The H.M.S. Pentland Firth was a 164.4 foot by 27.4 foot British Patrol Boat.  She was built in November, 1934, by the Cook, Wellington & Gemmel Co. Ship Builders in Beverly, England, and was originally owned by the Firth Steam Trawling Company Co. In 1939, she was purchased by the Royal Navy.

 On September 19, 1942, she was performing convoy duty on loan to the American Navy when she was struck by the minesweeper, USS Chaffinch, and sunk a few miles west of Ambrose Light Tower.  The wreck was determined to be a hazard to navigation and was quickly wire dragged clear to a depth of 50 feet.

 Today, The Pentland Firth lies in 70 feet of water, almost directly under the main shipping lanes within the Pilot's Triangle west of Ambrose Light Tower. She is low lying, and her steel plates overlap making good homes for lobster.