Swamped

The first thought you have is if there's another way to move the powder, without the cannons.

Due to the spell you cast, you can sense where the powder is. It's all been loaded into shells, but only a few of the shells are in the cannons already. Still, making that many shells move, even with magic, would be tricky. It would be easier if you had some way to move them all at once.

And then you happen to catch a glimpse of the empty powder barrel.

It's probably big enough to fit the shells in. Still, that leaves you with two main problems.

First, you need a way to move it. You can't use a flight spell, because the one affecting the ship would overpower it. A separate problem is that a lot of sailors might fall overboard if that spell activates again while they're incapacitated, which you have no thoughts on how to solve beyond destroying the opposing ship and hoping that stops the effect. You could lighten the barrel and throw it, but even if you got enough distance, it wouldn't have the necessary speed to ignite on impact.

Second, by doing it that way you'd be concentrating the powder in one place. That wouldn't be as effective as setting several smaller fires in different places on the ship. On the other hand, the damage to the spot where you did hit it would be more severe - perhaps it's just a matter of picking the right target.

Well, under the circumstances, it seems like a good enough idea to get started on it. You tell the greblings to put the shells they won't have time to use in the barrel. Now you just need to figure out where you're trying to move the barrel, and how you're going to get it there fast enough.