YOUR SPEED. YOUR DEVICE.
Boat Speed Over Ground: What a GPS Reading Means
GPS speed on a boat is speed over ground (SOG): how fast the boat moves relative to the earth. Speed through the water (STW) is how fast the hull moves relative to the water around it, and that water may itself be moving. Running with a current, SOG is higher than STW. Running against it, SOG is lower.
XpeedTwo speeds, two references
The International Maritime Organization's performance standards for ship speed and distance devices treat these as separate measurements. Resolution A.824(19) refers to the forward speed of the ship "through the water or over the ground", and it requires equipment that can show either one to indicate which mode is selected.
The difference comes from current. NOAA's National Ocean Service describes currents as the movement of water from one place to another, driven by wind, differences in water density and tides. Tidal currents are strongest near shorelines and in bays and estuaries.
A GPS receiver measures position over the earth, so a phone using GPS reports SOG. STW needs an instrument that measures the water moving past the hull, such as a ship's log.
How current changes the GPS reading
Take a boat moving at 6 knots through the water:
- With a 2-knot current from directly astern, SOG is 8 knots.
- Against a 2-knot current from directly ahead, SOG is 4 knots.
- With the current from the side, the boat is carried sideways. Its path over the ground differs from its heading, and SOG falls between those two values.
The same boat can therefore show different GPS speeds on the outbound and return legs of one trip, even at the same engine setting.
Knots and nautical miles
NOAA defines one knot as one nautical mile per hour, roughly 1.15 statute miles per hour. A nautical mile was set at exactly 1,852 meters in 1929 and is based on one minute of latitude. One knot is therefore 1.852 km/h.
Xpeed's boat mode starts in knots and shows distance in nautical miles. Changing units only changes the display. The underlying measurement stays the same.
What a phone can tell you on the water
GPS.gov gives typical smartphone accuracy as 4.9 meters under open sky and lists signal blockage from buildings, bridges and trees among the factors that reduce it. Reflections from nearby structures also cause errors. Under a bridge or close to a tall harbor wall, expect a weaker reading.
A browser can supply speed directly from the device or leave it empty. Xpeed shows the device's speed when it is valid. When it is missing, Xpeed estimates speed from the change in accepted positions and labels it as estimated. A dash means there is no recent valid reading.
The phone gives no information about current, depth, heading or other vessels. It cannot tell you STW.
Which speed answers your question
SOG answers questions about progress over the earth. Divide the distance to a point on the chart by SOG to estimate time to get there, as long as the current and your speed stay the same.
STW describes how the boat moves through the water. Comparing it with SOG shows how much the current is helping or slowing you. That comparison needs a through-water instrument on board as well as GPS.
Limits of this tool
Xpeed's boat speedometer is a passenger reference. It is not a navigation instrument and has not been field-tested on the water. Use proper marine equipment, official charts and local navigation guidance for anything that affects safety.
Questions and answers
Is GPS speed the same as my boat's knotmeter?
Usually not. A GPS reports speed over ground. A knotmeter or log measures speed through the water. The two match only when the water is not moving.
Why does my GPS speed drop when I turn into the tide?
Heading into a tidal current, the water carries the boat backward relative to the earth. Your speed through the water stays similar, but your speed over ground falls by the strength of the current.
Can Xpeed show the speed of the current?
No. Working out current needs both speed over ground and speed through the water. A phone measures only the first.
How do I convert knots to km/h?
Multiply knots by 1.852. For example, 10 knots is 18.52 km/h. The speed converter does this for you.