A DR runs on the course steered and the speed through the water. When the water itself is moving, the next fix lands somewhere else. The difference is the current: its set is the direction it flows, from the DR toward the fix; its drift is how far it carried you; its rate is drift ÷ hours since the DR was last set from a fix.

DR at noon (run from the 0600 fix): N 37°20.0′, W 066°10.0′. Noon fix: N 37°22.4′, W 066°00.0′.

From the DR to the fix: 073°, 8.3 nm. Over 6 hours that is a current setting 073° at 1.4 knots.

A DR with the current allowed for is an estimated position (EP). And when you know the current, you can steer to beat it: head up into it, so that it carries you back onto the line you want.

One hour, to scale. Draw the current from the start; from its end, swing the boat’s speed round until it meets the track. That line is the course to steer.

Track to make good: 160°
Head up into the current by: 19.5°
Course to steer: 179°
Speed made good: 5.7 kn
Steer the track instead and drift off it: 2.0 nm an hour

sin a = rate × sin(set − track) ÷ boat speed. Steer the track minus a: into the current.

Current sets: 070°Current rate: 2.0 knBoat speed: 6 kn

sin a = rate × sin(set − track) ÷ boat speed; course to steer = track − a

Track 160°, boat 6 knots, current 070° at 2 knots: sin a = 2 × sin(70° − 160°) ÷ 6 = −0.333, so a = −19.5°.

Steer 160° − (−19.5°) = 179.5°, about 180°. Speed made good: 6 × cos 19.5° + 2 × cos(−90°) = 5.7 knots.

A running fix is also moved by the current between its sights: the line you advance by course and speed alone is off by the drift. That is why a current worked from a running fix is close, but not exact.

Currents: set, drift and the course to steer