greeks. In the 1800s, they used this method again against the Ottoman empire. The Chinese utilized fire ships in the Opium Wars, while the British found them an effective weapon against the Spanish Armada, and they also played a role in the 1585 Siege of Antwerp.
The fire ship was an effective weapon historically because ships were made from materials that would burn, and more flammable materials such as pitch, tar, and fat were used in the maintenance and operations of ships. As ships began to be made from metals, and synthetic materials replaced things like hemp which would have required tarring, ships became less vulnerable to fire. As a result, the fire ship became much less useful. There are still potential applications for explosion ships, however, which could detonate explosions powerful enough to sink the enemy.