If an enemy captain has the "Light Repair" perk (the one which allows you to quickly repair your ship up to 10% hull if it's on the verge of sinking) then his ship is unsinkable. I've seen this twice. The most recent was while I was playing the "Elizabeth Shaw's Disappearance" side quest - I was continuing an "Ardent" game in free-play after completing the story and wanted to take the Dauphin Royale against the Flying Dutchman. I'd brought plenty of ammo and still had a lot left by the time the Dutchman was down to around 10% hull, then found that I couldn't finish it off.
The first time I had this problem was when I was developing the "Hornblower" side quest. This time it was El Supremo who had the perk and as a result, the Natividad did the same thing - I had hit it enough times to take it down to around 10% hull, then couldn't finish it off even after expending the rest of my ammo.
I don't think I've seen this happen with random ships, though most times I board them rather than sink them with cannonfire. But if the captain of the Temeraire ever gets the "Light Repair" perk, the first sea battle scene in the "Hornblower" storyline is going to be impossible to win.
It may be possible to sink an enemy ship which has "Light Repair", and it's certainly possible for you to be sunk if you have the perk. You'd need to hit the enemy ship hard enough to take it from greater than 10% hull straight to 0 in one salvo. And you'll sink if you don't activate the perk quickly enough before the next broadside hits you. But getting the Flying Dutchman from 11% to 0% in one salvo is probably not possible, and getting either the Temeraire or Natividad from 11% to 0% in one salvo from a frigate also isn't going to happen.
The first time I had this problem was when I was developing the "Hornblower" side quest. This time it was El Supremo who had the perk and as a result, the Natividad did the same thing - I had hit it enough times to take it down to around 10% hull, then couldn't finish it off even after expending the rest of my ammo.
I don't think I've seen this happen with random ships, though most times I board them rather than sink them with cannonfire. But if the captain of the Temeraire ever gets the "Light Repair" perk, the first sea battle scene in the "Hornblower" storyline is going to be impossible to win.
It may be possible to sink an enemy ship which has "Light Repair", and it's certainly possible for you to be sunk if you have the perk. You'd need to hit the enemy ship hard enough to take it from greater than 10% hull straight to 0 in one salvo. And you'll sink if you don't activate the perk quickly enough before the next broadside hits you. But getting the Flying Dutchman from 11% to 0% in one salvo is probably not possible, and getting either the Temeraire or Natividad from 11% to 0% in one salvo from a frigate also isn't going to happen.