I have a 29' homebuilt trawler and up until recently had been using a
autopilot that used the GPS for a heading sensor. Recently I upgraded to
a more refine unit (Raymarine EV-100) using an electronic compass for
heading have have been very pleased with the improvement. A couple weeks
ago is was passing through the canal between Kentucky and Barkley Lakes
while using the autopilot - something I have done dozens of times. I was
holding to the starboard side of the canal and was passing near one of
the large concrete bridge supports that are positioned on each side of
the channel. As I was directly beside it the autopilot suddenly pulled
hard to the starboard. Fortunately I was able to quickly disarm it and
swing back port before I ran into the shore but was very surprised by event.

I turned around and passed back by it 3 or 4 more times without the
pilot but watching the compass. I was surprised to see it swing over 90
degrees each time I passed by. Went to the center of the canal making
the supports an equal distance on each side and still saw a minor swing
of the compass. I am sure this has always happened but never noticed it
until the autopilot pointed me at the way too close rip rap covered
shoreline.

I am assuming there must have been enough metal in the bridge piling to
cause the variation but never would have thought it could produce a 90
plus degree compass swing. Is this the norm with a flux gate compass?

Ray Macke
http://www.egyptian.net/~raymacke/TG/TG01.html
http://www.egyptian.net/%7Eraymacke/TG/TG01.html

Yes.  The this will occur to some degree at bridges,  old submerged lock structures etc.

Joe Pica
M/V Carolyn Ann GH N-37
http://carolynann-n37.blogspot.com/
MTOA#3813, AGLCA #5485

I have always made it a practice to disengage the autopilot when approaching
bridges and would of course never keep it engaged when near barges,
stationary or otherwise.

Rich Gano
Calypso (1972 GB42)
Panama City, FL
https://sites.google.com/site/grandbanks42sale/

I agree.
My autopilot had a flux gate compass and I experienced variations but not to
the degree you did. I also noticed the current flow through bridges can be
so intense that it will force the autopilot to work harder to compensate
from the sudden push on either end of the boat. And for those reasons when
approaching a bridge I made it a practice to disengage the autopilot or at
least have my finger on the button to put it into standby.
R,

-----Original Message-----
From: Great-Loop [mailto:great-loop-bounces@lists.trawlering.com] On Behalf
Of Rich Gano
Sent: Saturday, May 30, 2015 9:14 AM
To: 'Ray Macke'; great-loop@lists.trawlering.com
Subject: Re: GL: Autopilots and Bridges

I have always made it a practice to disengage the autopilot when approaching
bridges and would of course never keep it engaged when near barges,
stationary or otherwise.

Rich Gano
Calypso (1972 GB42)
Panama City, FL
https://sites.google.com/site/grandbanks42sale/

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Several years ago while transiting the Adolphus Reach in the Bay of Quinte
on autopilot we slowed to let the ferry cross ahead of us in a typical rule
15 crossing situation.  Much to my surprise, the boat started  to follow the
ferry.  We were easily 200 from the vessel, yet the autopilot was affected.

T

-----Original Message-----
From: Ray Macke
Sent: Saturday, May 30, 2015 12:05 AM
To: great-loop@lists.trawlering.com
Subject: GL: Autopilots and Bridges

I have a 29' homebuilt trawler and up until recently had been using a
autopilot that used the GPS for a heading sensor. Recently I upgraded to
a more refine unit (Raymarine EV-100) using an electronic compass for
heading have have been very pleased with the improvement. A couple weeks
ago is was passing through the canal between Kentucky and Barkley Lakes
while using the autopilot - something I have done dozens of times. I was
holding to the starboard side of the canal and was passing near one of
the large concrete bridge supports that are positioned on each side of
the channel. As I was directly beside it the autopilot suddenly pulled
hard to the starboard. Fortunately I was able to quickly disarm it and
swing back port before I ran into the shore but was very surprised by event.

I turned around and passed back by it 3 or 4 more times without the
pilot but watching the compass. I was surprised to see it swing over 90
degrees each time I passed by. Went to the center of the canal making
the supports an equal distance on each side and still saw a minor swing
of the compass. I am sure this has always happened but never noticed it
until the autopilot pointed me at the way too close rip rap covered
shoreline.

I am assuming there must have been enough metal in the bridge piling to
cause the variation but never would have thought it could produce a 90
plus degree compass swing. Is this the norm with a flux gate compass?

Ray Macke
http://www.egyptian.net/~raymacke/TG/TG01.html
http://www.egyptian.net/%7Eraymacke/TG/TG01.html

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