The appropriate number of lights to use is a personal choice and depends on desired brightness and budget. There are no hard and fast rules to apply here. SHADOW-CASTER underwater LED lights have a very low profile and produce a wide beam pattern. Generally, Shadow-Caster recommends 2 lights mounted on opposite sides of the transom for boats up to 21 ft in length. As the size of the boat and beam increase, it is recommended that more lights be installed for continuous coverage. Usually lights are added 1 for every additional 1meter/3ft of transom width over 3meters/9ft. They can also be installed on the sides of boats for users that want the extra light coverage.