fire ship

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A military vessel loaded with explosives and combustible material and set adrift to destroy enemy ships or fortifications.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun historical A wooden ship set afire and then sent floating into an enemy flotilla, with the intent to set the enemy fleet afire too.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a weapon consisting of a ship carrying explosives that is set adrift to destroy enemy ships