T&T: Using Autopilots around nearby obstacles

[John Marshall]

Scott,
	I've cruised the PNW for a couple of seasons, always on my Simrad  
AP50 autopilot, and have never seen it glitch.

	The difference is that I don't use a fluxgate compass for heading,  
but rather a Furuno SC50 satellite compass. It has an triangular array  
of three GPSs that use the RF cycle in the GPS carrier frequencies to  
decode and report true heading, updating it as much as 10 times a  
second if desired, and uses 3-axis solid state angular rate gyros to  
carry you through brief periods of outage without alarming, like going  
under a bridge. It needs 3 satellites in view to get basic heading  
data, and 5 to cancel out the effects of pitch and roll. Accuracy is  
about 0.8 degrees.

	That said, it can output accurate pitch and roll data with many  
updates per second, given it has to use that data to adjust heading so  
its free of any influence from boat motion. A very neat box with no  
mechanical parts to wear.

	Highly recommended if you want to move beyond the antiquity, quirks  
and anomalies of magnetic compasses, given we are all becoming  
dependent on GPS in any case.

	With the exception of a few places where GPS reception is lost for  
many minutes due to terrain (like a very narrow gorge where you lose  
most of the sats), its been flawless. Of course, I lose GPS position  
data in those same places.

	When that happens, the SC50 continues to use the last data on the  
rate gyros, but which will eventually drift, leading to increasing  
error over time.
	
	Not really sure what the autopilot would do in that case, given that  
in such restricted gorges I'm hand steering anyway, given the walls  
are 2 or 3 boat lengths away.

	Of course, in such situations, chart plotters and Coastal Explorer  
and every other nav instrument alarms as well, given there's no  
heading, velocity or position data. If that outage were prolonged, I'd  
have to enable the backup fluxgate in the autopilot to get magnetic  
heading and keep the radars up.

	
I have no connection to Furuno other than being a satisfied customer.

John Marshall
Nordhavn 55-20 Serendipity



>
>
> Date: Sat, 31 Jan 2009 08:58:03 -0500
> From: "Scott E. Bulger" <scottebulger at gmail.com>
> Subject: Re: T&T: Using Autopilots around nearby obstacles
> To: "'TWL'" <trawlers-and-trawlering at lists.samurai.com>
> Message-ID: <005001c983ab$f04c0550$d0e40ff0$@com>
> Content-Type: text/plain; charset="us-ascii"
>
> A funny story, and applicable to both Mikes point as well as  
> Autopilots that
> Take a Holiday and rapidly steer you off course:
>
> In the Pacific Northwest I experienced a half dozen episodes where my
> autopilot would suddenly and rapidly steer the boat left or right.   
> Try as I
> might to isolate the source of these episodes I never could.  Coming  
> down
> the West Coast of North and Central America the boat did this about  
> 4 times.
> Once we arrived in Panama, and then for 4,000 miles on the East  
> Coast it has
> NEVER EVER done it again?   NO electronics changes were made and NO  
> other
> systems were modified, that I can think of.
>
> At one point during the FUBAR portion of the trip I was warning a  
> skipper
> who was passing me to give me a little extra room, because my  
> autopilot had
> shown this tendency to rapidly and unexpectedly change course.   
> Another
> Nordhavn skipper overheard me and later came up to me and asked  
> where my AP
> had made the rapid course corrections.  I told him the last two  
> places and
> he told me his boat (with Furuno gear) did the exact same thing.   
> Well that
> was the best info I ever got regarding what could possibly have been  
> the
> issue.  I postulate that at some point local magnetic disturbances  
> caused a
> deviation limit in the AP settings to be exceeded.  The AP sounds an  
> alarm
> and stops providing rudder input, but I'm not certain it returns the  
> rudder
> to center.  Therefore if the rudder was commanded hard over in an  
> attempt to
> deal with the disturbance, and then the AP says, I'm outta here,  
> your left
> with a rudder at 10, 20 or 30 degrees, in effect causing the rapid  
> course
> change.  Again, once in Panama and for 4,000 miles of East Coast  
> cruising
> this has not happened a single time.
>
> Back to Mikes point.  You would think after all this history of  
> rapid and
> unexpected course changes I would never be on AP in the ICW?  Well  
> just as I
> got use to seeing single digit depths under my boat, I also got  
> comfortable
> being on AP in the ICW.  However, I NEVER went under a bridge or  
> near a nav
> aid or other boat without either hand steering or standing at the  
> helm ready
> to take over.  If I had tried to hand steer all the way up and down  
> the East
> Coast without the aid of the AP I would have been driven crazy at  
> the end of
> the trip.  Mike would tell you I was crazy when we started, so what
> difference would it make?  Anyway, I must admit I am really curious  
> to see
> if the AP shows its previous tendency to do this now that Alanui is  
> on her
> way back to Seattle.
>
> Anyway, hope everyone has a great day, Scott
>
> Scott E. Bulger, ALANUI, Nordhavn 40II, Seattle WA