Hello, I'm not really a naval tactician. Land Battles are my forte, but I'm getting to be an alright sailor! :P I've found that using the smaller 'galley' & 'light galley' as literal suicide ships, is very useful, especially against larger fleets on a hard/very hard difficulty level. Simply keep them behind the target ship's line of fire, then pull up right next to it, and await a broadside. Being so small, this will more than likely cause the ship to explode, catching the target ship alight as well. It doesn't always work, but it at least damages the ship quite a bit.
It's quite simple actually, I plan to kill them all, then go back for the rum. Aye?