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First Lessons from DeWiggler

We’re entering the second year of public release of DeWiggler, and there is now enough data to make some general conclusions. Probably the single most important conclusion gleaned from the data analysis concerns the compass.

The first half of the DeWiggler tests calibrate the boat speed (Vs) and compass heading (Ms), so it is referred to as the VsMs test or “viz-miz.” This test only requires motoring around in a predefined pattern (no sails), so it has been performed more than the other half. This test also has the greatest initial effect on the operation of the instrument system. That is, only if the apparent wind calibrations aren’t too far off. Using previously valid or the default wind settings are usually a good start.

From the data gathered, the median value of existing compass peak-to-peak deviations was 6 degrees. The interesting discovery was that on compasses with deviations above the median, no amount of compensation would sufficiently remove the error. The error was thus due to the installation area, and not the lack of automatic compensation. To correct the problem on these compasses, it is necessary to move the compass location to eliminate the source of error, and then re-run the automatic compensation.

For instance, one boat had horrible deviation values, and no amount of compensation was removing that error. The compass was moved, and the compensation improved immediately. The owner spoke to the builder and designer, and found out that steel reinforcements had been embedded in the fiberglass under the original location of the compass! There was no external evidence of this to alert the installer, so the only way to really discover this was by examining the quality of data from the compass with DeWiggler.

An aside on installing compasses: I find that using a hand-bearing compass to scout installation locations works pretty well. By moving the compass in and out of the proposed installation area, you can see the deflection caused by any ferrous materials in the area. If the compass needle moves a lot, that is probably not a good location.

It is important to remove as much deviation as possible, as any compass deviation is completely carried into the wind direction figure calculated by the instrument system. That means that 10 degrees of deviation will create a 10 degrees error in the wind direction.

For the complete analysis of the first season’s DeWiggler results, see the document at http://www.ockam.com/OldSite/dewiggler/DeWigglerReport.pdf

We have also thought about how DeWiggler is used. There are likely to be some refinements in the coming months, especially in regards to changing calibrations before a race. It’s been well-noted that racers get really nervous about changing things just before a big regatta, so some changes in DeWiggler will be made to reduce the jitters caused by changing the calibrations. Also as previously mentioned, the VsMs test is by far the easiest portion of the DeWiggler test suite to perform. There is now separate pricing to run only the VsMs test.

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