Step onto a boat and speed suddenly gets measured in knots instead of mph or km/h — a unit almost nobody uses on land. There's a good navigational reason for that, and a second, less obvious distinction that matters even more once you're actually on the water: the difference between how fast you're moving and how fast you're getting somewhere.
What is a knot, and why sailors use it
A knot is one nautical mile per hour. A nautical mile, in turn, is defined as exactly 1,852 metres — and that specific number isn't arbitrary, it's one minute of arc of latitude along a meridian of the Earth. That connection to latitude is exactly why mariners adopted it centuries ago: a vessel's speed in knots ties directly to the same lines of latitude and longitude used on a nautical chart, making distance and time-of-arrival calculations far simpler than converting through an unrelated land-based unit. The convention has simply stuck, even with GPS doing all the heavy lifting today.
For reference: 1 knot equals exactly 1.852 km/h, or about 1.15078 mph. A 20-knot boat is doing roughly 37 km/h or 23 mph.
Speed through water vs. speed over ground
This is the distinction that catches a lot of people out. Traditional instruments — a paddle wheel sensor or a pitot-style log — measure your speed relative to the water around the hull. But water itself moves: tidal currents, river flow, and wind-driven surface drift all shift the water mass you're floating in. If you're doing 8 knots through the water but a 2-knot current is pushing against you, your actual progress over the seabed is only 6 knots. With the current behind you instead, it's 10.
GPS sidesteps this entirely because it measures your position relative to the fixed Earth, not relative to the moving water — what's properly called speed over ground. That's a genuinely different (and, for navigation and arrival-time planning, usually more useful) number than speed through water, and it's exactly what a GPS-based boat speed checker reports.
Typical boat speeds
Displacement-hull vessels — most sailboats and trawlers — are physically limited by hull speed, a function of hull length: longer waterlines support higher top speeds, but most cruising sailboats top out somewhere around 6 to 9 knots under power. Planing powerboats, which lift partly out of the water at speed, can comfortably exceed 25 to 40 knots, and high-performance offshore raceboats go considerably faster still. Knowing your boat's realistic displacement-speed ceiling helps you recognise when a GPS reading that seems "too low" is actually just physics, not a faulty sensor.
Using GPS as a boat speedometer
A phone-based GPS speedometer needs no paddle wheel, no through-hull fitting, and no calibration — it works the moment you have a clear view of the sky, on any boat from a kayak to a yacht. Because it reports speed over ground rather than speed through water, it's also the number that matters most for working out your actual time to the next waypoint or harbour.
The takeaway
Knots exist because they tie cleanly to nautical charts, not because the underlying physics of speed is any different on water. The more practically important distinction is speed through water versus speed over ground — and GPS, including this site's boat speed checker, always gives you the over-ground number, current and all.