If your boat has no working speed log — or you just want a second reading — your phone can measure your speed over water in knots, using GPS, for free.

How to measure your boat's speed

  1. Open the Boat Speedometer on your phone.
  2. Tap Start and allow location access.
  3. Your speed shows in knots (you can switch to km/h or mph).
  4. Watch your live speed, plus the top and average speed of the trip.

Everything runs in your browser and your location never leaves your device.

Why knots?

A knot is one nautical mile per hour, the standard unit for boats and aircraft. One knot is about 1.85 km/h or 1.15 mph. This tool starts in knots for marine use, but you can switch units any time.

Does GPS work well on the water?

Excellently. Open water gives an unobstructed view of the sky, so GPS gets a strong, steady signal and your speed reading is very accurate — usually within a fraction of a knot when you're moving steadily.

Speed over ground vs speed through water

One thing to know: GPS measures your speed over the ground (relative to the seabed/land). A traditional paddlewheel log measures speed through the water. In a current, these differ — for example, with a following current your GPS speed will be higher than your speed through the water, and against a current it'll be lower. For navigation and trip planning, GPS speed over ground is usually what you want.

What it tracks

Try it on your next outing — open the free boat speedometer and tap Start.