Your vessel is on a course of 255° T, at 14 knots. At 2126 a lighthouse is sighted dead ahead at a distance of 11 miles. You change course at this time to pass the lighthouse 3 miles abeam to port. What will be your ETA at this position off the lighthouse?
• Relative motion between your vessel and a fixed object (the lighthouse) • Using right triangles to model closest point of approach (CPA) with an abeam distance • Speed–time–distance relationship at a constant speed of 14 knots
• Sketch the situation from above: mark your position at 2126, the lighthouse dead ahead at 11 miles, and the point where you will pass 3 miles abeam to port. What geometric shape does that form? • How can you use the fact that the CPA to the lighthouse is 3 miles (abeam) and the current range is 11 miles to find how far you will travel along your new course before reaching CPA? • Once you know the distance you must run from the time of course change to the abeam position, how do you convert that distance at 14 knots into minutes to add to 2126?
• Be sure you’re treating the 3 miles abeam as a perpendicular distance from your track line to the lighthouse, not as distance along the track • Confirm you’re using the Pythagorean theorem with the correct sides: identify which side is the hypotenuse and which sides are legs of the right triangle • After finding the run distance, carefully compute Time = Distance / Speed at 14 knots and add that to 2126 without arithmetic errors
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