Windvanes 101Crash Course in Selfsteering Systems
The Air Vane Sensor
The air vane is of crucial importance. The more forceful its signals, the greater are the chances of any self-steering device to achieve course correction. Two basic type of vane are used: the vertical axis and the horizontal axis vane.
The horizontally pivoting air vanes give a much larger and more powerful signal for a given wind change than do vertically pivoting vanes of the same size and area. The performance of a horizontally pivoting vane is further improved by tilting the axis 10-20 degrees. The tilted axis causes the air vane to gradually feather back into the wind as it moves from its neutral, on course, upright position, making the corrections smooth and preventing over-steering.
A lot of people have trouble understanding the way the horizontally pivoting air vane works. Think of it as an upright paddle with a forward edge and an aft edge. The paddle is set so that the forward edge points into the wind when the yacht is on course. With the forward edge pointing straight into the wind there is equal wind pressure on either side of the air vane, keeping it in the neutral, upright position. If the boat gets off course the angle of the wind changes and the air vane gets hit on one side only, forcing it to yield and pivot on the horizontal shaft.
Regardless of the way an air vane pivots, weight is of crucial importance. The heavier the vane the more it will be influenced by the motion of the boat rather than by the wind. High aspect and airfoil shape aid the performance of an air vane as they will the performance of a sail or an airplane wing.
Using these criteria it is obvious that the air vane sensors of many vane gears are a far cry from the ideal and are sometimes the primary cause of weak performance.