Larry wrote in
:

Still, I'd like to add an autopilot IF (and only if) it can operate at
trolling speeds (about 1 knot or 1.5 mph). ComNav makes a few units
which look like they'd work, but I'd like to hear that from someone
other than the manufacturer or salesman, preferably someone who owns
one. The boat is equipped with a Garmin 540 combination
fishfinder/GPS
chartplotter which issues NMEA sentences.
--



If this is the only electronics in your system, I think your GPS, not
your autopilot, is your problem.

GPS only CALCULATES your direction of travel. The GPS signal only
allows its electronics to figure out your position every second. That
display that says your boat is pointed 085 degrees is only a guesstimate
of the GPS electronics because it looked at the last few seconds of
where you've been and where you are....a little 085 from where you were
a second ago.

At low speed, a GPS is damned near USELESS as a compass input for the
autopilot to follow and shouldn't be used. Too many sailboats try to
autopilot in poor wind conditions with the same wandering results
because the GPS' idea of direction of travel over ground is wandering,
too.

What you need for the autopilot is a COMPASS SENSOR. Some are also
called Fluxgate Sensors because they use a solid state magnetic field
sensor, not a real compass. This device will tell your autopilot which
direction the boat is pointed in EVEN AT ZERO MPH, always telling the
autopilot to follow a MAGNETIC COURSE from its information, not a
wandering GPS course from lack of information.

Under 3-4 knots, the GPS is useless as a directional compass sense
device because of its calculated guess. Use a real Compass Sensor.
Most good autopilots have at least a Fluxgate magnetic sensor as part of
their package. Unfortunately, most installations pay way too little
attention to the fluxgate's mounting position way too close to magnetic
objects mounted "out of sight" in lockers full of magnetic junk and
CURRENT CARRYING DC POWER WIRING, which always radiates a strong
magnetic field, which drags off the fluxgate or compass sensor's
calibration in some odd fashion.

The magnetic sensor, whether fluxgate or real compass, needs to be near
the CG of the boat's axii, all 3 of them. It needs to be away from all
power wiring carrying any kind of appreciable DC CURRENT and away from
ALL MAGNETIC OBJECTS, anything made of steel or brass. That's a pretty
tall order in a small boat, but very necessary to maintain compass
course accuracy at very low speeds.

On a rolling pitching boat, the solid state compass sensors are much
better than the mechanical ones, which get moving around from the motion
and give false readings. It's also best to get one that's "self
compensating", one that you simply turn in a circle a few times, slowly,
and it makes its own compensation chart which it stores to make more
accurate fixes.

http://www.maretron.com/products/ssc200.php
just as an example.....