Tap start and the page reads your live speed straight from your phone's GPS. The display toggles between mph and km/h, but sailors and powerboaters usually think in knots: one knot is one nautical mile per hour, equal to 1.852 km/h or about 1.151 mph. If you want to convert your reading, use the Knots to MPH and Knots to KPH converters linked below. Everything runs in your browser, so once the page has loaded you do not need cell signal to keep reading speed offshore.
SOG vs STW: the reading that actually matters
Traditional boat logs measure Speed Through Water (STW): a paddlewheel or pitot sensor in the hull senses how fast water is flowing past it. GPS measures Speed Over Ground (SOG): how fast your fixed position is changing across the planet. On a still pond the two numbers match. On real water they rarely do, because the water itself is moving.
Picture motoring into a two-knot tidal current while your log reads six knots through the water. Your true progress over the seabed is only four knots SOG. Turn around and run with that same current and your STW still reads six, but your SOG jumps to eight. For passage planning, ETA, and slack-water timing, SOG is the number that tells you when you will actually arrive. STW still matters for trimming a sailboat, but GPS settles the question of real-world progress.
Why a paddlewheel log lets you down
Through-hull speed sensors are mechanical and they live underwater, which is a hard life. Paddlewheels jam with weed, barnacles, and plastic; their bearings wear and start to under-read. Pitot tubes clog. Any of these means a log that reads low, reads erratically, or stops spinning altogether, often without you noticing until your distance run looks wrong.
A GPS speed readout has no moving parts in the water and nothing to foul. It is the obvious backup when your transducer fails mid-passage, and on a tender, RIB, or dinghy that never had a log fitted, it is simply the only speed instrument you have.
Who this is for on the water
Small craft are the sweet spot: aluminum tinnies and tenders with a bare console, inflatable RIBs, kayaks, canoes, and paddleboards where you just want to know if you are making three knots or five against the wind. Jet ski and PWC riders use it as a simple speed check, and sailing-dinghy crews like a number to chase upwind.
Paddlers in particular benefit because there is no instrument option at all on a kayak or SUP short of buying a dedicated unit. A phone in a clip on the deck gives you pace, and after the trip you know exactly how far and how fast you went.
Accuracy out on open water
GPS likes an open sky, and there is nowhere with a clearer view of the satellites than the middle of a bay or a lake. With no trees, buildings, or canyon walls to block the signal, marine GPS speed is typically accurate to better than two knots, often within half a knot to one knot once you have a steady fix and are moving.
Give the phone ten or twenty seconds after you start to settle on a position, and read speed while underway rather than drifting; at a near standstill GPS noise can make the number twitch even though you are barely moving. Marinas with tall masts and sheds nearby are the one place the fix can wander, so trust the reading more once you are out in open water.
Keeping your phone safe and signal-free navigation
A phone is not marine hardware. Drop it in a waterproof case or a sealed mount before you rely on it on deck, and clip or lanyard it so a wave or a wake cannot send it overboard. Treat spray, not just immersion, as the threat.
The good news is that GPS itself needs no phone signal. The satellites are global and broadcast everywhere, so once this page is open the speed readout keeps working far beyond cell range. To be safe, load the page at the dock while you still have data, then head out.
A speed readout, not a chartplotter
Be clear about what this is. It tells you how fast you are going and nothing more. It does not show depth, charts, hazards, tides, or your position on a map, and it is no substitute for a proper chartplotter, paper charts, or the basic safety gear every boat should carry.
Use it the way you would a handheld speed check: a quick, reliable number for your tender, your paddle craft, or as a backup when the boat's own log quits. For real navigation offshore, keep dedicated equipment aboard.
Frequently asked questions
What is the difference between SOG and STW?
Speed Over Ground (SOG) is how fast you are actually moving across the seabed, measured by GPS. Speed Through Water (STW) is how fast water flows past your hull, measured by a paddlewheel or pitot log. When there is current or tide they differ: against a two-knot current your STW reads two knots higher than your true SOG, and with the current behind you it reads two knots lower.
Does this show speed in knots?
The live display toggles between mph and km/h. For knots, read your speed here and convert it: one knot equals 1.852 km/h or roughly 1.151 mph. The Knots to MPH and Knots to KPH converters linked below do the math for you. Many dinghy, kayak, and jet ski users are happy thinking in mph anyway.
Will it work offshore with no phone signal?
Yes. GPS is a separate satellite system from your cell network, and those satellites cover the whole planet. Once this page has loaded, the speed readout keeps working with no signal, so load it at the dock and you are set for the trip out.
How accurate is GPS speed on the water?
Very accurate, because open water gives the clearest possible view of the sky. Expect better than two knots of error, often within half a knot to one knot once you have a steady fix and are moving. Accuracy is best underway; at a near standstill the number can twitch from GPS noise.
Can I use it as a backup if my boat's log fails?
Absolutely, and that is one of its best uses. Paddlewheels foul with weed and barnacles, bearings wear, and pitot tubes clog. A GPS readout has nothing in the water to foul, so when your transducer under-reads or stops, you still have a trustworthy speed over ground.
Is my phone okay to use on a boat?
Use a waterproof case or sealed mount and a lanyard or clip so it cannot go overboard. A bare phone is not built for spray or immersion. Protected and secured, it makes a fine speed readout for a tender, kayak, RIB, or jet ski, though it is not a replacement for a chartplotter.