Wednesday, October 16, 2013

GPS speed in jibes: the movie

After seeing my last blog entry with movies from practicing jibes on flat water, a fellow GPS addict asked me: "How fast was your minimum speed in these jibes?" Answering this question took me a while, since I wore two different GPS units that day: the Locosys GT-31 and the FlySight. Let's start with a movie that shows two jibes, overlayed with the GPS speeds:


The first jibe was planed through. The FlySight gives a minimum speed of 10.19 mph, which seems about right. But the GT-31 shows a minimum speed of just 5.19 mph, which is clearly too low.

The second jibe was into a lull, and therefore not planed through. The two GPS units show a minimum speed of 6.2 mph (FlySight) and 7.07 mph (GT-31). The positional data and the doppler speeds for the two devices are a lot closer than for the first jibe.

Unfortunately, I used only the GT-31 at the start of the session, when the wind was strongest. I added the FlySight after about 20 minutes, after seeing that conditions were perfect to lay down nice jibes. For the time where I wore both units, the minimum speed in my best jibe was 11.4 knots; the GT-31 had a much lower minimum speed of 9.3 knots. The discrepancies between the two units were larger if the jibe radius was small (35-40 m instead of 60-70 m for wide jibes), and if the positional GT-31 data seemed less accurate (as in the first jibe in the movie above). I looked at a number of jibes on the videos, and whenever the units disagreed, the FlySight seemed to be more accurate. No surprise here, this is what I saw the first time when I compared the units.

I have sailed East Bay only a few times in planing conditions, but my search for a better GPS for jibe analysis started after another East Bay session. Just like back then, the GT-31 results differ dramatically between the positional data and the doppler data, with the doppler-based results seeming too low. In contrast, the FlySight data give almost identical results with or without doppler. The more accurate FlySight data form a much better basis for further jibe improvements, especially when synchronized with GoPro footage. In my best jibes last Saturday, I kept about 50-56% of my entry speed through the entire jibe. That's decent, but I think there is definitely room for improvement. I have a few things that I want to try out - can't wait for the next flat water session!


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