Reading Boat Speed in Knots
Knots are nautical miles per hour, not a mysterious unit. Here is what boaters need to know about GPS speed-over-ground.
What a knot actually is
A knot is one nautical mile per hour. A nautical mile (about 1.15 standard miles, or 1.852 km) was originally based on one minute of latitude, which is why it remains the standard unit for marine and aviation navigation worldwide.
GPS speed-over-ground vs. speed-through-water
A GPS gives you speed-over-ground: how fast you are actually moving across the earth's surface. A paddlewheel or other through-water sensor measures speed relative to the water itself. In a current, these two numbers can differ — GPS reflects your true progress, while a through-water sensor reflects your speed relative to moving water.
Why this matters
- Fighting a current: your speed-through-water can read higher than your actual progress (GPS speed-over-ground)
- Riding a current: the reverse can happen, with GPS showing you moving faster than your engine effort alone would suggest
- For trip planning and ETAs, speed-over-ground (GPS) is what actually matters
Getting a clean GPS fix on water
Open water generally gives an excellent GPS signal, with few of the obstruction issues that affect GPS on land. Mount your device where it has a clear sky view, away from large metal structures that can interfere with reception.
Try it yourself
Boat Speedometer — free, no install, works in your browser.
Frequently asked questions
Why does my GPS speed differ from my boat's speedo?
Your onboard speedo usually measures speed-through-water, while GPS measures speed-over-ground — currents cause the two to differ.