Most likely you will need 3 things. The motor obviously and a gear
reduction unit with sprocket to fit the chain. Most DC motors turn too fast to use directly as an autopilot
. So a gear
reduction will change the speed to something useful.
Then the third thing you will need is a shaft brake. Typically brakes can be found that either hold when power is applied or when power is not applied. You will likely need one that holds when power is applied. Most autopilots have a brake output circuit for this. When the pilot is not turning the rudder
it will apply the brake to hold the rudder in position. The people that make gear reduction units also usually make brakes that bolt in line with the gear reduction. The brake usually goes between the motor and the gear reduction unit. When the autopilot is powered off the brake will release and you should be able to turn the wheel
Some autopilots use a manual transmission
that is operated by a cable or lever at the helm
. Alpha makes some pilots that are like this. You may want to look into this as it is simpler than the electric
brake. And it can reduce the resistance when manually steering
Any 2 terminal DC motor will work with an autopilot. The best kind would be a permanent magnet motor. You have to make sure the current
drawn by the motor is less than the maximum rating of the auto pilot. It does not require a huge motor. The size of the motor mostly controls how fast the rudder can turn from lock to lock. You don't want too strong a motor else it may break things as the rudder hits the stops. Depending on the efficiency of the gear reduction unit a typical motor for a 20M boat might be a 12V 20A motor (about 1/4 horsepower). The time for turning lock to lock might be 15-30 seconds for a typical autopilot. This is not super critical but you don't want it to be 2 minutes else the pilot may not be able to keep up with adverse conditions. Neither should it be 5 seconds because a motor that big will likely break things.
You may have to experiment
a little but it requires more mechanical skills than electrical
. The motor is super simple, it has just 2 wires.