Boosting Sails, in Pirate's Cove
Sails aren't strengthened; how would you track hits?

Because Blow me down! is a Battle card, it lasts only for the duration of the battle. Damage to any of the ship's sections is represented physically (e.g., moving a ring, turning a card sideways). Likewise, the results of repair/upgrade are represented physically and they require discarding a payment (e.g., gold, the Going on the account event card). If it made your sails temporarily able to take six more hits, how would that be represented without losing track of your sails' real strength?

(Note: This argument is weakened by the fact that the answer, "the same way as you'd track the legendary pirates' hull," is quite a reasonable counter-argument.)