You are enroute to assist vessel A. Vessel A is underway at 6 knots on course 133°T, and bears 343°T at 92 miles from you. What is the course to steer at 9 knots to intercept vessel A?
• Relative motion between two moving vessels • Solving for an intercept course using speeds and bearings • Using the law of sines or vector components in a speed triangle
• How can you represent both vessels’ motions (courses and speeds) and the line of sight between them on a relative motion (speed) diagram? • If you travel at 9 knots and vessel A at 6 knots, what must be true about the relative velocity vector so that you meet at the same point at the same time? • How does the initial bearing (343°T at 92 miles) help you set up the geometry to find the required course angle?
• Be sure you are measuring bearings and courses from true north (000°T) and using the correct directions on your diagram. • Check that your final course makes sense with respect to the initial bearing to vessel A (you must steer generally toward where it will be, not where it is now). • Verify that the ratio of speeds (9 knots vs 6 knots) and the geometry of your speed triangle are consistent (no impossible triangle).
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