In 1919 PD, he held the rank of Lieutenant and served as the Third Engineer of the heavy cruiser HMS Vigilant, assigned to the new RMN duty station in the Talbott Cluster. During the Battle of Monica, the ship took a direct hit in the bridge that killed Commander Osborne Diamond and the entire brigde crew. Lieutenant Gainsworthy, the senior surviving officer, assumed command and made contact with the formation's flagship HMS Hexapuma. Although he stated that he and his crew could save the Vigilant, Captain Aivars Terekhov, the squadron CO, ordered him to set scuttling charges and abandon ship. Gainsworthy executed these orders; Vigilant was destroyed, but her name was put on the List of Honor along with those of all ships in her squadron. (SI1)