The whole broadside will fire, however many guns are left that you haven't shot out. If the crew is below a calculated number the guns will load slower, and it depends on the number of undestroyed guns on that broadside. You probably see more effect from the captain's cannon skill. A ship with low crew won't sail slower, but *may* turn slower. I'd have to check the code again.
My personal experience is that grape isn't really very effective. I use it to reduce an enemy crew on a larger ship prior to boarding. I don't want to reduce the crew too far, because I'll need some of them to sail the ship later, so I stop firing at some point. If I don't intend to keep the ship, but to let it sink, then I may need crewmen on the other ship to replace losses to my crew. In some battles I'm getting as many crew losses from ball, in others I get reasonable losses from grape. But then I'm shooting at ships with a lot of crew to begin with.
From what you've described, and what I've seen, there may be something backwards in the calculation so that you get 3 or 4 kills from grape if the enemy crew is 400, and 20 or more if that same crew is around 200.
I'm not really a fan of reducing the sails, hull and crew of other ships as much as it's being done. The only reason I haven't messed with that code already is that there will be a lot of people who rely on enemy ships not running at full strength and they'd scream if we changed it.
I'll get into all that code and analyze it to make sure it's working properly at some point. If I remember right, that 32 gun ship should have 13 cannons on each broadside, each of which take 6 crew, and if the crew is below 78 (plus some number needed to sail the ship) the guns will load slower. If the ship is armed with carronades, it will take half that number. If you've shot out some of their cannons on that broadside, they'll require fewer crew. I don't remember if morale is a factor.
There's a similar situation with the hull. At some point you have to stop firing or you'll sink the ship.
Hook