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Many types of drive systems can be used.
A faster moving ram can drive in more difficult conditions, and react more quickly.
A more efficient ram (ball screw vs lead screw) makes a huge difference on efficiency.
The most basic brushed motor drive will move the rudder in one direction when power is applied, and the other way when power is reversed. The rudder can rotate at least 30 degrees over the full range. The rudder should move from end to end in 10 seconds or less.
If the ram is not powerful enough to compensate weather helm, it may still work if:
- attach the ram further out to gain more leverage. The range of motion lost is not important sailing upwind
- attach a bungee or spring to rudder to cancel weather helm allowing the motor equal resistance in both directions
It is possible also to drive a trimtab or servo oar from the autopilot.
A typical brushed ram
The motor must be powerful enough to move the rudder fast enough. This ranges widely depending on conditions. If the autopilot is only used in light conditions (maybe a wind vane takes over when it is stronger) then the motor size can be significantly smaller.