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Background -- Maximizing Speed to Windward

Without Fancy Instruments

What is Wrong with GPS VMG?

course

Why not just use your GPS?

Before we get into the method I want to address a question that had me doing the wrong thing for many races. The question is, why not just set your GPS to the next mark and sail for the maximum VMG on the readout? The simple answer is because the GPS doesn't know about the wind and it doesn't know you are going to tack. Consider the boat on the left. It is positioned so that the mark is directly upwind. Let us assume that the optimum course for this boat is 45 degrees to the true wind at a speed of 6 knots. That is that the boat should sail as long as the wind stays at 12 knots. If we put on the GPS, it will tell us to go 6 knots on a course 45 degrees to the wind. Perfect.

Now consider what the GPS is going to think about your course just before you tack. You are on the best possible course and you are approaching the layline. But the mark is 90 degrees off your starboard beam so your VMG is zero. You are not making any progress toward the mark as far as the GPS is concerned and it will want you to turn your boat toward the mark so that at least you get some motion toward it even though doing this would get you to the mark later.

The GPS just does not know you are going to tack. In fact, when you are half way to the point where you will tack, the GPS is going to tell you to pinch about 6 degrees which will slow the boat to almost 5 knots and you just took 4% off your real VMG to the wind. You may not notice the 4% but how would you like to have a handicap 30 points lower? Same thing. Your performance will suffer until you give up on the GPS for the leg but later you forget that you did and you repeat the same thing next week. At least that is what I did.

What the GPS would say for your optimum course

GPS_VMG
The graph on the right represents what the GPS would say about your VMG to wind if you sailed the optimum course. It would start out with the correct reading as you are traveling at 45 degrees to the wind. But that would go to zero just before you tack. And after you tack, you are heading directly to the mark so your VMG is your boat speed.

Can't I adjust the waypoint and still use the GPS?

No problem you say. All I need to do is set a way point in the GPS many miles upwind rather than setting a point at the next mark. That will in fact work, as long as you keep moving that point for every change in the wind direction of more than a couple of degrees. This assumes you know exactly what direction the wind is actually coming from. In practice, it just isn't going to work.

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