For most small powerboats, GPS is the boat speedometer you can trust most often. It reads the same way at 3 knots as at 40, has nothing in the water to foul, and needs no calibration. A pitot tube is cheap but coarse or dead at low speed and reads low when anything blocks it. A paddle wheel reads low speeds well but fouls and needs calibrating. The catch is that GPS measures speed over ground, while pitots and paddle wheels measure speed through the water, so in a current a healthy GPS and a healthy paddle wheel can disagree and both be right.
This page is about the hardware: how each sensor works, how it fails, and how to check one against another. For the current itself, see speed over ground vs speed through water.
How does each boat speedometer work? #
| Sensor | Measures | Works best | Common failure |
|---|---|---|---|
| GPS (dash gauge, chartplotter, handheld, phone) | Speed over ground | Any speed, any hull, no hole below the waterline | Can’t see current; a second or two of lag; needs open sky |
| Pitot tube | Speed through water, from water pressure | Planing speeds on outboards and sterndrives | Coarse or zero at low speed; clogged pickup or leaking hose reads low |
| Paddle wheel (impeller) | Speed through water, from wheel turns | Displacement speeds, sailing, trolling | Weed, growth or fishing line slows the wheel; needs calibration |
| Ultrasonic or electromagnetic log | Speed through water, no moving parts | Cruising and racing boats | Cost and a through-hull install; still needs calibrating |
GPS #
A GPS receiver doesn’t just subtract one position from the next. Most calculate velocity from the Doppler shift of the satellite signals, which is why the speed is steadier than the position dot on a chart. It reports horizontal speed across the Earth, so tide, river current and wind drift all show up in it. Nothing touches the water, so weed, barnacles, bubbles and water density make no difference, and trim or hull shape doesn’t change the reading.
Pitot tube #
A pitot is a small forward-facing hole, usually in the nose of an outboard’s gearcase or on a transom bracket. Water rams into it and a hose carries that pressure to the dash, where the “speedometer” is really a pressure gauge printed in mph or knots. The pressure rises with the square of speed, so at 10 mph it’s a quarter of what it is at 20 mph, and at trolling speed it’s tiny. That’s why many pitot gauges sit near zero until the boat is on plane.
Paddle wheel #
A paddle wheel has magnets in its blades and a sensor that counts pulses as it turns. The display turns pulses into speed with a calibration factor that’s only right for one boat and one mounting spot, because water speeds up and slows down around the hull. Weed, a barnacle, a plastic bag or a wrap of monofilament slows or stops it, and aerated water behind a hull step does the same.
Which is most accurate? #
| Pitot tube | Paddle wheel | GPS | |
|---|---|---|---|
| Idle and trolling speed | Poor or no reading | Good | Good, with a small wobble |
| Planing speed | Fair after calibration | Good if clean | Good |
| Current | Ignores it | Ignores it | Includes it |
| Weed, mud, barnacles | Reads low or zero | Reads low or sticks | Unaffected |
| Aerated water and chop | Needle bounces | Can read low | Recovers when the fix is steady |
| Calibration | Needed, and drifts | Needed, and drifts | None |
| Needs sky view | No | No | Yes |
As a rule of thumb, water sensors fail by reading low, and GPS fails by wandering. A GPS that holds a steady number with a good fix is rarely wrong about speed over ground. When a clean paddle wheel and GPS disagree in slack water at a steady speed, the paddle wheel’s calibration is the usual suspect.
For performance numbers such as prop comparisons or a top-speed run, GPS on reciprocal runs is the standard method: run the same stretch both ways at the same throttle and average the readings, which cancels current and wind.
Common pitot problems and fixes #
- Reads zero after launching or trailering: mud, sand, weed or an insect nest in the pickup or hose. Clear the hole with a thin wire or blow the line through gently, following the gauge maker’s instructions, since some gauges are damaged by pressure pushed back into them.
- Reads low everywhere: a partial blockage, a cracked or kinked hose, or a leaking fitting.
- Needle jumps around: the pickup is in aerated water, often from strakes or trim. It usually settles once the boat is fully on plane.
- Was fine, now wrong: check the pickup for impact damage. A bent tube changes how water enters it.
- Off by the same percentage at every speed: that’s calibration. Many digital gauges accept a correction factor.
Pitots are also scaled for one water density. Seawater is about 2.5% denser than fresh water, which shifts the reading by roughly 1%, small next to the problems above. Paddle wheels have one big fault, fouling. Many through-hull units can be pulled from inside the boat and replaced with a blanking plug while you clean them; follow the maker’s instructions, because water comes in fast.
How to calibrate a pitot or paddle wheel against GPS #
GPS makes a good reference as long as you cancel out the current.
