Question: Autopilot behaving badly - SailNet Community

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Old 03-11-2008
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Question Question: Autopilot behaving badly

Hi all,
I am a new owner of a 1988 Pacific Seacraft 31. It has a Raytheon ST4000 Plus Autopilot which works great when sailing with the engine off--holds a steady course in most conditions--but whenever I have the engine on, whether motoring or motor-sailing, fast or slow, the boat oscillates wildly from side to side and I have to shut the autohelm off. I am assuming the engine is interfering with the autopilot's compass. There is a decal on the forward bulkhead indicating the compass is located somewhere there, and of course,the engine is aft, so they are far apart. I consulted the owner's manual but there is no mention of this in the troubleshooting section. Any suggestions on how I should proceed in fixing this?
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Old 03-12-2008
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I am thinking that rather than the compass, the control head may be getting interference when the alternator comes on.
Is there a major difference in how your wheel feels under power vs. sail as the wheel pilot my just be having trouble contolling the added helm pressure that may boats get under power.
Control heads are no longer available but Raymarine fixes them for about $200 bucks + shipping.
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If the label is correct, I doubt it is the engine interfering with the compass. Is this using a wheel pilot or an under deck quadrant steering drive? What kind of mechanism is the autopilot using: hydraulic or mechanical? Is the oscillation random or is it predictable?

I think Cam maybe correct that you're getting some interference with the control head. One thing to check—Does it happen if the engine is running but the transmission is in neutral? If not, then it probably isn't electrical interference from the engine, but mechanical interference of some sort.
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It sounds to me like electrical interference as well. Raymarine is very emphatic about placing 'ferrites' in (or on) the the power leads going into the control unit. Is yours a factory install?
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Old 03-12-2008
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I also had some issues with my ST 7000 autopilot on a sail back to St Croix from St John. We were under sail only with 18 knot winds from the East heading 190. I had the full Genoa out as well as the main. The rudder indicator showed holding a few degrees to the starboard side and the swell was out of the NE. This seemed to cuase the boat want to turn back to Port after a big swell would pass and then the auto pilot would not keep up. I hand steered the last two hours and found myself constantly holding right rudder preasure then steering back to port every few seconds. I have hydrolic steering so I think the Auto pilot was just not able to keep up with the corrections needed. I am also wondering if I could have trimmed the sails better to help this problem.
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How's the reception on your onboard stero with the engine running? Sounds to me like your alternator is throwing electrical interference down the circuits.
If you hear the motor through your stero that's a pretty good verfiication of that.
Ferrites (small magnets) are one solution - and recommended for delicate electronics. Easy to retrofit as well - just go to radio shack, get a couple and slip them over the DC lines in the back of the power panel. Actual results may vary depending on how big the problem is.

The real solution is to root out the cause (again, likely the alternator) and fix it there with proper grounding and isolation.
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Ferrites aren't generally magnets Chuckles.. they're just lumps of iron ferrite...hence the name...
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oops, my bust - fingers going faster than brain. It's actually 'anti-magnetic' in that what it's doing is busting up the electromagnetic interference.

Longer explanations available. Don't believe the hype about them being used to prevent interference from getting out of a device, they block interference regardless of direction.
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I have a similar problem, except my ST does it with sail or motor. It seems to hold a course for a minute or two, and then it suddenly goes way off course to port or starboard. Is there anything I can do or is this a Raymarine fix?
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Jotun—

It would help if you gave some more information, like what model ST... They made five or six STx000 models at least. Tiller or wheel or under deck steering? Integrated or separate flux gate compass? What model control head? etc.
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a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
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