Hello this is my first post in about 15 years to digi-key.
I am gathering parts, information and knowledge to build an autopilot for my 18 foot boat.
Basically, to begin with it will steer my 7.5 Hp. trolling motor while fishing at low speed - typically 1-3 mph.
My heading, waypoint, and position information will come from my Humminbird fish finder/ sonar/ GPS through
a NMEA $GPAPB data sentence. This will go to an Arduino Mega and in turn to a set of IRF520 Mosfets to drive a pair of relays.
The relays control a 12 volt linear actuator to actually turn the outboard to correct course errors and change heading to a new waypoint.
I have a list of waypoints on a nearby lake where I want to fish and this will auto-navigate a preset course.
In time I would like to add depth sounding info to allow the boat to follow a given depth contour, also maybe a throttle control to offset
wind and currrent conditions if the GPS fix and speed is not effective.
QUESTION:
Is anyone else working on or has developed a similar project? Do you know of any appropriate Arduino sketches that I may not have found?
I am looking for info on the arduino GPS chip for what NMEA sentences it provides and also on the Arduino flux-gate magnetometer
for magnetic heading accuracy.
My main engine is a 188 Hp stern drive powered by a Ford 302 V-8. It can get upwards of 50 MPH but I don't think I would trust an autopilot at
that speed unless I was very high in an aircraft. 
Tnx es 73,
Jim VE4JI
Manitoba, Canada