Читать книгу SELF-STEERING UNDER SAIL - Peter Foerthmann - Страница 9

Autopilots depend on a compass. A steering impulse produced by the compass actuates an electric or hydraulic motor which extends or retracts a rod or hydraulic cylinder, moving the rudder so as to bring the boat back on course. The compass carries out a desired/actual value comparison and continues the steering operation until the vessel is back on the desired course. There is a direct relationship between

· the steering force;

· the speed with which the steering force is exerted; and

· the current consumption.

The physical constants between these factors are fixed, so the only relationship which matters on a sailing yacht, steering performance (output) / current consumption (input), is always a compromise. It is never possible to obtain maximum steering performance for minimum power consumption.

This gives rise to a dilemma, since an electric motor can be geared to produce either a lot of power slowly or a little power quickly (this is analogous to the way in which a car can manage a steep gradient slowly in first gear, but not at all in top).

Autopilots are distinguished by motor capacity, this automatically fixing the relationship between the force applied by the push rod and its speed of operation. Virtually all autopilot manufacturers rely on this proven arrangement, and none has yet produced an autopilot with more than one transmission ratio and the facility to change gear.

Such pronounced gearing-down of the force from the electric motor (to produce more force at the push rod) is not practical anyway, since the corrective movement of the rudder would then be effected too slowly to bring the vessel efficiently back to the desired course. Choose a low power consumption model for a relatively heavy boat, and the steering performance will be less than wonderful. Choose a powerful autopilot, on the other hand, and no battery in the world will be able to meet the power demand without regular recharging.