Sorted Speeds and Jibe Analysis

 I just release a new version of GPS Speedreader which has a couple of new tools to analyze sessions: turn analysis and sorted speed graphs. This post gives a few examples of how to use these functions. 

Let's start with the sorted speed graph - here is an example:

The y axis shows speed (in knots). The vertical lines correspond to 10%, 20%, 30%, and so on. So in this example, my speed was below 10 knots for about 80% of the time, and above 10 knots only 20% of the time. In other words, I spend most of my time slogging and waiting for wind. That's also obvious in the colored tracks and the speed graph for this session:

One of those days where the wind started out marginal and then dropped ... but it was fun in the gusts.

The sorted speed graph is quite useful to compare two sessions or parts of sessions. Here is an example:

This is from 1-hour periods from Lake George in February. The red line is from Kato, who set a 29.4 knot hour personal best (PB) on February 7th. The blue line is from my 25.9 knot hour PB on February 20.  Kato sails in a different league, and his hour is one of the fastest hours ever windsurfed (currently ranked #8 on the GPS Team Challenge rankings). So what does the graph tell us about why he was 3.5 knots faster?

His top speed in the session was only 1.5 knots faster than my top speed, so better top speed is not the main reason. If we go towards the left side of the graph, the difference get larger. In the middle, which shows our medium speed, Kato was about 3 knots faster. But the biggest differences are in the slowest 20 percent, on the left side of the graph. Kato spent only about 10 percent of his time getting up to speed (26 knots of more); in contrast, I spent about 20 percent of my time accelerating. To some extend, this is because I jibed more often: 30 times, compared to Kato's 23 times. But this is just a smaller part of the difference; the bigger part was that Kato's jibes were better, and that he accelerated faster after the jibes. I know that I am often am a bit slow getting back up to speed in jibes; I remember a set of slalom races in Hatteras where I had better straight line speed than the two guys who placed in front of me, but at every jibe mark, I'd see them pull away. So that's something I would need to work on to improve my hour. 

But why was Kato's speed in the main section of the graph higher than mine? The obvious answer is that he has mad skills, while my skills are more mediocre, but that does not give hints on how to improve. But since I happened to sail at the same spot when Kato did his hour, we can compare sail sizes: I was on 6.2, and Kato on 6.6 - basically a sail size larger. I remember this day very clearly, since it was the first (and only) time I sailed faster than 40 knots (41.4 knots/2 seconds). Obviously, I was well powered on my 6.2! But Kato, whom I remember as being a bit lighter than I am, was on a bigger sail, which allowed him to sail and accelerate faster. So a concrete goal towards improving my long distance speed numbers is to get more comfortable with larger sail sizes. That is something that I have done over recent years, at least on flat water, but there clearly is room for further improvements.

Now let's get back to the jibes. The new version of GPS Speedreader has a "Turn analysis" function that lets us take a closer look at jibes and tacks in a session. Here are my jibes during my 1 hour PB (as always, click on the image for a larger version):

For comparison, here are Kato's jibes:

To analyze jibes (and tacks), Speedreader looks for the center of a turn, and then examines the 50 meters before and after the turn. The first number to look at is the "Min. speed" column, which shows the minimum speed for the entire jibe. We both messed up one jibe, which shows up with a minimum speed below 1 knot. All my other jibes were above 5 knots, and in about half of my jibes, I maintained a speed of about 10 knots. But Kato kept a minimum speed of more than 11 knots in all of his dry jibes! In his best jibe, he was almost 5 knots faster than I was in my best jibe. So again, there is a lot of room for improvement for me! (To be picky: my GPS data were recorded at 5 Hz, while Kato's data were recorded at 1 Hz, which results in about 1 knot higher minimum speeds; the difference may even be a bit larger due to filtering in the GT31 he used. But even after taking this into account, his jibes were a lot better than mine. Also note that the jibe analysis methods work better with 5 Hz or 10 Hz data, and the program missed 3 of Kato's jibes - but those were just as good as the rest).

One difference between Kato's and my jibes shown in the tables above is the width. Most of my jibes were about 50-60 meters wide, while many of Kato's jibes were 70 or more meters wide. Looking at the tracks, Kato usually went slightly downwind for a bit longer before the jibe than I did; this is reflected in the higher entry speeds. But he also consistently kept at least 40% of his entry speed through the entire jibe, which I managed to do only in my best jibes. It looks like I have to go back to Oz for more jibe practice on flat water!

One of the reasons I added jibe analysis to GPS Speedreader is that I'd like to track my progress on wing jibes using GPS data. Learning wing jibes has been surprisingly hard for me, and I still have a lot to learn. So I'll show the jibe results from my lovely wife instead, who has been nailing wing jibes for a few years already:

If you compare the "% speed kept" column to the numbers for Kato's fin jibes, you can see she kept more than 70% of speed while wingfoiling, whereas most fin numbers are in below 50%. Keeping speed with the "motor off" is much easier when foiling than when windsurfing!

There are a couple of things to keep in mind when looking at the turn analysis in GPS Speedreader. First, keep in mind that turn analysis is new, and will sometimes give wrong results. A jibe may be characterized as a tack, or vice versa; may be missed by Speedreader; or what not really be a jibe or tack. Low frequency and less accurate (e.g. watch) GPS data are expected to have more problems, but some errors will also occur with accurate 5 or 10 Hz data. Hopefully, this will get better in future version, as the algorithms get refined with more data. 

Turn analysis requires that the wind direction is accurate. While Speedreader can often deduct the correct wind direction from the GPS data, this can be quite hard with some foil data. For example, if you always tack, foil through the tacks, and never jibe, it's likely that GPS Speedreader will have the wind direction wrong by 180 degrees. At other times, the error may be 90 degrees, or just a few degrees. For turn analysis, it's always a good idea to check the wind direction in the polar plot, and adjust it by hand if necessary.