New Feature: AI Body Movement Analysis for Tacking
We are excited to announce a major new feature on SailingMetrics: AI-powered body movement analysis for tacking. Starting today, every session you upload with video will automatically include a detailed breakdown of how your body moves through each tack, giving you a kind of analysis that was previously only possible with a dedicated coach on the water and a slow-motion camera.
This has been one of the most requested capabilities from sailors using the platform, and we think it changes what is possible for self-coached sailors and coached athletes alike. Here is what it does, what you will see in your analysis, and why we think it matters.
What We Built and Why
We have always been able to tell you what happened to your boat speed through a tack. The GPS data makes that straightforward. What we could not tell you until now was what your body was doing during those same seconds, and whether the way you move is helping or hurting the manoeuvre.
Body movement through a tack is one of the things coaches focus on most when they watch sailors from a support boat or the shore. How quickly does the sailor transfer their weight from one side to the other? Do they move early, late, or right on cue? Is the movement smooth and continuous, or does it happen in two stages with a hesitation in the middle? These are questions that have always required a human observer to answer. We wanted to change that.
To do it, we built a computer vision pipeline that runs an AI model over the video from your session. The model identifies your body in each frame and locates the key anatomical landmarks, from your shoulders and hips down to your knees and ankles. From those landmarks it estimates where your centre of mass is at every moment in time, and tracks how that point moves laterally across the frame through the tack. The result is a continuous measurement of your weight transfer, with a time resolution that no human observer could match.
The Centre of Mass Chart
The headline output of the new analysis is a chart that plots your estimated centre of mass position over time for every tack in the session, all overlaid on the same graph. You will find it in the upwind tab of your activity analysis, labelled Centre of Mass Position Through the Tack.
Each line covers roughly ten seconds either side of the tack, with the tack moment itself at the centre. The vertical axis runs from minus one to plus one, where zero is the midpoint of the boat frame as seen in the video. All tacks are aligned so that the weight transfer moves in the same direction on the chart, regardless of whether the tack was from port to starboard or starboard to port. On top of the individual tack lines, a bold red line shows your average movement pattern across all the tacks in the session.
The first thing most sailors notice when they see this chart is whether their tacks are consistent. If all the lines follow a similar path, your technique is repeatable. If the lines are scattered widely, different tacks are happening in very different ways, which usually means the manoeuvre is not yet automatic. Both are useful things to know, and neither is obvious from GPS data alone.
The shape of the lines themselves tells you about timing. A smooth, continuous rise from one side to the other, with most of the movement happening around the tack moment, is the pattern you are aiming for. Lines that show a long flat section before the tack started suggest the sailor was already anticipating, while flat sections after the tack moment suggest a delayed transfer. Lines that plateau in the middle point to a hesitation during the turn itself. Once you know what to look for, the chart is surprisingly readable.
Body Movement Speed
For each tack, the analysis also calculates a single number that summarises how quickly you transferred your weight. We call it body movement speed, and it is measured in terms of how much of the frame width your centre of mass covered per second at its fastest point during the tack. A higher number means a more explosive, dynamic transfer. A lower number means a more gradual, measured one.
This number appears on the video card for each tack, so you can immediately see which manoeuvres were fast and which were slow. It is also used in the scatter chart described below. On its own it is a useful benchmark, particularly for tracking how your technique evolves over a season or as you adapt to different conditions.
Speed Loss Versus Body Movement: The Scatter Chart
Below the centre of mass chart you will find a scatter plot that puts all your tacks in context. Each dot is one tack. The horizontal axis shows how fast your body movement was. The vertical axis shows how much speed the boat lost through the tack, calculated from your GPS data as the difference between your pre-tack boat speed and the minimum speed reached during the manoeuvre.
The goal, in theory, is to be in the bottom right corner: fast body movement and minimal speed loss. In practice, the scatter plot often tells a more interesting story. Tacks with very fast body movement sometimes produce significant speed loss because the timing was off even if the movement itself was quick. Tacks with moderate body movement can produce very little speed loss when everything was executed at exactly the right moment relative to the boat's heading.
This distinction is important because it separates how fast you moved from how well you timed the movement. Both matter, but they are different skills, and the scatter chart helps you understand which one is your limiting factor right now. If your fastest-movement tacks are also your best-speed-loss tacks, speed is the lever to pull. If the correlation is weak or even reversed, timing is the thing to work on.
Video Clips for Every Tack
Every detected tack comes with its own short video clip, automatically extracted from the full session footage. Each clip is labelled with the tack direction, the body movement speed, and the SOG loss for that manoeuvre, so the numbers and the video are always next to each other. You do not have to scrub through a long recording to find the moment you are looking for.
