To track how far your boat went, set a GPS app to knots so distance shows in nautical miles, press Start as you leave the dock and save the trip when you tie up. The app adds up the short straight lines between position fixes into a distance over the ground. A nautical mile is exactly 1,852 metres (about 1.15 statute miles) and a knot is one nautical mile per hour, so you can always check the total with distance = speed in knots × hours under way.
Open water is where phone GPS works best, with nothing blocking the sky. The things that decide whether the total is trustworthy are where the phone sits, how the app treats slow speeds and stops, and what “over the ground” means when there’s a current.
Why log trip distance on a boat? #
Boats don’t have odometers, and engine service runs on hours, so distance is easy to ignore. It’s still worth logging:
- Fuel planning. Divide fuel used by nautical miles covered and you get a real fuel-per-mile figure at your usual cruise. That’s the number the rule of thirds (a third of your fuel out, a third back, a third in reserve) needs.
- Knowing your range. A log of past runs tells you how far a full tank really takes you in typical conditions, not in a brochure.
- A record of where you went. A saved track shows your route, your speed along it and where you stopped.
The speed, time and distance formula #
Distance (nm) = speed (knots) × minutes ÷ 60. Turned round, the minutes to cover a distance are distance × 60 ÷ speed, so 12 nm at 8 knots takes 90 minutes. Many boaters remember it as “sixty D street”: 60 × D = S × T.
| Speed | 15 min | 30 min | 1 hour | 2 hours |
|---|---|---|---|---|
| 3 knots | 0.75 nm | 1.5 nm | 3 nm | 6 nm |
| 5 knots | 1.25 nm | 2.5 nm | 5 nm | 10 nm |
| 6 knots | 1.5 nm | 3 nm | 6 nm | 12 nm |
| 10 knots | 2.5 nm | 5 nm | 10 nm | 20 nm |
| 20 knots | 5 nm | 10 nm | 20 nm | 40 nm |
| 25 knots | 6.25 nm | 12.5 nm | 25 nm | 50 nm |
The nautical mile’s length, 1,852 m exactly, is set by international agreement and listed in NIST’s conversion tables. It’s close to one minute of latitude, which is why you can measure distance straight off a chart’s side scale. For converting speeds between knots, mph and km/h, see knots vs mph for boat speed.
How does a GPS app measure distance on the water? #
The phone gets a position fix about once a second, and the app adds up the straight line from each fix to the next. At 20 knots the boat moves about 10 metres between fixes, so even gentle curves are traced closely.
Good apps also filter noise, because a stationary phone’s position wanders by a few metres. Speedometer GPS ignores steps under 5 metres between fixes (10 metres once you’ve been sitting still), throws out any jump of more than 200 metres, and doesn’t count distance from fixes whose accuracy is worse than ±20 metres. That’s why a boat swinging at anchor doesn’t pile up fake miles.
The result is distance over the ground. In a current it differs from the boat’s log, which measures distance through the water; our explainer on speed over ground vs speed through water walks through the maths.
Honest limit: distance at slow boat speeds #
That 5-metre filter has a cost. A boat covers 5 metres a second at about 9.7 knots (11 mph). Below that, which covers most sailing, displacement cruising, trolling and paddling, a lot of real movement falls under the filter and Speedometer GPS’s trip distance reads short, sometimes badly. At 2 or 3 knots it can show almost nothing.
Speed, top speed and average speed aren’t affected. Two workarounds:
- Average speed × moving time. Both are on the saved trip. At an average of 5.2 knots over 3.5 hours of moving time, you covered about 18.2 nm.
- Measure the exported track. The app keeps the GPS points you recorded, and any mapping or navigation program that imports GPX can measure the track’s length. Saved routes are thinned to 500 points, which is plenty for a mostly straight passage but smooths out short tacks and tight circles.
If you mostly sail or troll and want an exact log, a chartplotter’s track or a dedicated tracking app is the better tool. Planing powerboats spend most of their time well above 10 knots, where the distance counts normally.
