Analysis of a Tack
IntroductionRegular readers of L-36.com know that I am writing an Android application for racing sailboats. I have had the pleasure this winter of collaborating with a Dutch sailor who is practicing this winter in hopes of winning his class in a major regatta this summer. With their 9th place finish two years ago and their 4th place last summer, we believe it is within reach. StartLine has had many analysis features added to offload some of the required calculations. Also, the unique ability to add routes quickly is an important feature for this particular race where they don't get the course until 10 minutes to the start. It is a real pleasure to get on the water race results as every race helps to improve the application. The last race highlighted the problem of tacking in current and an analysis was done to help in understanding what happened and to insure that the application was giving good advice.
This article is an analysis of a single tack. It uses GPS tracking data to calculate the current and from that to calculate the expected tacking angle. That is compared to the actual tacking angle to see why two additional tacks were required. The boat that is the subject of this analysis is not pictured to the left, but that is a pretty boat isn't it?
Before I start, I want to say a special thank you to my Dutch friend. We met on Google Plus when he expressed interest in StartLine. The changes that he as asked for have greatly improved the program. He is not shy in telling my what should change and his vast sailing experience means that he is virtually always right on. He wanted me to point out that the Dutch are not all skating on ice all winter, although looking at the last Olympics you realize they are pretty good at that.
He races every couple of weeks in weather not near as nice as what I have here in San Francisco bay where my boat sits waiting for Spring. In fairness to myself, winter in San Francisco bay is characterized by many days of no wind followed by a few days of way too much wind. There is an occasional day in winter where the sailing conditions are finer than we ever see even in the summer. Remember what Mark Twain said: "The coldest winter I ever spent was a summer in San Francisco."
The upwind leg is the leg of interest. We are analyzing the third from the last tack before the mark rounding. We want to know why the boat was not able to make the mark without the last two tacks. The ultimate goal is to come up with better on the water tools for calculating laylines when there is current.
Plotting the VectorsThe first step is to plot the two vectors representing the velocity of the port and starboard tack we read off the GPS data. For that I used a free CAD program that is super simple to use called SketchUp. I drew the vectors pointing straight up with a length equal to the speed, then rotated them the number of degrees required. For the current vectors, I then moved them. Creating a group out of the line and the triangle makes everything easier to move. When a second identical vector required such as the current vector, the program allows for duplication so you don't have to create the same vector twice. You can duplicate while you rotate or move a vector which makes for a very easy to use interface.
Finally we can plot where the boat over water would have had to be to get pushed by the current and end up where the GPS tracked it. The math for the GPS COG velocity is that it is the sum of the boat through the water and the current. What we are doing in the sketch is calculating the boat over water by subtracting the current from our GPS values.
Analysis of Target Tacking Angle
Compare with What happened on the water
FootnoteOne thing that should be pointed out about this particular analysis is that we were lucky that the current was more or less aligned with the boat heading. If the current had been at 45 degrees to the boat heading, we would not have seen any difference in boat speed. In that case, we would have had to assume that the boat was tacking through its target angle to find the current. That would not have worked here but would in most cases so this type of analysis can still be useful. I wanted to point this out in case you want to do this kind of analysis yourself.
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