The clips are particularly useful when combined with the charts. If the scatter plot shows one tack as an outlier with an unusually bad speed loss despite fast body movement, you can watch that clip immediately to see what was different. If the centre of mass chart shows a line that plateaued mid-tack, you can watch the corresponding clip to see whether that pause was a conscious hesitation, a moment of imbalance, or the boat being knocked by a wave at exactly the wrong time.
The combination of objective data and actual video in the same view is something we are genuinely proud of. The data tells you something happened. The video shows you what it looked like. Together they give you a much clearer picture of why a tack went the way it did, which is the part that actually leads to change.
Why This Matters for Your Sailing
Tacking efficiently matters more in some classes than others, but in almost every dinghy class it is a meaningful part of upwind performance. In a close race on a short course with frequent tacks, the difference between a fleet leader and a midpack sailor often comes down to how little speed each sailor gives away at every manoeuvre. Small, consistent improvements in tacking efficiency compound quickly over the course of a race.
The challenge with tacking improvement has always been feedback. Unless you have a coach with you on the water, or you are reviewing slow-motion video after the session with someone who knows what to look for, the feedback loop is very slow. You might feel that your tacks improved, but you cannot be sure. You might focus on the wrong part of the manoeuvre for weeks without realising it.
The body movement analysis shortens that feedback loop significantly. You can review a session within minutes of coming off the water, see exactly which tacks were good and which were not, and go back out the next day with a specific, measurable thing to work on. If you want to understand how tacking fits into the broader picture of upwind performance, the tacking training documentation covers the metrics and what to look for in your GPS data alongside the video analysis. That kind of precision in practice is what separates sailors who improve quickly from those who put in the hours without seeing the results.
For sailors who work with a coach, the analysis also changes the quality of the coaching conversation. Instead of the coach describing from memory what they saw from the shore, you can both look at the same chart and the same video clips together. The coach can point to a specific line and explain exactly what they want to change. The sailor can see the same thing. That shared reference makes the feedback much easier to act on.
Body Weight and Boat Speed: The Connection
It is worth taking a moment to explain why body movement timing has such a direct impact on speed through a tack, because understanding the mechanism helps you use the analysis more effectively.
When you tack a dinghy, the boat needs to slow down slightly, change direction, and then accelerate again on the new heading. Your body weight is a significant proportion of the total weight being moved through that process. If you move too early, before the bow has fully come through the wind, you load the leeward side of the boat at a moment when it needs to be as light and free as possible to complete the turn. The result is the boat stalling in the middle of the tack with no momentum to pull it through.
If you move too late, the boat comes out of the tack already heeled sharply to the new windward side. You lose speed to the heel, you have to sit out hard immediately before the sails are trimmed, and the boat is not in a position to accelerate cleanly. The difference between moving at the right moment and moving a second either side of it can easily be a boat length by the time you have rebuilt to full speed.
Getting this right consistently, across tacks in varying conditions during a race, is genuinely difficult. In light winds the movement needs to be smooth and measured to avoid killing the fragile momentum the boat carries into the tack. In a breeze it needs to be quicker and more committed to keep the boat flat through the turn. The body movement analysis shows you whether you are adapting appropriately to the conditions you were actually sailing in, rather than applying the same pattern regardless of what the wind was doing.
How to Get Started
The analysis works automatically for any session uploaded with video through the SailingMetrics app. The phone should be mounted in a fixed position on the boat, facing aft toward the sailor. A transom mount or a bracket at the base of the mast pointing backwards are both well suited to this. The app records the GPS and video together so they are already synchronised when you upload, and the AI processes everything after the upload completes.
Once the analysis is ready, open the activity from your account, navigate to the upwind tab, and scroll down to the body movement analysis section. The video clips appear first, followed by the centre of mass chart and then the scatter plot. If your session has fewer than two tacks with clear video data, the charts will not appear, but the individual clips will still be there for any tack where the footage was usable.
We recommend starting with a recent race or a training session where you felt your tacking was a weak point. Look at the centre of mass chart first to get a sense of your consistency, then check the scatter plot to see which tacks were genuinely efficient. Watch the clips of the outliers in both directions, the best and the worst, and see if you can spot what was different. From there, pick one specific thing to focus on and bring it to your next session.
What Is Next
This is the first version of the feature, and we are already working on several improvements based on early feedback. Better tracking in lower light conditions, extended analysis for gybing manoeuvres, and comparison views across multiple sessions are all on the roadmap.
We are also very interested in hearing from sailors and coaches who use the feature in the coming weeks. If you find something in your analysis that surprises you, or if you have questions about how to interpret a particular chart, get in touch through the contact page. The more sessions that get uploaded with video, the more we can refine the analysis and make it more useful for everyone.
Want to see what the analysis looks like before uploading your own session? Take a look at this example activity analysis. Create a free account and upload your footage to see what the AI makes of your tacks.