Hi Rick, If you are under sail alone, then you need a red, green and stern white light as navigation lights. The single masthead lamp take care of the problem. But, ................... if you decide to use your motor, then you have to show different navigation lights. Then you need a red and green plus a white stern light, and above them - on the mast - a white light shining forward that tells others you are under power.................. At this point you can't use your masthead tricolor light. The steaming light facing forward has to show a white arc of 225 degrees - the same arc covered by the red and green navigation lights. The aft side of the tricolor masthead light is the white sector - your stern light. I has to cover an arc of 135 degrees (67.5 degrees either side of the center line of the boat) The red and green navigation lights cover an arc of 112.5 degrees, from center of boat to 22.5 degrees abaft the beam on either side. So the tricolor light has red and green sectors, each 112.5 degrees, and the stern light sector of 135 degrees. The Regulations (1984) state: "Vessels under 7 meters (22 feet), and vessels under oars: Stern light visible 2 miles; side lights visible 1 mile; or in lieu of these she shall have ready at hand an electric torch or white lantern to show in time to prevent a collision." So a white all around light is all you need on an M15, which would cover being under power or under sail. Connie