A GPS speedometer on a boat measures speed over ground (SOG): how fast you’re moving past the seabed, the shore and the buoys. A paddle wheel, pitot tube or log measures speed through water (STW): how fast the hull moves through the water around it. In still water the two match. In a current, SOG is your speed through the water plus or minus the current, so if your log says 6 knots and the GPS says 4, you’re pushing into about 2 knots of current.
Neither number is wrong. They answer different questions, and most of the “error” people blame on a GPS or a boat speedometer is just the gap between them.
What’s the difference between SOG and STW? #
| Speed over ground (SOG) | Speed through water (STW) | |
|---|---|---|
| Measured by | GPS, from satellite signals | Paddle wheel, pitot tube or ultrasonic log |
| Tells you | Real progress toward where you’re going | How fast the hull moves through the water |
| Changed by current and tide | Yes | No |
| Changed by wind drift and leeway | Yes | Mostly no |
| Changed by weed, growth, bubbles | No | Yes |
| Distance it gives | Over the ground | Through the water |
| Best for | ETAs, fuel range, distance covered | Trim, sail trim, hull and prop performance |
Chartplotters with both sources often show them side by side. Most small boats, kayaks and dinghies have only one, and these days it’s usually the GPS in someone’s phone.
A simple way to picture it is an airport moving walkway. Walk against it and your legs do the same work while you cover less floor. The water sensor feels your walking speed; GPS sees the floor.
How does current change your GPS speed? #
Take a boat doing 6 knots through the water with a 2-knot current:
| Current relative to your course | Log (STW) | GPS (SOG) |
|---|---|---|
| Slack water | 6 kn | 6 kn |
| Straight against you | 6 kn | 4 kn |
| Straight behind you | 6 kn | 8 kn |
| Directly across, bow held on the same heading | 6 kn | about 6.3 kn, and you slide sideways |
The crosscurrent catches people out. Your speed barely changes, but your track drifts away from where the bow points. At 6 knots, a 2-knot beam current puts your course over ground about 18 degrees off your heading, which over an hour sets you roughly 2 nautical miles off your line.
Wind does the same to a light boat or a sailboat making leeway. It pushes you across the water, and GPS counts that as real movement, because it is.
Set, drift, heading and course over ground #
Navigators describe a current with two words. Set is the direction the current flows toward, and drift is its speed in knots. Heading is where the bow points, which your compass shows. Course over ground (COG) is the direction you’re actually moving across the bottom, which GPS reports. The angle between heading and COG tells you how hard current and wind are pushing you.
On a sailboat, that angle has two sources: current and leeway, the sideways slip the wind causes. You can separate them by comparing a GPS track in slack water (leeway only) with one in a known current.
Which speed should you use? #
- Arrival time: SOG. Distance remaining divided by speed over ground gives a realistic ETA, as long as the current doesn’t change on the way.
- Fuel range: both. Fuel burns by the hour at a given throttle, and how far that hour takes you depends on SOG. Punching into a current at 20 knots through the water but 16 over the ground, you use 25% more fuel per mile than the water-speed gauge suggests.
- Trim, prop and sail performance: STW. That’s what the hull and sails feel.
- Trolling: a lure reacts to water flowing past it, which is STW, so the same GPS speed runs a lure faster into the current than with it. Our guide to trolling speed by GPS covers how anglers deal with that.
- No-wake zones: many no-wake rules are about the wake you make, not a number on any instrument, and your wake depends on speed through the water. With a following current, GPS can show more speed than you’re making through the water. Where a numeric limit applies, check how your local rules define it.
How can you measure the current with GPS? #
This is the practical fix for “my speed readings don’t agree”. Four methods, from quickest to most thorough:
- Log minus GPS. If your paddle wheel or log reads correctly and you’re heading straight up or down the current, the difference between STW and SOG is the current along your line.
- Reciprocal runs. No log needed. Hold the same throttle or rpm, run a straight line one way and note the GPS speed, then turn and run back. Say you get 8.1 knots one way and 3.9 back: the current is half the difference (2.1 knots) and your speed through the water is the average (6.0 knots). This works when the current runs along your line, not across it.
