The Merrimac was a collier owned by the Navy during the time of the Spanish-American War and used to shuttle coal to the United States' ships the Texas, the Marblehead, and the Vixen. The collier broke down so often that it was suggested for a suicide mission by Admiral William T. Sampson. Assistant Naval Constructor Richmond Hobson modified the Merrimac to be operated by a crew of eight men and rigged it with explosives.
The plan was to sink the ship in the narrow opening of Santiago Harbor. The original date for the operation was the first of June, 1898, but due to technical difficulties four of the ten missiles aboard the Merrimac were not charged, and so the plan was called off. On the third of June the mission was attempted once again.
Hobson was given command of the attempt, and was joined by his crew of George Charette, Randolph Clausen, Osborn Warren Deignan, Francis Kelly, Daniel Montague, J. C. Murphy, and George F. Phillips. Almost immediately after embarking on their mission the collier began to act up, only three of the ten torpedoes responded to testing and the steering seemed to be more difficult than usual. Still, the Merrimac moved toward the channel. Upon closing with the narrow opening, a hidden Spanish gunboat opened fire on the small ship, damaging the steering and also striking the firing mechanisms of several of the torpedoes. Not to be swayed, Hobson and his crew continued into the channel and dropped the stern anchor. The command to fire the torpedoes was given, but only two missiles detonated and the collier did not sink. The bow anchor was dropped, but the collier continued to move, swept with the current.
The Merrimac was pulled between two Spanish warships, the Reina Mercedes and the Pluton, both of which immediately opened fire on the collier. Other Spanish vessels joined in, and the Merrimac finally began to sink, although not across the channel as had been intended.
Hobson and his crew were trapped. They could not swim back to their fleet since the current was pushing them into Santiago Harbor. Instead of blindly striking out, they clung to the slowly sinking hull of the Merrimac until they were captured by the Spanish.
While the mission was a failure, it was perhaps better for the U.S. that it didn't succeed. A success would have blockaded the Spanish in the harbor with full crews and guns, enabling them to protect their port and giving them a much better situation when it was time to negotiate peace.
Sources include http://www.spanamwar.com/merrimac.htm