Take a look at the ComNav 1101. I have installed them on a Hatteras 54 sportfisherman, a couple of CSY
44's and am putting 1 in my 12 ton yawl (Rhodes Reliant). Most head
units which talk the NMEA
lingo can command the A/P, but IMHO its the quality of the components down the line which really control dependability. Parts
such as rudder
feedback sensors, fluxgate
in the hydraulic reversing pumps are where you need good quality.
On sailboats a common complaint is that when changing from sail to powering configuration the A/P needs to be course-corrected to allow for the amount of helm
being carried to compensate for engine
torque or sail trim.
On you Twawler look for an A/P that will be robust enough to deal with long term quartering sea course corrections to allow for yaw. A good commercial
pilot should have multiple setting capability in thia area.
Once again IMHO I would go for the reversing pump/ separate ram combo at the rudder head
. If the steering
fails, you can come home on the independant A/P system.
Don't ask me how I know
Hope this helps