GPS speed test · Marine

Boat Speed Test

How fast is your boat going? Press Start for live GPS speed over ground in knots, mph or km/h — with top speed, moving average, trip distance in nautical miles and a speed alert for no-wake zones. No app, no wiring and no pitot tube to clog.

GPS off

0KNOTS

Max speed: —
Avg (moving): —
Distance: —
Trip time: —
Moving time: —
Heading: —

Set Speed Alert

Speed limitkm/h

Beep and vibrate when exceeding this speed.

Trip log

No points recorded yet

Knots by default
Speed over ground
Nautical-mile trip log
No-wake speed alert
Works offshore without data
GPX track export

How it works

Use Your Phone as a GPS Boat Speedometer

01
Protect Your Phone
Use a waterproof case or pouch and a lanyard. Spray and a wet deck end more phones than rough water does.
02
Open Before You Leave
Load this page while you still have signal, press Start and allow location access. It keeps running offshore without data.
03
Read Speed Over Ground
Keep the phone where it can see the sky — fiberglass and canvas are fine, metal is not — and read your speed in knots.

Marine instruments

How Does a Boat Speedometer Work?

Boats measure speed in one of three ways. Two of them measure your speed through the water; GPS measures your speed over the ground.

Pitot tube

The classic boat speedometer. A small forward-facing pickup — often a hole in the leading edge of an outboard’s gearcase, or a tube on the transom — rams water into a hose, and a pressure gauge on the dash converts that pressure into mph or knots. Simple and cheap, but the pressure grows with the square of speed — there is almost none to measure when you go slowly — so pitot gauges read poorly below about 10 mph, and the pickup clogs easily.

Paddle wheel

A small impeller under the hull spins as water flows past, and an electronic sensor counts its revolutions. Paddle-wheel logs are accurate at low speeds and common on sailboats, but weed and marine growth slow the wheel and make them read low.

GPS

A GPS boat speedometer — a marine GPS gauge, a chartplotter or this page — measures speed over ground from satellite signals. Nothing sits in the water, so nothing fouls, and it is just as accurate at 3 knots as at 50.

Navigation

Speed Over Ground vs Speed Through Water

Speed over ground (SOG) is how fast you are moving relative to the seabed — what GPS shows. Speed through water (STW) is how fast the hull is moving relative to the water around it — what pitot tubes and paddle wheels show.

On a lake they match. In a tidal river they don’t: a boat making 6 knots through the water against a 2-knot current covers ground at only 4 knots, and at 8 knots with the current behind it. For arrival times and fuel planning, SOG is the number that matters; for sail trim and hull performance, sailors watch STW.

Displacement hulls — most sailboats and trawlers — also have a natural limit called hull speed, roughly 1.34 × √(waterline length in feet) knots. Pushing past it takes far more power for very little extra speed.

Approximate hull speed by waterline length
Waterline length	Hull speed
15 ft (4.6 m)	5.2 kn
20 ft (6.1 m)	6.0 kn
25 ft (7.6 m)	6.7 kn
30 ft (9.1 m)	7.3 kn
40 ft (12.2 m)	8.5 kn

Hull speed ≈ 1.34 × √(LWL in feet). Planing hulls are not limited this way.

Benchmarks

How Fast Do Boats Go?

Typical speeds vary hugely with hull, engine and load, but these ranges are a useful guide.

Typical boat speeds
Boat	Typical speed
Kayak or canoe (paddled)	2–4 kn (2–5 mph)
Cruising sailboat	5–8 kn (6–9 mph)
Trawler or displacement cruiser	7–10 kn (8–12 mph)
Pontoon boat	15–30 mph top speed
Bass boat	50–75 mph top speed
Personal watercraft (jet ski)	up to about 65 mph
Cruise ship	about 18–22 kn

No-wake zones usually mean the slowest speed at which you can still steer — often around 5 mph or less. Check local rules.

Units

Knots, mph and km/h

1 knot = 1 nautical mile per hour = 1.852 km/h ≈ 1.151 mph.

Knots converted to mph and km/h
Knots	mph	km/h
3 kn	3.5 mph	5.6 km/h
5 kn	5.8 mph	9.3 km/h
10 kn	11.5 mph	18.5 km/h
15 kn	17.3 mph	27.8 km/h
20 kn	23.0 mph	37.0 km/h
30 kn	34.5 mph	55.6 km/h
40 kn	46.0 mph	74.1 km/h
50 kn	57.5 mph	92.6 km/h

A nautical mile is 1,852 m — one minute of latitude — so charts and knots work together.

Troubleshooting

Boat Speedometer Not Working? Common Causes

When a pitot-type boat speedometer reads zero, sticks or reads low, the cause is almost always in the water side of the system rather than the gauge:

Blocked pickup. Weed, sand, salt or marine growth plugs the tiny pitot hole. Clear it gently with a thin wire.
Water or debris in the hose. Disconnect the hose at the gauge and blow it through towards the pickup — never blow air back through the gauge itself, which can damage it.
Cracked, kinked or loose tubing. A leak anywhere bleeds off pressure and makes the gauge read low. Replace brittle hose and check the fittings.
Fouled paddle wheel. On impeller sensors, remove and clean the wheel, check that it spins freely and inspect the wiring.

While you sort it out, this page works as a backup boat speedometer. And if you are comparing a repaired gauge, remember that GPS shows speed over ground — test on still water, or run the same stretch in both directions and average the readings.

FAQ

Boat Speed Test FAQ

How accurate is a GPS boat speedometer?

At a steady speed with a clear view of the sky, phone GPS is typically accurate to within about 1 knot — and unlike a pitot gauge it stays accurate at low speeds. It can lag for a second or two when you accelerate hard or turn sharply.

Does GPS show speed over ground or speed through water?

Speed over ground (SOG): how fast you are moving relative to the seabed. A paddle-wheel log or pitot tube measures speed through water. The difference between the two is the current.

Do I need cell signal to use it on the water?

No. GPS satellites are received directly by your phone. Load the page while you still have a connection; after that it works offshore with no data.

Why does my boat speedometer read differently from GPS?

Pitot gauges are rough by design and read poorly at low speed, a partly blocked pickup reads low, and paddle wheels foul with growth. Both also measure speed through water, so a current makes them disagree with GPS even when they are working perfectly.

What is a knot?

One nautical mile per hour: 1.852 km/h or about 1.15 mph. A nautical mile (1,852 m) is one minute of latitude, which is why knots are used at sea and in the air.

Can I use it on a kayak, sailboat or jet ski?

Yes — anything that moves on the water. Kayakers and sailors get accurate low-speed readings, and PWC riders can set a speed alert. Keep the phone sealed in a waterproof case.

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