Admiralty is a set of free miniatures rules for the age of fighting sail. Its designed for 1/2400 miniatures. The authors write:

Admiralty is a set of miniature wargame rules that re-create battles from the age of the fighting sail. The game concentrates of the core features of period naval combat including sailing, cannon fire and boarding actions. The rules are geared to allow large battles to be fought over realistically scaled areas of “sea” as player struggle to pummel, dismast and capture their opponent’s vessels. The rules are split into six sections, each of which deal with a particular aspect of the game. Playing aids such as ship log, firing arc and the all-important combat chart can be gotten on the main Admiralty home page.