We sometimes get asked why PredictWind weather routing uses True Wind Angle (TWA) rather than Apparent Wind Angle (AWA), especially since most sailors steer and trim to apparent wind once they're underway.
That's true. Apparent wind is what you see on your instruments and it's extremely important for sail trim, steering and autopilot wind mode.
However, weather routing is solving a different problem.
When PredictWind calculates a route, it's trying to determine the fastest and safest path through the forecast weather over an entire passage. The routing engine does this using your boat's polar data, and boat polars are built around True Wind Speed (TWS) and True Wind Angle (TWA). Sail selection guides are generally based on TWA as well.
The reason is simple: true wind is the fundamental input that determines boat performance. Apparent wind is a result of the boat's speed and direction through that true wind.
As the boat accelerates, slows down, surfs, heels or changes course, the apparent wind changes too. Two identical boats sailing in the same true wind can have different apparent wind angles simply because one is travelling faster than the other. This makes AWA a moving target, whereas TWA remains the consistent reference used by polars, sail plans and routing software throughout the sailing industry.
There's also a practical sailing consideration. Offshore sailors often avoid steering purely to a target AWA because it's easy to end up chasing changing numbers rather than sailing the optimum course. AWA is excellent for sail trim and boat handling, but TWA is generally the better reference for routing strategy and performance targets.
For these reasons PredictWind routing is intentionally based on TWA. We believe it is the correct and most useful reference for route optimisation, and it keeps the routing aligned with the boat polars, forecast data and accepted offshore sailing practice.
While AWA remains an important onboard sailing instrument, it is not the reference used by the PredictWind routing engine, and we do not currently plan to change the routing output to display apparent wind angles.