How accurate is GPS trip distance offshore? #
Offshore is close to ideal for GPS. Position errors are small and mostly random, and on a long straight run they don’t add up to much. Accuracy suffers in a few predictable places:
- Tight manoeuvring. Idling around a marina or circling a fishing spot means short, slow segments, which the slow-speed filter hits hardest.
- Blocked sky. Inside a cabin, under a hardtop or beside a steel tower, the fix weakens. An accuracy radius of 5 to 10 metres is a strong fix.
- Cliffs and structures. High rock walls, bridges and big ships reflect signals and nudge the position around.
- Heat. A phone baking on a console can overheat and shut down, which ends the log.
GPS distance vs chart distance vs your log #
- Chart distance is the straight lines between your planned waypoints. Your real track is almost always a little longer, because you steer around traffic, bear off in waves and wander.
- GPS distance is how far you moved over the ground, including every push or setback from the current.
- Log distance, from a paddle wheel or other through-hull sensor, is how far you moved through the water.
With a 2-knot current behind you, a boat making 6 knots through the water covers 8 knots over the ground, so over three hours the log says 18 nm and GPS says 24. Neither is wrong.
How to log a boat trip with a phone #
- At the dock, open the app, set the units to knots and wait for the accuracy indicator to show a strong fix. Plug the phone into a 12 V USB outlet or a power bank.
- Mount it in a waterproof case where it can see the sky and you can read it from the helm without leaning.
- Press Start as you leave. Live speed shows as soon as the app opens, but trip distance only counts from Start.
- At a lunch or fishing stop, press Stop to pause the trip, and Start again when you move on. Trip time keeps running through the pause; moving time doesn’t.
- Keep the app on screen. Speedometer GPS only uses location while it’s open and doesn’t track in the background, which is why its keep-screen-on setting is on by default. If the app does get closed, it offers to resume the trip when you reopen it.
- Press Save back at the dock. The trip keeps its distance, top speed, average speed while moving, trip time and moving time, plus the route and a speed chart, and you can export it as GPX. Up to 100 trips are kept on the phone, and the Trips screen adds them into a lifetime odometer, handy for season totals.
Speed comes from the satellites, not the phone network, so all of this works offshore with no signal. If you’re entering a speed-restricted zone, set the speed alarm in whole knots before you leave; the readout turns red and the phone buzzes and beeps when you pass it.
Fuel planning in nautical miles #
Divide distance by fuel used to get nautical miles per gallon or per litre. Use distance over the ground, since that’s what gets you home, and remember that the same run against the current burns more fuel than with it. If your trips are mostly slow, work the distance out from average speed and moving time rather than trusting a short GPS total.
What a phone speed app can’t do #
A speedometer app is a speed display and trip log, not a chartplotter. It has no charts, depth, AIS or tide data, so it doesn’t replace charts, a VHF radio or proper navigation gear. Keep the phone dry in a case with a lanyard, shaded, and charged. If you want to check your boat’s own speed sensor while you’re out, see GPS vs pitot tube vs paddle wheel.
Frequently asked questions #
How do I measure the distance I travelled by boat? #
Use a GPS app or chartplotter with a trip log. Start it as you leave and stop it when you arrive, with the units set to knots so the total comes out in nautical miles. At slow speeds, check it against average speed × time.
How do I calculate distance from knots and time? #
Multiply speed in knots by hours, or by minutes and divide by 60. At 7 knots for 45 minutes: 7 × 45 ÷ 60 = 5.25 nautical miles.
Does GPS work offshore without cell service? #
Yes. A GPS receiver listens to the satellites directly and doesn’t need a cell connection. Maps and some app features need data, but GPS speed and distance work in airplane mode far offshore.
Why is my GPS trip distance different from the chart? #
Your real track is never as straight as the planned legs, and current changes how much ground you cover in a given time. Some apps, including Speedometer GPS, also filter small movements, which makes slow trips read short.
Can I export my boat track to a chartplotter or map? #
Yes. Speedometer GPS exports saved trips as GPX, a standard format many chartplotters, navigation apps and mapping programs import. Saved tracks are thinned to 500 points, enough to show the route clearly.