The direction (set) and speed (drift) of a current, and how the moving water carries the boat off its course.
A current is described by two values: its set, the direction it flows toward, and its drift, its speed in knots. A current setting 090° at 1.5 knots carries everything floating in it eastward by 1.5 nautical miles every hour. Note the difference to wind, whose direction is the one it comes from.
How it affects the boat
The boat moves through the water, and the water moves over the ground. Both add up as vectors:
course and speed through the water + set and drift = course and speed over ground
- Current from astern: faster over the ground, same direction.
- Current from ahead: slower over the ground.
- Current from the side: the boat is set off its course, sometimes by a lot at low speed.
Measuring it on board
With a GPS and a log you can calculate the current directly: compare speed and course over ground with speed through the water and heading, corrected for leeway. What's left over is the current. Manta does this with your instrument data: speed over ground against speed through the water reveals the current.
For planning, tidal stream atlases and current charts give the expected set and drift by hour and place.
Manta does this for you
Plotter data on your iPhone over WiFi
Connect your plotter or a WiFi gateway, and Manta shows your instruments live and records wind, depth and speed through the water on every trip.
