Navigation lights intended for use on power driven vessels may be different from navigation lights intended for use on sailing vessels. Although the horizontal arc of visibility is the same for all lights, the vertical divergence (i.e. vertical arc of visibility) requirements for lights on vessels under sail are larger to accommodate greater heeling. Manufacturer labeling may not discriminate between the different requirements. Navigation lights that claim compliance with the navigation rules1 may meet the vertical visibility requirements for a power-driven vessel, however, they may not comply with the vertical visibility standards for sailing vessels. Manufacturer labeling may not indicate that the lights are designed for use on power-driven vessels only.
Annex I (COLREGs section 10 and Inland 33 C.F.R. part 84.16 “Vertical sectors”) prescribes the degrees and intensities that navigation lights must meet on the vertical plane. Many boat owners may not be aware of the +/- 25° vertical light divergence requirement for sailing vessels, a 17.5° increase from the power-driven vessel standard. Installing a navigation light, designed for use on a power-driven vessel, on a sailing vessel may result in the light losing visibility when the vessel heels beyond the narrower +/- 7.5° vertical divergence angle established for power-driven vessels. A sailing vessel operator in this situation would likely not realize that the sailing vessel’s lights were not visible when heeling beyond 7.5°.
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