The GPS Speedometer team · Apprexy StudioAn iPhone can measure your boat's speed with a GPS speedometer app set to knots: the phone tracks its own position by satellite and turns the change into speed, with no sensor fitted to the hull. Bear in mind that GPS gives speed over ground, which differs from speed through the water whenever there is current. Get everything ready before you leave the dock, then secure or stow the phone and look at the numbers once you have stopped.
Knots: the unit used on the water
On the water, speed is usually given in knots. One knot is one nautical mile per hour, and a nautical mile is 1,852 metres, a little longer than a land mile. Nautical charts measure distance in nautical miles too, so knots make it easy to relate your speed to the distances you plan with.
GPS Speedometer shows speed in km/h, mph or knots. Choose knots and the phone matches your charts and any instruments on board.
Speed over ground and speed through water on a boat
A boat moves through a body of water that may itself be moving. A tidal stream, a river's flow or a wind-driven drift carries the hull along with it. That gives two different speeds:
| Speed over ground (SOG) | Speed through water (STW) | |
|---|---|---|
| What it measures | Movement across the earth's surface | Movement relative to the surrounding water |
| Typical source | GPS, including a phone | A log or paddlewheel sensor on the hull |
| Changed by current | Yes | No |
| Useful for | Arrival times, progress along a route | Trim, sail settings, engine performance |
A phone only knows where it is, so it can only give speed over ground. Say your boat moves through the water at 6 knots and you head into a 2-knot current: GPS will show about 4 knots. Turn around and it shows about 8, even though engine and hull are doing exactly the same work.
Neither number is wrong. They answer different questions. If you want to know when you'll reach the harbour, speed over ground is what counts. If you're comparing how the boat performs from one day to the next, a reading taken in slack water, or the average of two runs in opposite directions, gets closer to its speed through the water.
What a phone can't replace on board
A phone app is a handy second source of speed, not marine navigation equipment. It doesn't replace a chartplotter, compass, depth sounder, VHF radio or paper charts, and it isn't safety equipment. Lifejackets, a reliable way to call for help and whatever else is required where you go out still come first.
A phone can also fail at the moment you need it: a flat battery, a screen that overheats in the sun, water damage, or a slip over the side. Rules on equipment, speed and conduct on the water differ from place to place, so check what applies where you boat.
Keeping a phone dry and charged aboard a boat
- A waterproof case. An iPhone's water resistance is not permanent and isn't designed for spray, rain and the odd dunking. A sealed case or pouch made for immersion, ideally one that floats or comes with a lanyard, is worth having. Salt water is harder on seals and ports than fresh, so rinse and dry the case after a trip.
- A secure place. Either fix the phone in a marine mount that won't shake loose as the hull slams into waves, or zip it into a dry bag or a closed locker. A phone left loose on a seat tends to end up on the floor or in the water.
- Charge before you go. Start with a full battery. GPS, a bright screen and direct sun all drain it faster than usual.
- Backup power. A power bank in a dry bag, or a USB outlet if the boat has one, keeps a long trip going. Charging adds heat, so shade the phone on a hot day.
- Heat and cold. An iPhone that gets too hot may dim its screen or pause until it cools, and cold air on the water drains the battery faster.
Setting up GPS Speedometer before you cast off
Do all of this while the boat is tied up or at anchor, before anyone takes the helm.
- Allow location. When iOS asks, let GPS Speedometer use your location. If you declined earlier, the setting lives under Settings › Privacy & Security › Location Services (older iOS versions list it under Privacy); choose the app, allow access while using it, and keep Precise Location on.
- Choose the boat profile. The app has seven vehicle profiles; pick the boat.
- Switch to knots. Change the unit so it matches your charts and instruments.
- Check the gauge. The live readout with the Digital Clean gauge is free. With Premium you can double-tap the gauge to switch to another theme that is easier to read in glare.
- Start recording. Trip recording is a Premium feature. Tap record now, at the mooring, so the whole outing is logged.
- Let the signal settle. Give the phone an open view of the sky for a moment. Readings straight after opening the app are the least steady, and a cabin roof or canopy can weaken the signal.
- Secure or stow the phone. Lock it in its mount or put it away in a dry bag. Recording carries on in the background, but fully closing the app ends it.
Once you are under way, leave the phone alone. Whoever is at the helm keeps their eyes on the water, other boats, swimmers, buoys and the weather, not on a screen. If the phone slips, beeps or goes dark, it waits until the boat has stopped and is safely tied up or at anchor.
You don't need mobile coverage for any of this. GPS doesn't use a data connection, so the speedometer and trip recording keep working far from shore. The speed-limit alerts are built on road data, so they have no role on the water: follow the markers and local rules instead.
Recording a boat trip and reviewing it ashore
Back at the dock, or once the anchor is holding, open the trip. With Premium, GPS Speedometer keeps each one in your history with the distance covered, the drive time (the time spent moving), average and max speed, a speed chart and the route on a map.
On a boat, those numbers tell you more than raw pace:
- The route shows where you actually went over the ground, including any sideways drift from current or wind.
- The speed chart shows current at work. On an out-and-back trip at the same throttle, one leg is often faster than the other; the difference comes from the water moving, not the engine.
- Average speed over ground is the useful figure for planning: it tells you roughly how long a similar trip will take in similar conditions.
- Max speed is worth a second look. A brief spike after the signal drops, under a bridge or alongside a large vessel, is usually the GPS catching up, not the boat.
FAQ
Can my iPhone replace a boat's speed log?
No. A log on the hull measures speed through the water, while the phone measures speed over ground by GPS. The two differ whenever there is current, and the phone is no substitute for proper marine instruments or safety equipment.
Does GPS Speedometer work out of mobile range?
Yes. The speedometer, gauges and trip recording work offline, because GPS doesn't need a data connection. Only the automatic speed-limit lookup needs internet, and it relies on road data anyway.
Which features are free?
The app is free to download, and the live speed readout is free with the Digital Clean gauge. Premium unlocks the other gauge themes, HUD mode, trip recording and history, the speed-limit alerts, the drag timer and the Picture-in-Picture widget.
Why does my speed change when I turn around?
Because GPS measures speed over ground. Heading into a current, the water carries you back and the reading drops; heading with it, the reading rises, even at the same throttle.
Written by the GPS Speedometer team with the help of AI. What the article says about GPS Speedometer comes from the app itself; menu names in other apps can change between versions.