[3] On the evening of 18 June 1918, Unit F was on an antisubmarine patrol in the Strait of Otranto when it became involved in a friendly fire incident.

The submarine chasers challenged the British destroyers with recognition signals flashed several times by blinker light.

Defender and Nymphe immediately flashed lights at the submarine chasers, which ceased fire, went alongside the destroyers, and discovered their identities.

Discussing the incident in a letter to the British Admiralty, the commander of United States Naval Forces Operating in European Waters, Vice Admiral William Sims, wrote: "While it appears that, under attendant circumstances, the commanding officers of the submarine chasers were justified in opening fire on the destroyers, I nevertheless wish to express regret that the incident occurred, and that H.M.S.

[5] S.C. 151 was involved in another friendly fire incident on 20 June 1918, when the British destroyer HMS Acorn mistakenly opened gunfire on her.

[6] S.C. 151 also saw service towing lighters for the collier USS Jason at Spalato in the new Kingdom of the Serbs, Croats, and Slovenes.

[1] The United States entered World War II on 7 December 1941, and in 1942[1] the U.S. Navy acquired Usona as part of the "splinter fleet" or "splinter navy," a wide variety of wooden boats the United States Armed Forces acquired for war service.

[8] Hostilities concluded on 15 August 1945, and in June 1946 the U.S. Navy transferred YP-191 to the United States Maritime Commission for disposal.