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.
naval
Fleet of Battle Ancient Galleys Wargames Rules
Fleet of Battle is a set of free wargames rules for playing out battles between fleets of ancient galleys. Think Ben Hur. There also are several pages of free print-and-play galleys for use in lieu of lead miniatures. The rules require a knowledge of the commercial Field of Battle wargames rules.
Ironclad Draughts One Page Ironclad Rules
Ironclad Draughts is a set of one-page rules for playing out battles between ironclads.
Periplus Ancient Naval Game
Periplus is a free two player wargame that abstracts a battle between two fleets of ancient greek warships.
Wooden Ships and Iron Men Lite Rules
Wooden Ships and Iron Men Lite is a set of free wargames rules The author writes that Wooden Ships and Iron Men Lite are:
A rules set that focuses on fighting small squadron actions in the late 18th and early 19th Century using Pirates of the Spanish Main/Crimson Coast/Revolution (hereafter called PoX) ship models. I hope this variant is almost as simple and as colorful as Battle Cry and Memoir ’44 and captures the spirit of the Age of Fighting Sail