How fast the boat moves through the water around it, measured by the log, the speed the sails and hull actually produce.
STW (speed through the water, often just boat speed) is the boat's speed relative to the surrounding water. It's measured by the log: a small paddlewheel or an ultrasonic sensor in the hull. Because current moves the water and the boat together, STW shows what the sails, hull and engine produce on their own.
Why it matters
- Performance. STW is the honest measure of how well the boat sails. Polars and VMG are based on it, not on GPS speed.
- True wind. Instruments calculate the true wind from the apparent wind and STW.
- Current. Compared with SOG, it reveals the current.
Calibration
A log needs calibrating: paddlewheels foul with weed and growth, and the flow around the hull differs from tack to tack. A log reading 10 % low makes every true-wind and polar value wrong with it. The classic check is to motor a measured distance in both directions in slack water and compare with GPS.
Shown in knots; in NMEA 0183 it travels in the VHW sentence, in NMEA 2000 in PGN 128259.
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.
