USCGC Blackthorn (WLB-391) was a 180-foot (55 m) seagoing buoy tender (WLB) which sank in 1980 in a collision near the Tampa Bay Sunshine Skyway Bridge, resulting in 23 crew member fatalities.

Blackthorn was initially assigned to the Great Lakes for ice-breaking duties, but after only a few months, she was reassigned to San Pedro, California.

Meanwhile, the tanker Capricorn, owned by Kingston Shipping Company and operated by Apex Marine Corporation of New York, was standing (traveling with right-of-way) into the bay.

Blackthorn's captain, Lieutenant Commander George Sepel, had departed the ship's bridge to investigate a problem with the newly installed propulsion shaft.

Unable to make radio contact with Blackthorn, Capricorn's pilot blew two short whistle blasts to have the ships pass starboard-to-starboard.

The Commandant of the United States Coast Guard, Admiral John B. Hayes, approved the report of the marine board of investigation on the collision between Blackthorn and Capricorn.

Additionally, Admiral Hayes pointed out that attempts to establish a passing agreement by using only radiotelephone communications failed to be an adequate substitute for exchanging proper whistle signals.

The marine board found evidence of violation of various navigation laws on the parts of Capricorn's master and pilot.

The Commandant also acted on various safety recommendations made by the marine board concerning training and equipment aboard Coast Guard vessels, and navigation considerations in Tampa Bay.

Seaman Apprentice William "Billy" Flores was originally from Carlsbad, New Mexico and attended Western Hills High School in Fort Worth, Texas.