Chart Navigation & Plotting · Set & Drift
The difference between a vessel's course/speed through the water and its course/speed made good over the ground is caused by:
- ACompass deviation
- BChart projection error
- CSet and drift of the current (and leeway)✓ Correct
- DIncorrect variation
Explanation
The current (set and drift) plus wind-driven leeway push the vessel off its water track, so the course over ground (track) and speed over ground differ from heading and water speed. The vector triangle of water track, current, and ground track resolves the difference.
Authority: Bowditch, Pub. No. 9 (current triangle)
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