Monday, April 11, 2011

Getting started with GPS surfing

At the recent Windsurfing Magazine board test in Hatteras, several testers expressed interest in using a GPS while sailing, so here is a short guide to get started.

1. Get a GPS
You'll need a waterproof GPS to track your speed. The best GPS for windsurfing is the Locosys Genie GT-31. You may be able to save a few dollars by using a different model, but the GT-31 has the best battery life, supports a SD card for saving years of GPS data and easy transfer to computers for analysis, and has the best accuracy. It's the recommended device on both and the GPS Team Challenge (see below). You can get it from the Midwest Speed Quest web site,, and

2. Get a waterproof arm band
While the GT-31 and other GPS units are waterproof, they will not live through many windsurfing crashes, so you need to get a waterproof armband to put them in. The Aquapac large armband 212 works well. Available from the Midwest Speed Quest web site and
3. Prepare your GPS
If you bought your GPS unit with the arm band and the memory card from the Midwest Speed Quest web site, you should be all set to go. Otherwise, you should update your firmware, and change a few settings by following the instructions at

4. Get analysis software
There are several programs out there to analyze your data. I happen to use GPS Action Replay Pro, which works on Mac OS X and Windows. It's free to try for 60 days, a license costs about $40. The images I show below are from GPS Action Replay Pro. Several web sites mentioned earlier and below also have links to other analysis software.

Now you're ready to go sailing! After sailing, simple remove the card from the GT-31, put it into a card reader, and transfer the file(s) for the day onto your computer. Open the file in the analysis software and analyze! I'll give some examples in a minute.

5. Join and/or the GPS Team Challenge
Check out, a web site where you can join for free and post your sessions to compare them with thousands of other speed surfers from all over the world. Even if you're not aiming to be on the top of the rankings, it's a very useful site to track your sessions and results.

If you have a few buddies who are also into speed surfing in your area, why not form a team on the GPS Team Challenge and compete with teams from all over the world? Teams compete in 5 disciplines: 2 second top speed, 5 x 10 second averages, nautical mile, 1 hour, total distance, and alpha. "Alpha" is 500 m run with a jibe in the middle, and the ends of the run have to be close together (so you can't just go downwind - jibe - downwind). A team can have as many members, but only the best two results in each discipline on a given day count. That's a great system, because it does not hurt your scores to have slower members on your team. If you live in an areas that already has a speed team, you can consider just joining an existing team. Currently, there are only two active teams in the US: the "Fogland Speed Surfers" in the Rhode Island-Cape Code area, and "The Speedsters" in the South West.


Now to some examples of what you can do with GPS data. I will be using a data that Billy generated during the board test when he used a GPS unit for the first time. He was out on a Fanatic Falcon 111 with a Severne 6.3 m sail in 20-25 mph winds in Avon, Cape Hatteras. Here's a screen shot of his tracks:

The top shows where he sailed, with the speed (in mph) encoded by color. His top speed was close to 30 mph, very nice for the chop we had in the sailing area. The lower track shows his speed; the dips in the speed are jibes. You can see that almost all of Billy's jibes where dry, and that he carried considerable speed through his jibes (more than 10 mph for most of them, that's a fully planing jibe). The software makes it easy to zoom in on a particular jibe:

Here's a screen shot of the "Jibe analysis" in GPS Action Replay, sorted by minimum speed:

If you're working on your jibes, you can use the minimum speed and the "Score" as objective measures of how well you did.

Another function you'll use a lot is the speed analysis - here's an example:

As I said, this was Billy's first sailing with a GPS unit. His tracks were pretty much just going back and forth at a right angle to the wind. For top speed, however, it's necessary to go on a deep downwind course, which in turn requires some steeper upwind courses to get back. Here's an example of GPS tracks from the Maui speed challenge:

The top speeds of above 40 mph in these runs where all reached at angles above 130 degrees to the wind, and the tracks have a characteristic S-shape. You can also see that the speed runs were parallel to the dominant wave direction in Kanaha. The best speeds are possible if you have perfectly flat water, for example right behind sand banks or in offshore winds. I find 43 mph in Kanaha extremely impressive...
So much for today. Get a GPS unit and join the fun! You'll probably find that you'll be sailing at angles that you rarely sailed at when just blasting back and forth, and might just find a new way to increase your fun on the water.