Speed Through the Water (STW) (STW)

A vessel’s speed relative to the water immediately around it.

A water log may measure STW from paddlewheel, electromagnetic, ultrasonic or pressure-based sensing. It differs from speed over the ground, which includes the effect of tidal stream or current and is normally derived from GNSS.

Local flow around the hull, fouling, bubbles, heel and calibration can affect the reading. Comparing STW with ground velocity can help estimate current, but only when direction, sensor errors and leeway are considered.