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Title: Wild swings on Nav—not NMEA related!
#14
I'm driving a 48' commercial fishing boat. The multiplexer is mostly there as a convenient way to go from USB to the autopilot's rx terminal. It is also a backup route to put an alternate GPS feed into the computer. Right now I'm using a BU-353 in one of the laptop's USB ports. The other USB port goes to the multiplexer. Besides the autopilot (which is rx only) I had only one thing turned on, and that was a Furuno FCV-585 color sounder.

It was the sounder that caused the feedback loop last year. It is tx/rx and there is an option to retransmit whatever is received. The Furuno owner's manual was not very clear about the proper setting, but after realizing that I was re-transmitting the steering information (the feedback loop) I turned the option off and never had that steering problem again.

The feedback loop problem occurred while traveling in a straight line. This problem mainly occurs at a waypoint switch along a route. I don't recall a time when it happened while going to a single waypoint (things are often hectic when it happens—that detail might have escaped me) but if it is happening while going to a single waypoint, it is only happening for some period immediately after I switch to nav. That would be the single line equivalent of when it happens on a route. Since the sounder is relatively quick to kill or restart, that is the box that I'm planning to disable the next time this happens.

The VHF radio is a red herring. Remember, I'm in Alaska and there are long periods of time when nobody else is in radio range. A VHF radio can definitely screw up certain navigational electronics though. I saw a fancy Sperry gyrocompass once that would spin like a frisbee if a handheld VHF was keyed too close to it. I've also seen a SSB radio screw up a hard disk whenever it was keyed on a mid-4megs frequency.

My autopilot also has settings to control the turn rate. Those settings don't do anything to control the new direction of travel. They only control how long it takes to swing around to the new heading. Normally my autopilot does a pretty good job of turning on to a new heading. The 100°+ turn that I described in my first post overshot the new course bearing line only because of the extreme angle of turn and it corrected itself rapidly when it came back to the course line. When the problem occurs the autopilot display clearly shows that it is receiving a new heading that is way off the proper bearing. In fact, since the wrong heading is usually 40-60° off the proper bearing, the autopilot usually does a great job of turning to the wrong heading.

On a ComNav autopilot an alarm will sound if the rudder angle sensor doesn't detect movement after about 3 seconds. I suspect that Sea Slacker would have heard an alarm in the event of a rudder indicator failure. The strong sinusoidal swings are described in my autopilot owner's manual too. I have a setting that is the equivalent of gain for straight line control, and a second that controls the turn rate when a new course is entered into the autopilot. My wild swings aren't sinusoidal though. The boat doesn't oscillate. It just goes off in the wrong direction until the XTE builds to at least 0.1NM.
Edmund
 
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Messages In This Thread
[No subject] - by - April 1, 2010 20:32
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[No subject] - by - April 3, 2010 17:46
[No subject] - by - April 3, 2010 20:35

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