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Old 11-07-2006, 22:12   #16
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Now thats what I'm talk'n about! Thanks Rick for nailing down all those uncrossed T's and undotted i's
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Old 12-07-2006, 09:37   #17
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Rick while I agree with your basic premise, more conversations with the MFG have helped. Basically while many points everyone made are valid and I appreciate all the input, I took some of these conscerns to Nexus. They said "Yes this is mostly all true but in the real world as opposed to the theoretical, compass performance is affected so little it can't be measured in most cases". Also as low in the boat as I have it installed it, it is at foot level when standing on the cabin sole, will reduce it's motion and acceleration more than if I had installed it on the centerline which would have had to be considerably higher. All in all it will even out performance wise.

I appreciate all the comments as it gave me more ammunition to talk to the people at Nexus and bounce those ideas off of them.

By the way, while it is not well known, the Nexus system is great and their support is wonderful. This Fluxgate compass was broken - internally when I bought the boat (unknown to me), a year later when I went to install it I called the dealer and he told me to open it up and let them know what I was seeing. Well the mounting for the glass compass ball was broken and the wire had come disconnected, also there was a broken corner on the outer case. I got an e mail that same day from Nexus up in Vancouver that said send us the compass. So about two weeks later my compass appeared at my door all fixed and tested in a brand new case and they didnt even ask me to pay the shipping!..That is some service considering that I didnt even buy the thing originally. Also they have been invaluable in helping me with other parts like extension cables and advice. I know that when it comes time to get other stuff they will be first on my list.
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Old 12-07-2006, 10:26   #18
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Always good to hear about good service (and also bad). Amazing how some can get it so right, and others so absolutely wrong!
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Old 12-07-2006, 10:36   #19
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As alanperry discovered, itís not just the horizontal distance off centreline that causes compass perturbations - but the distance from the radial centre of rotation (in any direction).
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Old 12-07-2006, 13:20   #20
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Not just theoretical

The real heart of a well operating autopilot is the compass input. When you discover one day (often in heavier sea conditions) that the autopilot seems downright sluggish in keeping the boat on track or it wanders all over the place you need to remember these details of what can cause heading errors.

Another detail often overlooked is that if your compass is marginally able to measure the earth's magnetic field in the presence of other influences you may have a good autopilot heading in one direction and not in another. Flux gate compasses are not nicely linear in this regard, especially in locations where the earth's magnetic field is actually vectored at an angle into the earth, not nicely parallel to the surface, like most people assume is the case.
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Old 12-07-2006, 14:57   #21
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The real heart of a well operating autopilot is the compass input
For a really good autopilot, the compass input is just the start, The real basis for accurate course keeping is the rate gyro input. This not only tells you if you are going off-course, but how fast the boat is turning, thus the brain can apply an appropriate amount of wheel for the conditions.
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Old 12-07-2006, 17:28   #22
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Well, the real world is about to be tested soon and we will see what happens with the heading and the compass response time.

Mounting of any piece of hardware on a boat is always an excercise in compromise. I don't know of any boat that is set up precisely to allow you to mount everything you have, or may acquire in the future, in the optimum place.

Given that my boat rolled out of it's yard 27 years ago I feel lucky to find a good place to mount my chartplotter!

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Old 13-07-2006, 16:49   #23
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rate gyro input

I agree with Talbot that if one has a rate gyro to input to an autopilot it will give you the best response time in keeping the boat on track, especially in following seas.

But those babies are expensive so far even if available. Some day relatively soon the the new solid-state gyro will be used. Don't know why they aren't prevalent now because they already are affordable in model helicopter controls and other radio controlled aircraft.
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