Module wind 

The wind triangle: converting between apparent and true wind.

Which true wind

There are two, and mixing them up is a real source of wrong tactical numbers. Both come out of the same formula; the difference is entirely in what the caller passes as the vessel’s motion:

• Pass speed through water and heading and you get the wind relative to the water – the “sailing wind”. This is the one that sets the boat’s trim and the one polar tables are indexed by.
• Pass speed over ground and course over ground and you get the wind relative to the ground, which is what a shore observer or a weather forecast means.

In a tideway the two differ by the whole current vector. This module takes whatever it is given and names the parameter vessel_speed_kn rather than pretending to know which one the caller meant.

Structs

Wind

Wind speed and its angle off the bow, signed positive to starboard.

Functions

angle_off_bow_deg

The wind angle off the bow, given the vessel’s heading and the compass direction the wind blows from. The inverse of direction_deg.

apparent_from_true

Apparent wind the vessel would feel given the true wind and its own motion. The exact inverse of true_from_apparent.

direction_deg

The compass direction the wind blows from, given the vessel’s heading and a wind angle off the bow.

true_from_apparent

True wind from measured apparent wind and the vessel’s own motion.