The Gamewright rules on tied pirates includes this slightly confusing (to me) wording:
If two piratesare locked in battle over a merchant ship for too long, weaker pirate ships may join the battle and eventually take the ship for themselves! Note: Attack strength is measured by adding together all skull and crossbones on same colored pirate ships attacking a merchant ship.
At first I thought this implied there was some condition where a weaker pirate ship could take a merchant ship where two other pirates were tied, but I think all this is really saying is that it's possible for a third player to take a merchant ship by playing (perhaps over several turns) enough pirate cards over several turns to exceed the value of the tied ships. Is that right?