To time your boat from 0 to 30 mph, start a stopwatch the moment the throttle goes forward from idle and stop it when a GPS speed reading reaches 30 mph, on flat water, in both directions, several times. The catch is that a phone updates its GPS about once a second and most apps smooth the speed, so the readout trails the boat by a second or two while it accelerates. That’s a big error on a run that only takes a handful of seconds.
A phone is still useful for this. It won’t match a dedicated high-rate data logger, but if you time every run the same way, it will tell you reliably whether a new prop, a trim change or a lighter load made the boat quicker.
What are hole shot and 0-30 time? #
- Hole shot is how quickly a planing boat climbs out of the water and gets on plane from a standstill. It’s what you feel as the bow rises and then drops.
- 0-30 mph time is the time from a standstill to 30 mph. It’s the figure most boat tests quote, because it captures the hole shot and the acceleration after it in one number.
- Time to plane is sometimes quoted separately, measured to the moment the boat is fully on plane.
30 mph is 26.1 knots or 48.3 km/h, if you’d rather work in those units. There’s a conversion chart in our mph to km/h guide, and more on why boats use knots in knots vs mph for boat speed.
Why does a phone GPS app struggle to time acceleration? #
Three things add up:
- Once-a-second fixes. If a new speed only arrives every second, the moment you cross 30 mph can fall anywhere in that second.
- Smoothing. Raw GPS speed jitters, so speedometer apps average several recent readings. An average of the last five one-second readings sits about two seconds behind a speed that’s changing quickly. We explain the effect in why GPS speed lags behind your gauge.
- Your reaction time, at both ends of the stopwatch.
Here’s what that does to a run. Say your boat gains a steady 5 mph every second and truly reaches 30 mph in six seconds. A readout running two seconds behind shows 20 mph at the six-second mark and doesn’t show 30 until about eight seconds. Your stopwatch says 8.0 when the answer is 6.0.
Real boats don’t accelerate evenly. They’re slow while climbing onto plane and then pick up quickly, so you can’t subtract a fixed number to correct it. What you can rely on is consistency. If every run is timed the same way, the lag is roughly the same each time, and a run that’s half a second quicker really is quicker.
How do you time a boat’s 0-30 mph run? #
Safety first #
- Choose open water with plenty of room, well away from no-wake zones, swimmers, moorings and other boats.
- Put everyone in a seat and warn them before you open the throttle.
- Wear the engine cut-off lanyard or wireless fob. In some places, including the US for many small boats, it’s a legal requirement.
- Have a passenger keep a lookout and do the timing, so the helm watches the water, not the phone.
The run #
- Pick flat, calm conditions. Wind chop and swell change the result more than you’d expect.
- Set the boat up the same way each time: similar fuel level, the same people and gear aboard, engine warmed up, and a note of your trim and tab settings.
- Mount the phone where it can see the sky, open the speedometer app, set the unit to mph and wait for a strong GPS fix before you start.
- Come to a complete stop at idle with the boat pointing into clear water.
- Have your passenger film the phone’s readout with their own phone, with the helm in shot if possible. Video is more precise than a thumb on a stopwatch.
- Open the throttle and run until the readout passes 30 mph, then throttle back.
- Turn around and repeat in the opposite direction. Current and wind push your GPS speed up one way and down the other, and averaging a pair cancels most of it.
- Do at least three pairs and average them.
Back at the dock, step through the video frame by frame: note the frame where the throttle moves and the frame where the readout first shows 30. The difference is your time, including the display lag.
Getting closer to the true time #
If you want an absolute number rather than a comparison, you need unsmoothed data. A GPS app that exports its recorded track (GPX) with a timestamp for every point lets you work out speed from the change in position between points in a spreadsheet. At one point per second that’s still coarse, but it removes the smoothing delay. For numbers you can compare with published boat tests, a dedicated GPS data logger that records ten or more times per second, or a radar gun operated from another boat, is the right tool.
Why does your GPS speed differ from the boat’s speedometer? #
Most boat speedometers measure speed through the water, with a pitot tube or a paddlewheel. GPS measures speed over the ground. On a lake with no current and a calibrated sensor they should be close. With a current they won’t be: running against a 2-knot current, GPS shows 2 knots less than the water speed, and running with it, 2 knots more. That’s another reason to time runs in both directions.
Pitot and paddlewheel sensors also need calibration and can foul with weed or growth, while GPS doesn’t care about either. Neither is wrong; they’re measuring different things.
What affects a boat’s 0-30 time? #
When you compare runs, change one thing at a time.
- Load. People, fuel and gear all add weight, and weight slows the hole shot.
- Propeller. Pitch, diameter, blade count and cup change how hard the boat launches versus how fast it tops out.
- Trim and tabs. Most boats launch best trimmed down, then trimmed out once on plane. Your own boat’s best setting is what these tests will reveal.
- Hull condition. Growth on the bottom adds drag.
- Engine condition, air temperature and altitude, which all affect power.
- Water conditions. Chop, swell and wind, which is why calm days give repeatable numbers.
Timing runs with Speedometer GPS #
Speedometer GPS shows live GPS speed in mph, knots or km/h (with nautical miles for distance in knots), plus your heading and altitude, and it works with no mobile data, which matters offshore. Its readout averages the last five one-second readings, so it’s steady to read on a bouncing boat and, as explained above, about two seconds behind during hard acceleration. It’s well suited to cruise speed and top speed: the maximum is taken from the smoothed value, so a single glitchy reading can’t give you a fake top speed, though a very brief peak may be trimmed slightly.
Start a trip before your runs and save it afterwards, and you get a speed chart for the whole session and a GPX export with a timestamp on every point, which you can use for the spreadsheet method above. For harbours and channels with speed limits, its speed alarm has knots presets from 5 to 30 and buzzes and beeps when you pass the one you set. One limit for slow boating: its trip distance ignores movements under 5 m between fixes, so below about 10 knots it can read short, although the speed readout isn’t affected. For more on how recording rate changes GPS distance, see why GPS distance varies by sampling rate.
Frequently asked questions #
What is a good 0-30 mph time for a boat? #
It depends entirely on the boat, engine, prop and load, so there’s no single good number. The useful comparison is against a published test of the same boat and engine, or against your own earlier runs timed the same way.
Is phone GPS accurate for measuring boat speed? #
For steady speed, yes: with a clear sky it’s usually within a couple of km/h of your speed over the ground. It’s less suited to timing acceleration, because the readout lags a second or two, and it measures ground speed, so current affects it.
How do I convert knots to mph? #
Multiply knots by 1.151 to get mph. So 20 knots is about 23 mph, and 30 mph is about 26 knots.
Why does my GPS top speed differ from my boat’s speedometer? #
Your boat’s speedometer usually measures speed through the water, and GPS measures speed over the ground, so any current makes them differ. Pitot and paddlewheel sensors also need calibration. Average runs in both directions for the fairest GPS figure.
Does smoothing affect GPS top speed? #
A little. Averaging several readings trims very brief peaks, so a short burst may record slightly under its true maximum. For sustained top speed on a long, flat run, the effect is small.