How fast the boat moves over the seabed, measured by GPS, the speed that decides when you arrive.
SOG (speed over ground) is the boat's speed relative to the seabed, as measured by GPS. It includes everything that moves you: sails, engine and any current. That makes it the speed that matters for passage planning and arrival times, but not for judging how well the boat sails.
SOG and speed through the water
Speed through the water (STW) comes from the log and measures the boat's speed relative to the water around it. The difference between the two is the current:
- SOG higher than STW: the current is pushing you along.
- SOG lower than STW: you're sailing against the current.
Comparing both, together with COG and heading, gives the current's set and drift.
Good to know
SOG is given in knots. Because it's derived from successive GPS positions, it gets jumpy at very low speeds, for example at anchor or when manoeuvring in harbour.
Manta does this for you
Sailing trip charts for wind, depth, speed and current Pro
Every trip comes with charts of everything Manta recorded, from wind and depth to heel, current, waves and performance.
