The Trireme Battles blog offers a set of free wargames rules for trireme battles.
Rules: Naval
Naumachiae Ancients Naval Rules
Naumachiae is a set of rules for ancients naval warfare for 1/12000 miniatures.
La Felissima Armada Naval Rules
La Felissima Armada is a set of free wargames rules for naval skirmishes between England and Spain in the Elizabethan Age.
Island Mayhem Pirate Rules
Island Mayhem is a set of rules for playing games in which 25mm – 28mm Pirates are searching for treasure.
Shot and Sail Napoleonic Naval Rules
The prolific Jim Wallman offers a set of free wargames rules for Napoleonic Warfare called Shot and Sail. In addition to the pdf fo the rules, you’ll need the zipped graphics of ship cards.