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 speed with which the steering force is exerted; and
• the current consumption.
The physical constants between these factors are fixed, so the only relationship that matters on a sailing yacht - steering performance (output) / current consumption (input) - is always a compromise. It is never possible to obtain maximum steering performance using minimum power.
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 relates to a car managing a steep gradient slowly in first gear, but not at all in top gear).
Autopilots are distinguished by motor capacity. This automatically fixes 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 systems with variable speed motor drives are very seldom seen. 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.
To identify the appropriate autopilot it is necessary first determine the maximum rudder torque for the boat in question; the critical factors here are rudder size ( length and width ), counterbalance ( distance from the centre of the rudder post to the leading edge of the rudder ) and speed potential of the boat. The rudder torque can either be calculated or worked out empirically, that is by actually measuring the force on the tiller or wheel. If the maximum load on the rudder exceeds the maximum torque of the drive unit, failure is inevitable. Choose a low power consumption model for a relatively heavy boat, and the steering performance will be less than wonderful. Choose a system which will be constantly at its limits and it will need replacing long before an overdimensioned one. Choose a powerful autopilot, and no battery in the world will be able to meet the power demand without regular recharging. Every compromise has its price!
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.