- Pick calm water near slack tide, away from river flow if you can.
- Hold a steady throttle on a straight line for at least a minute and note the settled GPS speed and the gauge speed.
- Turn around, run the reciprocal course at the same throttle for the same time, and note both again.
- Average the two GPS speeds. A current along your line adds to one run and subtracts from the other, so the average is your speed through the water.
- Divide the GPS average by the gauge average to get a correction factor, and enter it as your gauge’s manual describes.
- Repeat at a second speed to confirm the correction holds.
Example: run one gives the log 6.0 knots and GPS 7.1. Run two gives the log 6.0 and GPS 4.7. The GPS average is 5.9 knots, so the log reads about 1.7% high and the correction factor is 0.983. A crosscurrent doesn’t cancel this way, so choose a line that runs with and against the flow.
Is GPS boat speed accurate in chop and waves? #
Mostly, yes. In chop, GPS speed jumps because the boat really does slow as it climbs each face and surge down the back, and GPS reports that one second at a time. A phone high on a console also swings in a small arc as the bow pitches, which adds a fraction of a knot that averages out over each wave. What isn’t real is a sudden spike when the fix gets weak from spray, reflections or a steep pitch angle.
| What you see | What it usually means | What to do |
|---|---|---|
| Speed swings in rhythm with the waves | The boat is really surging | Read the middle of the swing |
| A sudden jump and back, and the accuracy indicator drops | A weak or reflected fix | Ignore that spike |
| Speed falls steadily as seas build | You’re genuinely slowing in head seas | Decide whether to ease off |
To get a fair number in rough water, hold the throttle steady for 30 to 60 seconds, read the midpoint of the swing, and compare the trip average over a few minutes rather than single readings. Leave top-speed runs for flat water. A water sensor doesn’t do better in chop: a pitot’s pressure spikes with every wave, and a paddle wheel can spin in aerated water when the hull lifts. In a following sea, speed spikes well above cruise mean the boat is surfing, which is a sign to slow down.
Which speedometer does your boat need? #
- Runabouts, bowriders, bass and bay boats: GPS. An accurate number at every speed and nothing to maintain.
- Trolling and slow fishing: GPS for a repeatable speed, plus a sense of the current so you know what the lure feels. See trolling speed by GPS.
- Sailboats: a water-speed log for trim and performance, and GPS for progress. They answer different questions.
- Performance testing: GPS on reciprocal runs. Timing acceleration has its own quirks, covered in boat 0-30 mph time.
- Tight budget or no dash space: start with a phone before you buy hardware.
Using a phone as a second boat speedometer #
A phone in a waterproof case is the cheapest way to check a pitot or paddle wheel, or to get a speedometer on a boat that has none. Speedometer GPS shows speed in knots, mph or km/h, averages the last five one-second readings so the number settles at a steady throttle, and shows a colour-coded accuracy indicator so you can tell a solid reading from a guess. Fixes worse than ±20 m aren’t used for speed or distance at all. It works offline, and its speed alarm, set in whole knots, buzzes and beeps when you pass a harbour limit.
It reads speed over ground from the phone’s own GPS and doesn’t connect to your boat’s instruments, so it can’t show speed through the water. Its trip distance also reads short below about 9.7 knots, because steps under 5 metres between fixes are dropped; speed readings aren’t affected. Mount it with a view of the sky, not deep in a console or under a steel T-top, use a lanyard in rough water, and set the units and alarm at the dock.
Frequently asked questions #
Is GPS or a pitot tube more accurate on a boat? #
GPS, for speed over ground at almost any speed. A calibrated pitot can be close at planing speed on clean water, but it’s unreliable at low speed and drifts. Its one real advantage is that it measures speed through the water, which GPS can’t.
Why does my pitot speedometer read zero at low speed? #
Pitot pressure rises with the square of speed, so at idle and trolling speeds it’s too small to move the gauge. A zero at planing speed is different: that usually means a blocked pickup or hose.
Is GPS or a paddle wheel more accurate? #
They measure different things. GPS measures speed over ground and is rarely wrong with a good fix. A clean, calibrated paddle wheel is accurate for speed through the water, but it fouls easily and then reads low. In a current, both can be correct and still disagree.
Does a GPS speedometer work under a T-top or hardtop? #
Usually. Fibreglass and canvas let GPS signals through with little loss. Metal frames, radar arches and electronics boxes overhead can weaken reception, so if the accuracy indicator shows a poor fix, move the phone or antenna.
Can a phone replace a boat speedometer? #
For speed over ground, yes, as long as it sees the sky, is mounted securely and stays charged. It can’t measure speed through the water, and on long days it needs a waterproof case and a 12 V USB outlet.