This is a replacement LED bulb for Aquasignal Series 20 bow and stern lights that used 9mm bayonet bulbs. These lights were commonly deck mounted on earlier 26X boats. If you have a rail mount bow light or a transom mount stern light this is not the right bulb replacement.
Navigation Light Requirements
Under sail at
night, light requirements:
The operator of a sailboat operating under sails at night
shall, from sunset to sunrise, display: sidelights (red - green) and. sternlight
(white). If less than 20 meters in length, the three lights may be combined at
or near the top of the mast.