Raymarine Marine GPS System manual Twa Corrections Screen

RayTech Navigator User’s Guide

Figure 5-22. Twa Corrections Screen

It’s important to note that depending on your instrumentation system’s damping, the Twa value can take up to 30 seconds to “settle down” after a tack/gybe. Whether you add or subtract the value depends upon whether the instruments have either lifted or headed during the tack. The same convention applies to a downwind gybe, whereby a lift takes you away from, and a header takes you closer to, the bottom mark. If you apparently get a lift after tacking, you must subtract that correction and add a header correction to the table.

Here’s an example of Twa calibration:

After tacking five times, the average wind direction changes 6 degrees tack-to-tack in 10 knots of wind. This has the effect of showing a lift and therefore 3 degrees can be subtracted from the upwind correction table for 10 knots of wind. Keep in mind that some tacks/gybes can yield an incorrect calibration value if the wind has actually shifted, so this must also be factored into the calculation process. Additionally, calculations will be affected if the boat has been set up differently tack-to-tack (such as varying runner loads, more backstay, etc).

5-68