An armed yacht was a yacht that was armed with weapons and was typically in the service of a navy. Their speed and maneuverability made them useful as patrol vessels. In the United States Navy armed yachts were typically private yachts expropriated for government use in times of war. Armed yachts served as patrol vessels during the Spanish–American War and the World Wars. In the latter conflicts, armed yachts were used as patrol vessels, convoy escorts, and in anti-submarine duties. In the United States, yachts were purchased from their owners with the owners given an option to repurchase their yacht at the close of hostilities.
During World War II, the US navy commandeered small vessels, including private yachts, and requisitoned them to patrol US coastal waters and the Gulf of Mexico against German U-boats. In the Gulf, the boats ventured out for 12 miles. The US Coast Guard Auxiliary was formed to man the vessels, but their crews were unexperienced and untrained. They had absolutely no navigation equipment, and, consequently, the crew often had no idea where they were. The boats were equipped with radios, which, in theory, were to be used to report any U-boat sightings. In practice, however, reporting a U-boat's position was impossible due to the crew's lack of bearings. The men on board spent their time scanning the horizon with binoculars, having been given pictures and silhouettes of the different types of U-boats so that they would be able to identify them.
Some of these boats were armed with a .50 caliber machine gun in the bow and four depth charges on racks in the stern, although actually attacking a U-boat was probably suicidial. The lopsided battle would have conceivably ended with the U-boat using its deck guns to blow up the patrol yacht (since scoring a hit with a torpedo was improbable).
The Royal Canadian Navy also commandeered and used armored yachts and other such vessels for anti-submarine patrols, having 12 of them.