Hold your intended track when a current is pushing you sideways. Enter the track you want, your speed through the water, and the current’s set and drift; get the heading to steer, the correction angle, and your speed over ground.
Use this when planning a leg across a tidal stream or river current and you want to steer a heading that keeps you on the line to your waypoint instead of being carried off it.
Set is the direction the current flows toward; drift is its speed in knots — from tidal current tables, a current atlas, or your own observed set and drift. Track and set should use the same reference (both true or both magnetic).
If the current changes during the leg, split the leg into parts and solve each one.
Split the current into two parts relative to your track: the part pushing you across the track and the part pushing you along it. To stay on the track, point the bow up-current just enough that the cross-track part of your own speed cancels the cross-track current. That angle is the arcsine of cross-current divided by boat speed.
Speed over ground is what is left of your boat speed along the track after crabbing, plus (or minus) the along-track current. A fair current speeds you up; a foul current slows you down — and the crab angle always costs a little speed.
cross = drift × sin(set − track) · along = drift × cos(set − track)
correction = asin(−cross / boat speed)
heading = track + correction
SOG = boat speed × cos(correction) + along · time = distance / SOG
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| Track | Speed | Set | Drift | Steer | Correction | SOG |
|---|---|---|---|---|---|---|
| 90 | 6 | 180 | 1.5 | 076° | −14.5° | 5.81 |
| 90 | 6 | 0 | 1.5 | 104° | +14.5° | 5.81 |
| 90 | 6 | 90 | 1.5 | 090° | +0.0° | 7.50 |
| 90 | 6 | 270 | 1.5 | 090° | −0.0° | 4.50 |
| 45 | 5 | 135 | 2 | 021° | −23.6° | 4.58 |
| 200 | 7 | 250 | 1 | 194° | −6.3° | 7.60 |
With the calculator defaults (Desired track (°) = 90; Boat speed through water (kn) = 6; Current set (° toward) = 180; Current drift (kn) = 1.5; Leg distance (nm, optional) = 12), the arithmetic runs like this:
cross-track current = drift × sin(set − track) = 1.5 × sin(90°) = 1.500 kn
along-track current = drift × cos(set − track) = 0.000 kn
correction = asin(−cross / speed) = asin(-0.2500) = -14.48°
heading = track + correction = 90 + -14.48 = 75.5°
SOG = speed × cos(correction) + along = 5.809 + 0.000 = 5.81 kn
time = 12 / 5.809 h = 2 h 04 min
Result: 076° heading to steer. Correction −14.5° (to port of the track). Speed over ground 5.81 kn. 12 nm takes about 2 h 04 min.
Now change Current drift (kn) to 3 and leave everything else alone:
cross-track current = drift × sin(set − track) = 3 × sin(90°) = 3.000 kn
along-track current = drift × cos(set − track) = 0.000 kn
correction = asin(−cross / speed) = asin(-0.5000) = -30.00°
heading = track + correction = 90 + -30.00 = 60.0°
SOG = speed × cos(correction) + along = 5.196 + 0.000 = 5.20 kn
time = 12 / 5.196 h = 2 h 19 min
Result: 060° heading to steer. Correction −30.0° (to port of the track). Speed over ground 5.20 kn. 12 nm takes about 2 h 19 min.
Doubling the drift to 3 kn more than doubles the correction (about 30° instead of 14.5°) and cuts speed over ground to about 5.2 kn. Strong cross-currents get expensive quickly; timing a crossing for slack water often saves more than steering cleverly.
Current tables give predicted set and drift at reference stations; local headlands and channels can be very different. Watch a buoy or a lobster-pot float as you pass — its wake shows the real set — and update the drift if your track starts to wander.
On a long leg through a changing tidal stream, it is often better to steer one constant heading worked out from the net current over the whole leg than to chase the track hour by hour. The boat is carried off the line and back again, but sails a shorter distance through the water.
Leeway is separate. A sailboat close-hauled or a high-sided powerboat in a crosswind slides sideways through the water, and that needs its own correction on top of the current.
Units: Degrees for directions, knots for speeds, nautical miles for distance.
Where the model stops: Constant current and boat speed over the leg. Leeway, changing tidal streams, and steering error are not modeled.
Not a substitute: Use current predictions, charts, and observed position fixes. Keep a margin off hazards on the down-current side of the track.
| Current (kn) | Boat 4 kn | Boat 6 kn | Boat 8 kn |
|---|---|---|---|
| 0.5 | 7.2° | 4.8° | 3.6° |
| 1 | 14.5° | 9.6° | 7.2° |
| 1.5 | 22.0° | 14.5° | 10.8° |
| 2 | 30.0° | 19.5° | 14.5° |
| 3 | 48.6° | 30.0° | 22.0° |
Degrees to steer up into a current at right angles to the track. Slow boats need much bigger corrections, and the correction grows faster than the current.
Set is the direction the current flows toward, in degrees. Drift is its speed in knots.
Either, as long as track and set use the same reference. Convert the resulting heading to compass with variation and deviation if needed.
Crabbing into the current points part of your speed across the track, so less of it moves you along the track.
That page works backwards from where you went to find the current. This one starts from a known current and tells you how to steer.