Speed Through Water (STW)

The velocity at which a vessel moves relative to the water it is travelling through, excluding the effects of current or tidal flow.

Speed through water refers to the rate at which a vessel progresses through the water itself, as opposed to its speed over the ground, which includes the influence of currents and tides. It is a critical measurement for navigation, fuel efficiency, and performance assessment. This speed is typically measured using a log, such as an impeller log or electromagnetic sensor, which detects the movement of water past the hull. Unlike speed over ground, which is often determined via GPS, speed through water provides insight into how the vessel is interacting with the water mass it is in. For example, if a vessel is moving at 6 knots through the water but there is a 2-knot current against it, its speed over ground would be only 4 knots. Understanding this distinction is essential for accurate passage planning, collision avoidance, and maintaining optimal engine performance. Speed through water is also used when calculating true wind speed and direction, as it helps correct for the vessel’s motion through the water.