True Wind

1. True Wind

2. Suspect True Direction Data

3. Suspect True Speed Data

4. Coordinate Systems

5. Definitions • True Wind (T) – Wind vector with • speed and direction referenced to fixed Earth and • direction from which the wind blows referenced to true north (not magnetic north). • Apparent Wind (A) – Wind vector with • speed referenced to the vessel and • direction from which the wind blows referenced to true north. • Platform-Relative Wind (P)– Wind vector with • speed and direction referenced to the vessel and • direction from which the wind blows referenced to the bow.

6. Definitions • Course Over Ground (C) • Vector motion of the vessel with a speed and direction to which the vessel is traveling referenced to true north. • Heading (hθ) • The direction to which the bow of the vessel is pointing referenced to true north.

7. Definitions • Zero-line-reference (Rθ) • The angle between the zero line on the wind vane (anemometer) and the bow of the vessel. • Measured clockwise from the bow.

8. Impact of 180˚ Error in Rθ

9. 1D Example of True Wind Vector Math • Vessel is stationary on a day with true wind blowing directly over the bow.

10. 1D Example of True Wind Vector Math • Vessel moves forward inducing additional wind relative to the vessel.

11. 1D Example of True Wind Vector Math • A = T + M • T = A – M • M = –C • T = A – (–C) = A + C

12. Calculation Details • Calculate the apparent wind in math coordinates. A’θ = 270˚ – (hθ+Rθ+Pθ) |A| = |P| • Adjust the course over ground to math coordinates. C’θ = 90˚ – Cθ

13. Calculation Details • Calculate the components of the true wind vector. Tu = T’u = |A| cos(A’ θ) + |C| cos(C’ θ) Tv = T’v = |A| sin(A’ θ) + |C| sin(C’ θ) • Calculate the true wind speed and direction from which the wind is blowing. |T| = (Tu2 + Tv2)0.5 T θ = 270˚ – arctan(Tv/Tu) • NOTE: Arctan function must have a range of -180˚ to +180˚.

14. Averaging Errors

15. Turning Ship Alters Apparent Wind

16. Incorrect Averaging Method

17. Correct Averaging Method

18. Questions?