You wish to make good a course of 230°T while turning for an engine speed of 12.5 knots. The set is 180°T, and the drift is 1.7 knots. What course should you steer?
• Current set and drift and how they affect your course over ground versus your heading through the water • The current triangle / current vector diagram (one side is ship’s speed through the water, one side is set & drift, and the resultant is course and speed made good) • Converting between course to steer (CTS) and course made good (CMG) when current is pushing you from a known direction
• Sketch the vector triangle: start with the desired track (230°T) and speed made good, then add the current (set 180°T, drift 1.7 kn), and see which way your heading must point so the resultant lies along 230°T • Think about where a current from 180°T pushes your vessel: to the left or right of your desired track? Then decide whether you must steer into or away from the current to stay on 230°T • Check whether the correction in course should be a small or large angle, given that the current (1.7 kn) is relatively small compared with your speed through the water (12.5 kn)
• Confirm that you are using speed through the water (12.5 kn), not speed over ground, for the ship’s vector in your triangle • Make sure the current vector (set 180°T, 1.7 kn) is drawn in the correct direction (toward 180°T, not from 180°T) • After estimating the angle of correction, verify that the chosen answer would give a resultant course made good close to 230°T when you mentally (or roughly) add the vectors
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