Tools for a Calculated Approach

GPS-enabled starting tools can help put your boat right on the line, but using them properly takes practice. "New Gear" from our April 2011 issue.

January 28, 2012
Sailing World

Velocitek ProStart

Courtesy Velocitek

GPS-based devices for gauging the starting line were once the luxury of high-end programs. But the new generation of GPS units from Velocitek (at right) and RockBox (from Rock City Marine) deliver this technology at reasonable cost, and it’s really changing the mechanics of starting a race by allowing more consistent and predictable starts. For classes that allow them, GPS-enabled starting devices may reduce the number of general recalls, saving time and hassle for everyone.

I’ve used both the RockBox and the Velocitek ProStart, and they’re both very good devices. They’re easy to use, reliable, and have rapid position updating, which is critical. Remember that GPS accuracy is finite, so expect variations of up to 6 to 10 feet. Knowing this, you should exercise common sense and not rely solely on the starting unit to determine the precise line position, especially if you’re at an end of the line (see “How Close Are You, Really?” at the end of this article). I’ve found that line sights (transits) are nearly impossible to use in a crowded fleet because they’re likely to be obscured. Having an instrument that tells you where the line is can be tremendously valuable.

Using these devices is simple. First, find a good place to mount the unit on your boat, one where the helmsperson can easily read the display. Then, ping each end of the line by pressing the specified buttons on the unit. If it’s too crowded to get to these points, you can be on an extension of the line.


After you’ve pinged both ends, the units create a virtual starting line and display the distance between your boat and the line. If you’re below the line, the distance shown is perpendicular to the line, not the direction in which you’re headed. If you’re outside the starting marks, the RockBox will show the distance directly to the nearest end. The ProStart creates an infinite line passing through the two points you pinged, so it doesn’t matter where you are relative to the starting marks. It will always show your perpendicular distance to this infinite line.

Use it or lose it
Now that you have this great information, how do you use it to get a better start? First, let’s review the basics of positioning in the final minute before the start. Normally, you want to be as close to the line as possible, but still have enough distance for good acceleration. Each boat has a minimum speed that it can efficiently sail as you approach the line. This determines how close to the line you want to set up. This minimum boatspeed is unique for each boat, and also dependent on wind speed and sea state. For example, in a Melges 32, you can sail at about 4 knots in 15 knots true-wind speed and smooth water. Slower than that, and you’ll lose flow on the keel and rudder, start to slip sideways, and then need to put the bow down to create flow again. In general, the lighter the wind, the slower you can go and still maintain flow. In 6 knots of wind, the M32 can sail at about 2.5 knots and still maneuver effectively. In waves, you need to go a little faster.

On each boat I sail, I create a table of minimum speed for each condition. Then I add to that table the optimum distance to be off the line at various times before the start, say 60, 40, 20, and 10 seconds. This is the critical information you need to position your boat correctly as you approach the line, other boats aside. It’s really useful to practice starting with the GPS, and develop your table empirically from this practice.


Keep trying to go slower and higher into the wind during the period from 60 to 20 seconds, until you define how slow you can go without losing flow in each condition. Also pay close attention to how long you need to get up to full speed in the final seconds, both in time and distance. Normally, you don’t have the luxury of bearing off to a beam reach to build speed, because there are boats to leeward. So practice your high build technique, where you bear away 5 to 10 degrees below closehauled. This way, you will maintain your gap to leeward, which is critical in the minutes after the start.

If you position yourself in the way described above, you’ll have better control of your destiny, especially if you start near the middle. This is because you control the water ahead of you, and nobody will want to be ahead of you for fear of being too early. You still need to defend your space in the traditional manner, from both port-tack boats and boats reaching across your stern. But as long as another boat is not directly in front of you, you can use the high and slow technique, combined with good management of time and distance, to approach the line in a controlled way. If you can build speed at the right time, and be close to the line at the gun, you can usually live in that lane for awhile.

This method comes with a few caveats. It’s safest to ping the line after the 5-minute gun, or at least when the race committee reports the line is in position. Otherwise, the committee can adjust the line afer you’ve set your waypoints.


Secondly, remember that the distance displayed is perpendicular to the line. Therefore, if the leeward end is favored, an upwind course is a wider angle to the line, and it will take longer to cover the same “distance.” Thus, if the pin is favored, you can be a little closer than your table might suggest, and the distance will close more slowly. Conversely, if the weather end is favored, you have to begin your approach further back, and you’ll close more quickly, even at the same boatspeed.

How Close Are You, Really?
We asked Alec Stewart, of Velocitek, about the accuracy of the GPS, and whether 6 to 10 feet was an acceptable fudge factor. He tells us that “in practice,” WAAS GPS accuracy can be as good as plus or minus 3 feet in North America. Also, he adds, “because the ProStart is used to measure your position relative to the line, rather than your absolute global position, the accuracy can be even better than 3 feet when you first ping the line. This is because, on a global scale, your boat, the race committee boat, and the pin are all close together. As a result, the GPS errors in measuring your position, the position of the race committee boat, and the position of the pin tend to be similar in direction and size. This makes the errors cancel out when the ProStart calculates your position relative to the line.

“For example, if the GPS solutions you had when you pinged the race committee and the pin put both the RC and pin 10 feet further north than their true locations and your current GPS solution puts your boat 10 feet further north than its true location, the unit’s distance-to-line measurements will not be affected because shifting everything north by 10 feet doesn’t change any relative distances.


“Over time, the error in measuring your boat’s current position starts to be substantially different in size and direction from the error that was present when you pinged the race committee and pin, diminishing the benefit of this effect. This is why it’s important not to ping the ends too long before the start of the race.”


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