Orpheus and the difficulty of identifying a friendly submarine

On 19 March 1917, Orpheus nearly sank the British submarine J1 during a sweep. J1 carried a British flag and was expected to identify herself to friendly forces, including by a flare when necessary. Her crew did not see the destroyer until shells began falling nearby, while the flag was too small for the destroyer to identify confidently. The incident prompted a reassessment of how submarines should act on the surface near friendly forces. Recognition procedures had to work before the approaching ship began its attack.