Togo is a set of free wargames rules for naval battles during the 1904 – 1905 Russo-Japanese war.
Rules: Naval
Schnell Rules For Schnell Boote!
Schnell Rules is a set of fast play rules for WW2 coastal forces.
American Civil War Riverine Actions
Mike Callan offers this set of free wargames rules for American Civil War Ironclads. The set originally was published in Wargames Illustrated 14.
Trireme Ancient Naval Rules
Trireme, from Andy Watkins, is a set of free naval wargames rules. These are of the one-page variety, which means that there is much to recommend them. I also like the staggered movement system based on boat speed, which reminds me of the old Car Wars system.
Tough Men With Iron Balls: Piquet For Age of Sail
Tough Men With Iron Balls is a set of free wargames rules for fighting 7 Years War and Napoleonic Naval Battles.