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.....

On Sun, 27 Apr 2008 03:57:55 +0000, Larry wrote:

away from
ALL MAGNETIC OBJECTS, anything made of steel or brass.

Since when is brass magnetic?

Casady

The autopilot I had used a fluxgate compass. The trolling problem was
the result of incorrect response. It would overcompensate at low speeds
because of the amount of time it took between issuing a command (it
turned the wheel which directed the jet) and the boat moving. There was
no rudder feedback nor could it have been added easily. While there
were multiple levels of sensitivity which could be set, none of them
worked at low speed with the result that the craft overcompensated. I
think the unit was a Raymarine Sportpilot.

Normally, one mounts the fluxgate compass as close to the bow as
possible, but (naturally) away from anything ferrous like an anchor.
The only purpose of a GPS might be to plot a long course, but, as I
said, the main reason for adding an autopilot would be for trolling
single-handed.

I've been boating for over 50 years, and have sailed all around the
Carribean, so I'm not a novice. I've owned boats from 14 to 38 ft.
and am an EE so don't be afraid to get technical.
--

Larry
rapp at lmr dot com

On Sun, 27 Apr 2008 10:49:18 -0400, Larry wrote:

The autopilot I had used a fluxgate compass. The trolling problem was
the result of incorrect response. It would overcompensate at low speeds
because of the amount of time it took between issuing a command (it
turned the wheel which directed the jet) and the boat moving. There was
no rudder feedback nor could it have been added easily. While there
were multiple levels of sensitivity which could be set, none of them
worked at low speed with the result that the craft overcompensated. I
think the unit was a Raymarine Sportpilot.

Normally, one mounts the fluxgate compass as close to the bow as
possible, but (naturally) away from anything ferrous like an anchor.
The only purpose of a GPS might be to plot a long course, but, as I
said, the main reason for adding an autopilot would be for trolling
single-handed.

I've been boating for over 50 years, and have sailed all around the
Carribean, so I'm not a novice. I've owned boats from 14 to 38 ft.
and am an EE so don't be afraid to get technical.

Give it a rudder so it can become the happy little second-order system
it was born to be.

Goofball_star_dot_etal wrote:

Give it a rudder so it can become the happy little second-order system
it was born to be.

Well, you can't add a rudder to a jet boat. I could have connected the
sensor to the tube used to steer the craft, but since the the entire
coupling was underwater, it would have been quite a production and I
still don't know that it would have worked correctly.
--

Larry R
rapp at lmr dot com

(Richard Casady) wrote in
:

Since when is brass magnetic?

Casady



http://www.kayelaby.npl.co.uk/genera...2_6/2_6_6.html

On Sun, 27 Apr 2008 21:44:36 +0000, Larry wrote:

(Richard Casady) wrote in
:

Since when is brass magnetic?

Casady

*****

http://www.kayelaby.npl.co.uk/genera...2_6/2_6_6.html

This helpful table compilation from NPL confirms that brass is
essentially non magnetic, as it happens.

Brian W

On Sun, 27 Apr 2008 17:06:41 -0500, Brian Whatcott
wrote:

On Sun, 27 Apr 2008 21:44:36 +0000, Larry wrote:

(Richard Casady) wrote in
t:

Since when is brass magnetic?

Casady

*****

http://www.kayelaby.npl.co.uk/genera...2_6/2_6_6.html

This helpful table compilation from NPL confirms that brass is
essentially non magnetic, as it happens.

I tried to follow the link and got the runaround. Thank you for
getting there and giving us a report. Maybe there is somthing wrong
with the SSSW [Suck****SoftWare].

Casady

On Sun, 27 Apr 2008 21:44:36 +0000, Larry wrote:

(Richard Casady) wrote in
:

Since when is brass magnetic?

Casady



http://www.kayelaby.npl.co.uk/genera...2_6/2_6_6.html

That website has nothing to say. It lists magnetism in a table of
contents but when you click on it, it goes back to a previous table of
contents. There seems to be nothing there.

Casady

Richard Casady wrote:

On Sun, 27 Apr 2008 21:44:36 +0000, Larry wrote:


(Richard Casady) wrote in
t:


Since when is brass magnetic?

Casady



http://www.kayelaby.npl.co.uk/genera...2_6/2_6_6.html


That website has nothing to say. It lists magnetism in a table of
contents but when you click on it, it goes back to a previous table of
contents. There seems to be nothing there.

Casady


It showed magnetic susceptibility of brass as 1, steel as 150.

Less than 1% as magnetic as steel?


Richard