Work out the current acting on the boat. Compare where the boat is pointing and how fast it moves through the water with the course and speed the GPS shows over the ground. The difference is the current: its set (the direction it flows toward) and its drift (its speed).
On passage, to measure the current you are actually in rather than the tidal atlas estimate, and then to feed that into the course to steer for the next leg. It is also a good check on instruments: if the log is calibrated and there is no current, the two vectors should match.
Course through the water is the heading, corrected for leeway if you are sailing. Speed through the water comes from the log (paddlewheel). Course and speed over ground come from GPS. Average each over a few minutes rather than using a single reading.
Each motion is split into east and north components. The ground vector minus the water vector is the current.
current east = SOG × sin COG − STW × sin CTW
current north = SOG × cos COG − STW × cos CTW
drift = √(east² + north²); set = atan2(east, north)
Crossing a tidal stream. Heading 090° at 5 kn through the water, GPS shows 100° at 4.5 kn. Current: 0.97 kn setting 216.0°T.
Fair tide. 045° at 6 kn, GPS 045° at 7.5 kn: drift 1.50 kn, set 45.0°T. A current dead astern only adds speed.
Foul and across. 180° at 6 kn, GPS 195° at 4.8 kn: drift 1.84 kn, set 317.7°T.
| GPS course / speed | Drift (kn) | Set (°T) |
|---|---|---|
| 90° / 6 kn | 1.00 | 90.0 |
| 90° / 4 kn | 1.00 | 270.0 |
| 80° / 5 kn | 0.87 | 355.0 |
| 100° / 5 kn | 0.87 | 185.0 |
| 110° / 5.5 kn | 1.89 | 174.9 |
| 70° / 4.5 kn | 1.72 | 333.4 |
Toward. A current setting 180° flows to the south. Wind is the opposite: named for where it comes from.
Enter the set and drift into the course to steer calculator to correct your heading for the next leg.
Mixing magnetic and true courses invents a current. Keep both in true.