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Advanced Navigator Functions
5-67
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.