Navigation lights aimed for use on power driven vessels may differ from navigation lights intended for use on sailing vessels. Despite the fact that the horizontal arc of visibility is the same for all lights, the vertical divergence requirements for lights on vessels under sail are larger to accommodate greater heeling.

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However, manufacturer labeling may not discriminate between the different requirements. Navigation lights that claim to comply with the navigation rules may meet the vertical visibility requirements for a power driven vessel, but they may not comply with the vertical visibility standards for sailing vessels. Manufacturer labeling may not tell 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 know the +/- 25 degree vertical light divergence requirement for sailing vessels, a 17.5 degree increase from the power-driven vessel standard. Installing a navigation light, designed for use on a power driven vessel, on a sailing vessel may lead to the light losing visibility when the ship heels beyond the narrower +/- 7.5 degree vertical divergence angle established for power-driven vessels. A sailing vessel operator in this situation would likely not know that the sailing vessel’s lights were not visible when heeling beyond 7.5 degree.

This means that if your sailboat does not have the correct lights and it heels over a certain degree, it may not be seen by other vessel operators.

This important as operators may not know that other ships cannot see them because of the heel of the ship. Failure to operate with the correct navigation lights may create a situation where operators mistakenly believe another mariner can ascertain the vessel’s aspect or operational condition, which increases risk of collision.

USCG also add that although a navigation light designed for a sailing vessel will not meet the vertical visibility requirements for a power driven vessel, it does not considered as a commensurate safety concern.

In addition, manufacturers must be aware of the larger vertical visibility requirement for lights installed on sailing vessels. Similarly, sailing vessel operators and vessel repair facilities should make sure that the installed lights comply with the applicable requirements in Annex I.

As not all navigation lights are designed in the same way, operators must ensure that when they install a navigation light, it should be USCG certified for the length and type of boat. Such information should be readily available from reputable sources such as the light or vessel manufacturer.

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