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.”