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Deck: Rules of the Road

Question 1,608 of 1,025

BOTH INTERNATIONAL & INLAND You see a red sidelight bearing NW (315°). Which direction may the vessel be heading?

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Comments from mariners who studied this question.

Ti Tiatia81 · Dec 6, 2018
Confusing. How do I determine course with a single bearing? Also, is this relative or true?
ad adjective_noun · Dec 6, 2018
You know that a red sidelight shows from dead ahead to 22.5 degrees abaft the port beam. So they must be heading between 135 (the reciprocal of 315, this assumes you're looking at their bow), and 247.5, which would be if you're looking right at the intersection of where their red side light and stern light would be showing. 135 plus the 112.5 degree range of a sidelight gives you the 247.5. Only one answer falls between 135 and 247.5. True bearings.
go golivi · Dec 6, 2018
Tiatia81, for me these problems are best solved by drawing them out. Start with a sketch of your boat in the center (heading any direction), then scribe a line out 315 degrees (NW) true. Draw the other vessel pointed back down that bearing line (other ship pointing 135 degrees). On that heading you would actually see both red and green sidelights, so for you to only see the red sidelight the other vessel would need to be heading slightly to the right, or roughly 137 degrees. You know the sidelight has an arc of visibility of 112.5 degrees, so the other ship could be heading anywhere between 137+112.5=249.5 degrees. So the other ship could be headed anywhere between 137 and 249.5 degrees, and the only answer choice that falls into that arc is 180 degrees. Hope that helps.
Ti Tiatia81 · Dec 7, 2018
Thank you!