Using GPS as a tactic

Sailing Skills

Michael Blackburn shows how to use trackers to improve your results.

GPS tracks of your sailing races – "snail trails" – can be a great learning tool for your tactics and strategy.

Many big regattas now use tracking to help fans follow racing and competitors to analyse it afterwards. For instance, here is a link to the 470 men’s medal race tracking at the 2011 Sail Melbourne http://goo.gl/cNEBz.

I’ve taken a screen shot from the TracTrac website showing the fleet shortly after the top mark in that medal race as they run towards the bottom gate marks.

The Kiwi Snow-Hansen brothers were forced to gybe shortly after rounding the top mark in third place - they were being covered by the trailing boats (McNay and Conway). The Kiwis dropped to fifth by the bottom mark and finished fifth, while McNay and Conway finished second.

In this case the lesson for the Kiwis would have been to protect their path on the longer gybe to the next mark.

Tracking Yourself

Even if your next club race or regatta hasn’t organised tracking you can do it yourself using a variety of GPS units.

You can do it with a stand-alone GPS like the cheap Garmin eTrex 10 (http://goo.gl/tDGl3). You set it to record while you race, then load the track to your computer and see it on Garmin’s maps or Google earth.

Or, if you have a smart phone, get a waterproof pouch, a GPS tracking app and away you go. You can even get a free app (eg, Outfront for iPhone; OruxMaps for Android) that will send your live position to a website (mapmytracks.com) so your mum can watch at home.

If possible, you should track a few boats in the race at once for greater context to your snail trails. The above apps can do that with a little setting up. Or, there are package solutions like TracTrac and TackTracker.

The Australian Sailing Team uses compact QStarz GPS units to record tracks from multiple boats and the TackTracker software (Race Editor version - tacktracker.com) to download GPS data and play back races. The software has built-in maps to help visualise any land effects on the racing.

There is some cost to this set-up ($265 for a GPS and software), but the sailing-specific nature of TackTracker offers greater functionality than just using an app and a map (like variable replay speed and the ability to add marks). You can also replay races on TackTracker apps for iPhone and iPad.

Using the Trails

You won’t know exactly what you’re going to learn from seeing your tracks until you see them. Here are a few ideas of how to use them and what you could discover.

A key thing to do is to compare the angle of your track at different points on a beat to examine how well you are keeping track of the shifts (especially in oscillating winds). You can hold a ruler to the screen to compare the angle of the port or starboard track you sailed at the start of a beat to the top of a beat. TackTracker also gives bearing information for each boat, so that will be more accurate than the ruler.

Examine extra distance sailed on the reaches and runs to decide if sailing further was prudent given pressure and wind angle. See how well you tacked in time with any shifts.

Analyse your positioning with respect to other boats - did we risk too much or not enough?

Play back at high speed (eg, 10-15x) to see the game play out quickly and the impact of earlier key moves. At the same time, try to factor in any differences in boat speed that might have affected tactical outcomes. Remember that GPS tracks drawn on a blue background on a screen hides the causes of speed and angle differences caused by chop, lulls and gusts. If it was today’s or yesterday’s race, try to recall specifics of what the wind was doing as you look at the tracks.

Upwind, straighter track lines indicate steadier winds and/or steadier steering (tip: set the track record frequency to 1s or less).

If the wind is persistently shifting or affected by geography it can also be nice to see those effects from the impassive view point of an orbiting satellite.

A Lesson Learnt

One of the first things I discovered about my own positioning after reviewing track info was that I always tacked quite soon after leading around a bottom mark. I figured I didn’t want to get too close to a corner.

My GPS tracks showed that in the big picture I was a long way from the corner at that stage of the beat and that I needn’t worry. I could happily continue on to get well wide of dirty air and wash from boats still on the run before tacking.

The GPS will also give you your boat’s speed and it can be fun to know how fast you go when sailing a moth or skiff, but soon becomes passé in most boats. Your primary test of how fast you are going in small boats will always be the comparison to nearby boats sailing in the same wind.

Also, since they’re accurate to about +/-4m, position reports won’t be definitive for boat-on-boat situations or start lines.