Speed Over Ground vs Speed Through Water: A Practical Guide for Recreational Sailors
Speed over ground (SOG) is how fast your boat moves relative to the earth, measured by GPS. Speed through water (STW) is how fast the hull moves through the water, measured by a log. The difference reveals current, leeway, and other factors that affect passage planning, fuel range, and collision avoidance. Understanding both is essential for accurate dead reckoning and safer coastal cruising.