I'm assuming you mean navAl combat; navEl combat probably wants a different forum.
I made up some rules for ship-vs.-ship stuff a long time ago. The main thing you need to realize is that a "combat round" is going to be SLOW - quite possibly several minutes long - as large sailing ships are often slow and always unmaneuverable; this applies until-unless someone gets boarded and hand-to-hand fighting begins. Also, unless the sea is flat calm (almost never) any spellcasting should be at risk of failure and any ship-to-ship missile fire by individuals should be at minuses; less so if the shooter is a trained sailor.
For ship's weaponry I assume ballista fire instead of cannon as my game doesn't have gunpowder. Then you just need to assign odds of hitting the target ship based on range, size, speed, etc., then if shot hits where the hit occurs and what happens to that ship (if anything), which can be one of:
- rigging or spar or mast damaged (or, very rarely, dismasting; with consequent damage to other things/people)
- sail(s) punctured or torn or damaged
- hull punctured (above or below waterline)
- one or more sailors hurt or killed
- direct hit on a significant individual (e.g. captain, helmsman, etc.)
- one or more ballistae/cannons disabled
I give a bonus to hit if a ship's fire is being co-ordinated by a gunnery officer, and a (stacking) penalty if the sea is particularly rough.
Lan-"hope this helps as a start"-efan
