Submarines and the Jolly Roger after counter-piracy

May a submarine fly the Jolly Roger after completing an anti-piracy mission?

Yes. The flag marks a successful operational return, so an effective counter-piracy mission qualifies.

The exchange traces the Royal Navy submarine custom to HMS E9's First World War success under Max Horton. A submarine flies the flag after surfacing on its return to port; it does not carry it while submerged. The Jolly Roger marks a successful operational patrol and may carry symbols for the boat's achievements. Because the custom is tied to mission success rather than a particular type of target, it can apply after an effective anti-piracy patrol. The target need not be an enemy warship.