Drift
The speed at which a current is moving; the magnitude component of set & drift.
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In depth#
How this term is used in the rules, and where it tends to trip people up on the exam.
Drift is speed, not direction
In current calculations, drift is the current's speed. Set supplies the direction toward which the water flows. Keeping those two quantities separate prevents a common plotting error: a current setting 090° at 1.5 knots moves the vessel eastward at 1.5 nautical miles per hour.
Applying drift to a navigation plot
Multiply drift by elapsed time to find the length of the current vector. A drift of 1.5 knots acting for two hours produces 3 nautical miles of displacement. Plot that distance in the direction of set from the DR position; the endpoint is the estimated position.
Exam questions#
Is drift the same as set?
No. Drift is the speed of the current, while set is the direction toward which the current flows.
How far will a 2-knot drift move a vessel in 30 minutes?
One nautical mile. Multiply 2 nautical miles per hour by 0.5 hour.
Sources#
- NGA Pub. No. 9 — The American Practical Navigator, Vol. I, §803 (retrieved 2026-08-09)
Related terms#
See Drift in a real exam question.
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