Windvanes 101Crash Course in Selfsteering Systems
Mechanical Windvane Selfsteering
In a mechanical windvane self-steering gear the sensor is not a compass but a wind sensitive vane. This vane sensor or air vane is set for a desired point of sail. The sensor is connected to a steering device of some kind. When the angle of the wind relative to the course of the boat (the apparent wind) changes, this change is registered by the air vane, which activates the steering device to return the boat to the selected point of sail.
Windvane self-steering does not steer a constant compass course but a constant point of sail. When using the vane gear in practical life you first sail the boat on the desired compass heading. Then you trim and balance the boat for this course. After the boat is balanced the vane sensor is set and the vane gear is engaged to steer the boat at the point of sail which corresponds to your desired compass course.
It follows that if the wind changes direction the vane gear, steering at a preset angle to the wind, will cause the yacht to change course away from the desired compass heading. This property seems to provoke a feeling that vane gears cannot by "trusted" like autopilots. That, however, is a faulty assumption. On a sailboat it is actually preferable to use a self-steering device that steers by the apparent wind, keeping the boat from gybing or backing its sails, which an autopilot could do in a wind change. If it is extremely important to steer a straight compass course while sailing, both an autopilot and a vane gear have to be supervised. A change in wind direction will require retrimming in both cases to stay on the desired heading.
Out at sea the wind does not change often or drastically in direction. Moderate deviations can be tolerated. The vane gear will steer more efficiently as it steers by the wind, keeping the boat at optimum trim when you go upwind and minimizing the chance of an accidental gybe when you go downwind.