Originally a private yacht and later a navy training vessel, Sapphire was built in 1929, and named Margo by George Lawley & Son, Neponset, Massachusetts.

In September 1941, Sapphire left Norfolk with orders to patrol and perform anti-mine operations in the area of the Panama Canal.

There, throughout World War II, the ship supported various Submarine School programs, but was primarily engaged in training prospective commanding officers in attack procedures and in testing sound equipment.

As Storekeeper First Class Ed Hickey, a crewmen on Sapphire, commented, "We knew we were a sitting duck for any U-boat that we found since a converted yacht is no match even though we had some depth charges and a few other armaments.

[2] On 21 May 1942 a German U-boat attacked the American 3,282-gross register ton cargo ship Plow City in the Atlantic Ocean about 30 nautical miles (56 km; 35 mi) off Bermuda, sinking her with two torpedoes.