Thursday, June 30, 2022

Small Steps and Fast Foils

Foiling jibes are hard, if you ask me (just don't ask Nina). I've spent countless our watching instruction videos; have taken camps and private lessons with one of the best foil instructors in the world; bought new foil boards hoping that they would solve my jibe problems; and tried jibing on the foil often enough that I discovered dozens of ways to screw it up. But I still cannot foil through jibes consistently. 

Here are some of the things that I remember from various lessons and videos:

  1. Use a wide board, it's easier (my Stingray 140 qualifies!)
  2. Take a big step close to the rail to start the turn. That's suggested in many videos, except that Nico Prien suggested a smaller step closer to the center line.
  3. When it's time to switch feet, step heel to heel, bringing the old front foot to the back foot. 
  4. Move the old back foot straight into the new front strap.
Can you spot the problem? When I followed step 2 and took a big step, then my crashes in step 3 would be virtually guaranteed, and often scary (like the one where I fell onto the foil with the back of my head .. in case you wondered why I now always wear a helmet). I've blogged about my problems with step 3 before, step 4 remained a pipe dream - I count it as a success when I step on top of my foot straps, instead of landing in front of them, as I usually do (which is 100% effective at dropping the board onto the water).

One issue that I could never wrap my head around was this: if I take a big step to the outside rail, and then try to bring my other foot heel-to-heel, that's also a pretty big step. While taking the second step, all my weight is near the edge of the board, and the board is above the water - meaning it is very tippy. Maybe I'm just to slow? But whatever it is, the board would tilt as if I was trying to start a rail ride, and I'd have to hope that my fall would be away from the foil.

Things finally started to click when I watch a video about the One Hour Classic at Lake Garda:

About 45 seconds into the video, the boat close to the jibe mark captured great footage of several foil jibes. Everyone is on a wide board (#1), and everyone does takes a wide step to the carving rail (#2). But when it comes to moving the old front foot to the back, nobody follows rule 3! Instead of a single step, every single racer takes multiple steps with the new back foot. The first step is a small step out of the front strap, a bit to the back and towards the center of the board. This is immediately followed by the carving foot sliding forward a little bit into the foot strap. After that, there are one or two additional small movements of the back foot that bring it to the other side of the board. Except for the first step onto the carving rail, all foot movements are small.

Considering that the flying board is very reactive, and that we don't want to unsettle it while switching the feet, these multiple small movements make a whole lot more sense than a single big step! I went back to look at some of the instructional videos I had watched, and in most of them, the instructor also takes multiple steps to move the old front foot to the new side. They just don't ever talk about it. All those foilers who learned the foil jibe relatively quickly must have either noticed it, or figured it our on their own. But I know I'm not the only foiler who has a hard time learning to foil through the jibe!

So I'm excited about trying this our in my next foil session (and to try some ideas I have about the tack footwork, too). But I also found it quite exciting to read that at the One Hour Classic, foils were significantly faster than fins. The winner in both fin and foil categories, Jordy Vonk, completed 21 laps in the foil race, but only 19 laps in the windsurf race. The recorded top speeds were also faster for the foils, with 33.59 knots vs. 31.84 knots for fins. Just as amazing was the average speed of 27.4 knots for the hour. On the GPS Team Challenge, the fastest 1-hour result ever posted for the US was about a knot slower (26.46 knots by PWA sailor Taty Frans on slalom gear). So an average speed of 27.4 knots with 20 jibes in Lake Garda chop is pretty amazing! 

Saturday, June 25, 2022

GPS Speedreader 2

 I have just posted a new version of GPS Speedreader at Since this version adds a feature specifically for dual GPS units, like the ESP/e-ink loggers I mentioned in my last post, I changed the version to 2.0. This version also has a few additional new features, including support for the FIT file format, which is used my many GPS watches. In this post, I'll explain some of the new features, and give examples where they help to improve the accuracy of GPS speeds.

