Your vessel is on a course of 297° T at 11 knots. At 0019 a light bears 274.5° T and at 0048 the light bears 252° T. At what time and at what distance off will your vessel be when abeam of the light?
• Relative motion of bearings when passing a fixed object (light) on one side • Using time between bearing changes to find closest point of approach (CPA) and time of abeam • Speed–time–distance relationships at a constant speed of 11 knots
• Are the bearings of the light drawing ahead or abaft, and what does that tell you about whether you have already passed abeam? • How much time elapses between the first and second bearing, and how far do you travel in that time at 11 knots? • How can you use the change in bearing to construct a right triangle that gives you the distance off at abeam and the time from one of the observed positions to abeam?
• Confirm that speed in knots × hours = distance in nautical miles is used correctly for each time interval. • Check that your assumed geometry makes the bearing at abeam exactly 90° off your ship’s heading (i.e., either 027° T or 207° T in this case). • Verify that the time you compute for abeam is after the first and second observations, consistent with how the bearings are changing.
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