I have a Benmar AP on my yacht,and I wonder is it possible to change the control to something more modern,that is that will interface with my other instruments,but keep the motor which is both very powerful and reliable?
I would do it by making the new autooilot operate a couple of relays then switching your motor...I would think you would not need an autopilot control to be of a size to opperate a large boat as it will only be required to work the relays...
It would work fine with a set of proper relais, but time has moved on and modern semi conductors (MOS FET) do a much better and more reliable job than good old relais. The fast switching time of these also made it possible to safe power by pulse width modulating the drive. This is a fast on-off sequence which on average takes less power consumption achieving the same torque.
A good example of a modern course computer would be the Raymarine type 150 or type 400(G), of which you can find the manuals conveniently online at their site.
The 400 for example could run drive units up to 40A at 12V or 24V. It's not cheap and might be overkill in that case.
You may have a drive unit that has separate winding for turning left or right, in this case you would have a big minus wire, and two positives that are tuned on alternatively by two relais. (That is aside of two more (smaller) wires for the clutch). In such a case you might need some extra diodes. If it is a regular motor that reverses when you reverse polarity on it's two poles, it will be entirely straight forward.
You will need to establish type of drive motor, voltage and max current draw, and then go look for the right box.
with a bit of thought and effort, just about any brand could be used, but keeping it simple with help with reliability in the future.
We have recently specified a Raymarine system in a similar senario as yours, where the client wished to keep his existing hydraulics. The new Raymarine course computers are very versatile when it comes to other brands of pumps and this would be one of the first we would look at.
If you need any help getting a specification together, we would be happy to help, and naturally quote for supply at a sensible price too!