Here’s a very involved tutorial on making a pre dreadnought battleship from scratch.
De Bellis Navalibis is a set of free wargames rules for playing ancients naval wargames.
Iron and Steam is a set of free wargames rules for pre-dreadnought Victorian naval actions.
Iron and Steam is a set of miniatures rules for naval combat of the American Civil War. The rules feature a gun-by-gun combat resolution, written orders and simultaneous movement. Traditionally played with 1/600 scale models the combat scale is closer to 1/2400 scale, though the game is generally model scale agnostic. Also included are over 100 pre-filled ship record sheets, turning radii and arc-of-fire templates as well as a 2 page QRS to facilitate fire resolution.
By Jeff Peach & Les Booth offer a set of free wargames rules for ACW River and Coastal Warfare
Trierarch is a set of free wargames rules for naval warfare in the classical era. The game is based on DBA, with measurements expressed as “ship lengths.”