"Intelligent Averages" for dual GPS units

When a speedsurfer in Belgium developed a cheap DIY GPS unit with an e-ink display that can be mounted on both sides of a boom, making it easy to check the current speed when windsurfing, my friend Mike from the GPS Team Challenge suggested to calculate speeds as averages of the two GPS units. In theory, the average of two separate measurements should be more accurate than a single measurement. We discussed a few ideas about how to calculate the averages, and came up with "intelligent averages". Let's first look at some speed data from a recent foiling session:

The two GPS units mounted on each side of the boom gave nearly identical speeds for the vast majority of points. There were some ups and downs, but they were well synchronized between the units, which indicates that they were probably real changes in speed. Sometimes, these speed changes may be due to boom movements - pumping, for example, is easy to spot. In the graph above, the little spike as the speed drops is due to moving the rig during a tack. When data look like in the graph above, speed averages will be very similar between the two units, and the averages will also be very close.

But that's not always the case. Here is another example from a top-10 second run from a different session:

Here, the green speed track shows a couple of pronounced spikes. The little inset shows the speeds and error estimates for the first spike. For the two points where the green speed is about 1.5 knots higher than the red speed, the error estimates are roughly 50% higher for the green data. Since one of the two tracks is clearly more accurate than the other, it does not make sense to calculate a plain average - instead, GPS Speedreader uses the only speed of the more accurate data. That's the "intelligent" part of the "intelligent average" (the blue lines).  Not really hard, so maybe "not dumb" would be a more accurate description, but "intelligent" sounds better.

In the example above, there was a reason why the green GPS had worse data than the red: the green GPS was using only 2 satellite systems (GPS + GLONASS), while the red GPS unit used 3 satellite systems (GPS + GLONASS + Galileo). In the region shown, the additional 5-6 satellites from the Galileo system helped the GPS to be more accurate.

The effect of the artifact in the picture above was small, between 0.1 and 0.2 knots over 10 seconds. But sometimes, we can see much larger differences between two boom-mounted GPS units:

This section is from a crash, where the boom side that the red GPS was on ended up under water, and the boom side with the green GPS stayed above water. Under water, the red GPS lost track of most of the satellites, at times only being down to just 3 or 4 satellites. In contrast, the green GPS stayed above water the entire time, and kept tracking 16-17 satellites. The green GPS correctly reported that my speed dropped from 15 knots to almost 0 in less than a second - but the red GPS kept reporting speeds around 15 knots, eventually even increasing to 27 knots. Very interesting, considering both GPS units were attached to the same boom!

This is very typical behavior for u-blox GPS units in windsurfing crashes. Keep in mind that GPS chips were developed for cars, not for speedsurfing. Cars very rarely end up under water, and when they do, GPS accuracy will be the last thing the driver worries about. But cars often drive under bridges or through tunnels, where they loose GPS reception for a short time. In these situations, the "dead reckoning" behavior that the GPS chip shows is perfectly reasonable - most likely, the car will keep going at about the same speed. 

But in windsurfing crashes where the GPS ends up under water, things are different, and GPS chips that "fantasize" about steady or increasing speeds can be a problem. If you look closely at the data in this example, you can see that the error estimates rose quite slowly after the crash - it took almost 8 seconds before the +/- estimates reached 4 knots. An error estimate of 4 knots happens to be the default value for speed accuracy filters in older versions of GPS Speedreader, since this threshold makes sense for Locosys GPS units like the GW-60 watch. But for u-blox based GPS units, it it much too high, as this example shows (and I have seen examples where crashes led to incorrect GPSTC postings, and at least one false personal best).

In GPS Speedreader version 2, this problem is addressed by using different accuracy filter thresholds for u-blox based data. For typical u-blox data (5 Hz or higher), the accuracy threshold has been reduced from 4.0 to 1.2 knots. In the example above, this filters out the speeds in cells with a red background (the blue line is where the error estimates exceeds the old threshold of 4.0). The file format is used to determine which threshold should be used - .ubx and .aoa files use the u-blox threshold, other file formats use the higher (Locosys) threshold.

While it is uncommon for "dead reckoning artifacts" after crashes to lead to errors in top speeds, these artifacts can often distort the total distance for a session. The use of dual boom units and the "intelligent averages" practically eliminates this problem, since at least one of the two units will typically keep good satellite reception, and therefore report accurate speeds.