- The drift test. In open water with nothing downstream to hit, take the engine out of gear and let the boat drift for a minute or two. The GPS speed is roughly the drift and the direction you move is the set. Wind pushes the boat too, so this reads high on a breezy day.
- The predictions. Tide and current tables give expected strength and timing for many harbours and channels. In US waters, NOAA publishes them at Tides & Currents; elsewhere, national hydrographic offices and tidal stream atlases do the same job.
If the reciprocal runs still don’t match your log in slack water, the problem is the water sensor, not the current. Fouled paddle wheels read low, pitot tubes are unreliable at low speed, and every log needs calibrating for its hull. Our comparison of GPS, paddle wheels and pitot tubes walks through calibrating one against GPS.
How does current change trip distance? #
A GPS logs distance over the ground. A log records distance through the water. In a current, they drift apart quickly.
Say you motor for three hours at 6 knots through the water against a 1.5-knot current. The log records 18 nautical miles; GPS records 13.5, because that’s how far you got. Coming home with the current behind you, the same three hours cover 22.5 nautical miles over the ground. For fuel and arrival times, distance over ground is the one that matters. For engine hours and hull performance, distance through the water tells you more.
To see what the current cost you, record each leg as its own trip. Comparing the outbound and return legs at the same throttle shows the current’s effect in plain numbers.
Planning around tides and currents #
- Do the time math before you go. A 16-nautical-mile run at 6 knots through the water takes 2 hours with a 2-knot current behind you (8 knots over ground) and 4 hours against it. The same fuel burn per hour buys twice the distance.
- Cross narrow places near slack water. Inlets and passes usually run hardest mid-tide.
- Watch for wind against current. Waves get short and steep, and GPS shows your speed dropping on every face. That’s real, not a glitch.
Using a phone as a boat GPS speedometer #
A phone makes a decent SOG display for small boats, kayaks and dinghies with no instruments, and a quick independent check on the ones you have. Speedometer GPS shows speed in knots with distance in nautical miles, has a heading readout and a colour-coded accuracy indicator, and works offshore with no signal because GPS doesn’t need data. Its speed alarm can be set in whole knots for a harbour limit before you leave the dock. Saved legs keep the route and can be exported as GPX.
Know its limits before you rely on it:
- The heading is course over ground, taken from GPS movement and only updated above about 2 knots (1 m/s). In a cross-current that’s your track, not where the bow points. During a slow drift, watch the shore rather than the heading.
- Trip distance reads short below about 9.7 knots (11 mph). The app ignores steps of less than 5 metres between one-second fixes to filter out drift at anchor, and at displacement, trolling and paddling speeds a lot of real movement falls under that. Speed and average speed aren’t affected, so for slow legs multiply average speed by moving time, or use a chartplotter’s track. Boat trip distance in nautical miles has the details.
- It can’t measure speed through the water. For STW you need a hull sensor.
It’s a helper, not a navigation aid. Use it alongside charts and your boat’s instruments, and see knots vs mph for boat speed for setting up units.
Frequently asked questions #
Is GPS speed over ground or through water? #
Always over ground. GPS measures your movement across the Earth’s surface, so any current carrying the water is part of the reading. For speed through the water you need a paddle wheel, pitot or log, or you can estimate it by averaging GPS speeds on reciprocal runs.
Why is my GPS speed higher than my boat’s speedometer? #
Either the current is running with you, or the water-speed sensor is reading low. Pitots under-read at low speed and fouled paddle wheels slow down. Run a reciprocal course at constant throttle: if the GPS average matches your speedometer, it was current.
Why does my knotmeter read higher than my GPS? #
You’re probably heading into a current, which lowers your progress over the ground while the log still reads your speed through the water. If the gap is still there when you average runs in both directions, the log is calibrated high.
Does current affect the distance a GPS logs? #
Yes. GPS distance is how far you travelled over the ground, so a current behind you adds distance for the same time under way and one against you takes it away. Your log’s distance through the water doesn’t change with the current.
Do sailors race on SOG or STW? #
Both. Speed through the water shows how well the boat and sails are performing. Speed over ground, and velocity made good toward the mark, shows real progress. When the tide is running, much of racing tactics is finding current that helps your SOG.