How about a card that you cast on an enemy and the crew on the fleet/ship get way laid by a brothel for 3 hours. make it a common card.
Ship is stuck in port for that time. but the ship/fleet has its usual number of crew if attacked.
It just keeps them in port for 3 hours.
it would stop runaways and/or give you a chance to runaway.
possible extra, if a fleet is under FFJ or HN it could reduce the time of them by half. this was an after thought.