Sailing GPX data analysis — from a Strava, watch or phone track
You don't need a racing instrument to find out what your tacks cost you. Upload a GPX and Sail Smarter works the speed and course out of the positions in it, then does the same analysis it does for a Vakaros or a Velocitek: every tack and gybe found and costed in metres of lost ground, the session split into upwind and downwind legs with speed and VMG for each, a polar diagram, and the tide worked out from where the boat actually went. It runs in your browser, and the file isn't stored.
Free to use.
What a GPX actually contains
This is the whole of it, and being straight about it is the point: a GPX is a list of positions and times. Everything else has to be worked out, and some things can't be worked out at all.
| What the analysis needs | Where it comes from in a GPX |
|---|---|
| Position and time | Recorded directly — this is what a GPX is |
| Speed over ground | Derived: how far the boat moved across a six-second window, fitted rather than differenced between neighbouring fixes |
| Course over ground | Derived the same way, from the direction of that movement |
| Wind direction | Estimated by bisecting your upwind tack headings, and yours to override or refine |
| Heading | Course over ground stands in — there is no compass in a GPX |
| Heel | Not available — no sensor behind it |
| Speed through water | Not available — no paddlewheel behind it |
| Start line, gun, OCS, time to line | Not available — none of it is recorded, which is why this is the training tool and not the race one |
Why a six-second window and not the gap between two fixes: consumer GPS position error is a couple of metres, and a boat at five knots moves 2.6 m in a second. Between neighbouring fixes the error is larger than the movement, so speed comes out as noise and course comes out as nothing at all. Fitting a line through six seconds of positions averages that error down while staying comfortably inside a tack, which takes eight to fifteen seconds end to end.
What you get out of it
Everything that doesn't depend on an instrument you haven't got.
- Every tack and gybe, costedNot just counted. Each one is measured against a ghost of your own boat that carried on without turning, and the answer is in metres — two boat lengths, half a length, gone.
- Manoeuvres compared side by sideAll your tacks overlaid on one chart, aligned and rotated so "up" is toward the wind, plus a speed trace through any you pick.
- Leg by legUpwind and downwind legs found automatically, each with distance, duration, average speed, VMG and wind angle.
- Polar diagramYour speed at every wind angle, from your own session — the shape you actually sailed, not a designer's prediction.
- Tidal vectorsThe current across the racing area, worked out from the difference between where the boat pointed and where it went.
- A wind direction you can trustEstimated from your tacking angles, then re-read off the beat you actually sailed and offered back as a refinement, with the numbers behind it shown.
Getting a GPX out of your app
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Strava
Open the activity on the website, click the three dots at the top right, and choose Export GPX. The phone app doesn't offer it — use a browser.
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Garmin, Coros, Suunto
Each has an export on the activity page in its own web portal. Any of them gives you a GPX with one fix per second if the watch was recording at that rate.
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A phone in a dry bag
Most tracking apps export GPX. A phone's GPS is no worse than a watch's, and it's what a lot of people already have on board.
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Check the logging rate
One fix per second is what this works well on. A watch in battery-saver mode logging every ten seconds will miss manoeuvres — the analysis tells you if that's what it's been handed rather than quietly under-reporting.
Where this sits against a racing instrument
Honestly: a Vakaros Atlas or a Velocitek puck will tell you more. This tells you a great deal for the price of an export.
| Feature | GPX (Strava / watch / phone) | Vakaros or Velocitek |
|---|---|---|
| Tack & gybe detection and cost | Yes | Yes |
| Leg breakdown, polars, tidal vectors | Yes | Yes |
| Speed and course | Derived from position | Measured by the instrument |
| Heading | Course over ground only | Compass or orientation sensor |
| Heel analysis | No | Yes |
| Start line, OCS, time to line | No | Yes |
| Two-boat tuning comparison | Yes — against another GPX or an instrument | Yes |
| Video overlay | Yes, without heel and heading | Yes |
See the devices and file formats page for the full comparison, including what each instrument adds.
Sailing GPX data analysis — FAQ
Can I analyse sailing GPX data?
Yes. Sail Smarter Training Session Analysis reads a GPX track directly and works out speed and course from the positions in it, then detects your tacks and gybes, costs each one in metres of lost ground, breaks the session into upwind and downwind legs with speed and VMG for each, and draws a polar diagram and estimated tidal vectors. It runs in your browser, so there is nothing to install.
How do I export a GPX from Strava?
Open the activity on the Strava website, click the three dots at the top right, and choose Export GPX. Garmin Connect, Coros, Suunto and most phone tracking apps have an equivalent export. The file is a list of positions and times, which is all this needs.
What can a GPX not tell you about a sailing session?
A GPX records position and time and nothing else. There is no compass, so heading is course over ground; no heel sensor, so heel analysis is unavailable; no paddlewheel, so there is no speed through water to compare against speed over ground; and no start line or start gun, so there is no start analysis, OCS check or time-to-line. Speed and course are derived from how far the boat moved between fixes rather than measured, so expect them close to but not identical to a dedicated instrument's.
How does it know the wind direction from a GPX?
It estimates one by bisecting your two upwind tack headings, the same way a sailor does by eye, and you can override it with the direction you remember. Once the session has been analysed it reads the wind back off the tacking angles actually sailed and offers that as a refinement, with the two mean tack headings and the angle between them shown so you can judge it. Applying the refinement and letting it settle takes a step or two.
What GPS logging rate does sailing GPX analysis need?
One fix per second, which is what most watches and phone apps record by default. A tack takes eight to fifteen seconds, so a log with a fix every ten seconds has too few points inside each turn to detect it reliably. If a file is logged that sparsely the analysis says so rather than quietly missing manoeuvres.
Try it with your own track
Sign in, upload a GPX, and see what your tacks cost.