Raymarine Marine GPS System manual True Wind Angle Calibration

Advanced Navigator Functions

Here are some important guidelines for successful Awa offset calibration:

∙Choose a day with no wind shear, minimal gradient, and a steady True Wind Speed.

∙Ensure that Navigator’s DataTrak utility is online and operational.

∙Sail upwind at your optimum close-hauled angle.

∙Complete four to six tacks in steady wind conditions.

∙Use the “wand method” of averaging in the charts to ascertain the difference in Awa on each tack.

Once you have determined the Awa offset, enter it into the Mast head Unit offset parameter within your instrumentation system. Use half the difference between the wind angle on each tack. This keeps the Awa symmetrical tack-to-tack, because Laylines are predicted based upon the Awa.

True Wind Angle Calibration

True Wind Angle must be calibrated because of the way instrumentation systems measure Awa. Since Awa is measured relative to the top of the mast, errors can be introduced by the mast-top twisting under various wind conditions as a result of mainsheet, topmast, and runner loads. Awa calculation accuracy is also affected by air flow over the sails (and would be entirely different in the absence of sails) and by wind shear as discussed in the previous subsection.

To calibrate Twa, begin by performing at least six tacks/gybes each day to correct for Twa and Twd. The simplest way to arrive at a value is to watch the Twd before and after a tack, halve the difference, then add or subtract this value to/from the wind correction table for that wind strength. You can use the Twa Corrections screen (Figure 5-22) within Polars to keep track of these corrections.

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