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.
galley
Capitana Wargames Rules For Galley Warfare In The Mediterranean
Ray Lucas offers Capitana, a set of naval rules specifically for 16th Century galley warfare in the Mediterranean. They were written for the NLWG 2009 convention game.
Nauta Ancient Naval Wargame Rules
Nauta is a set of free miniature wargames rules for playing out ancient naval battles. The files are hosted here:
15mm War Galleys
Here are a set of instructions for building 15mm models of ancient War Galleys from the Jackson Gamers.
Corvus Ancient Naval Rules
Corvus is a set of free wargames rules for fighting ancient sea battles.