I posted this a few days ago on another thread about converting a
linear drive to rotary.
The difficult part of
DIY autopilots is the
electronics, the mechanical
parts are a piece of cake.
The image is of the second or third attempt at converting a
Raymarine ST4000 from an unreliable and expensive toy into a
cheap and reliable, if a bit agricultural,
autopilot.
The first attempt involved the
emergency repair of the
wheel drive by using the
gearbox from a
cheap Chinese
battery screw driver to replace the
Raymarine box which had tiny nylon gears which stripped.
The second got rid of the 7 mm wide belt and used the gearboxes out of two cheap Chinese
battery drills to increase the torque output. The 7 mm belt was replaced by a 25 mm wide auto timing belt for which I made a sprocket out of alloy using a drill press and small lathe. The
wheel drum was made from a Regal cookware frying pan and I have to say it did a hell of a lot better job as an
autopilot part than it ever did as a
cooking utensil.
The third is the one in the image which used a 24V
electric bike
motor to drive a bike chain reduction and a larger sprocket driving the timing belt. By this transition I had got it down to pretty well bullet proof. The 300 watt 24 volt
motor running on 12 volt provided sufficient torque and was derated enough to blow the fuse before any damage to the motor.
My
new boat has a
Simrad hydraulic drive autopilot and whilst it is extremely well made and appears reliable after about 6,000 nm of coastal cruising I don't get anywhere near as much gratification from it as I did from the agricultural
DIY job.
If you are handy with tools this is probably your best solution.
If you go the DIY
route a good source of motors is the ones from aftermarket 12V radiator fans.
I highly recommend Regal cookware as suppliers of wheel drums (I actually emailed the president of Regal complimenting him on the utility of his frypans but the curmudgeonly beggar declined to answer)
12 mm pushbike chain and sprockets are readily available and the larger sprockets required are easily made from alloy plate on a drill press.
I used the timing belts with semi circular teeth as it made the sprockets easy to manufacture.
On the left hand side beside the drum is a lever in the disengaged position which lifts the motor and loosens the belt which then slides on the drum allowing hand
steering. The lever and push rod are made out of scrap pieces of alloy section and solid rod respectively.
The brackets which hold the drum onto the wheel spokes are made from scrap alloy angle iron. I used the original Raymarine mounting brackets but they were plastic and the sun eventually got them.