Simrad rate compass going bad?

[HeadedToTexas]

The old Simrad AP16 on Escape does great at times and weaves like a drunk driver at times and it seems to be getting worse. My recent experience was running from Sandusky about 10 miles north to Put In Bay. The autopilot held a true course up to the Buckeye Reef buoys, but then was 30-40 degrees off after making the turn west into PIB. And it wouldn't simply hold the course, it wavered all over the place.

I recently installed a SCAD sensor on our holding tank and the rate compass is mounted right on top of the holding tank. The SCAD technician explained that their sensor system is only powered for 300 milliseconds every five minutes and that they didn't think the two metal leads stuck to the holding tank would create any compass interference. Interesting coincidence though and I find it hard to believe it's not related.

Before I wade through the Simrad calibration procedure, can anyone steer me straight on this?

 

[Choices]

If it is on top of that plate you found the problem.

The compass is usually very sensitive to metal mass fluctuations.

A friend's Defever did a hard 90 when passing under a big metal ICW bridge, putting him into the side of a passing barge.

There are several threads on here about that.

 

[twistedtree]

Does your autopilot display show the heading? Assuming it does, watch what it's showing for heading. If that suddenly become unstable then you know it's a bad heading. Could be either a failing rate compass, or the tank sensor. Perhaps turn off/disconnect the tank sensor when underway and see if the problem goes away? It doesn't take much to mess up a rate compass, and you can't "see" the interference so it's really hard to diagnose other than by trial and error.

 

[HeadedToTexas]

I believe the rate compass has been there since Mariner installed it in 2007, but I suppose it's always been a problem simply chalked up to "that was weird."

Now that I think about it, the symptom I have noticed is that it holds a heading most of the time, but can't hold some headings. Perhaps that's when north is over the metal plate?

Is there a spec for how far from such a plate the rate compass is to be mounted?

 

[HeadedToTexas]

...and the sensor in the holding tank plate is the original that is powered on with a switch (and therefore off except when reading the tank level, which is virtually never while underway). The new SCAD sensors are stuck to the back side of the tank.

 

[C lectric]

I googled aluminum, is it magnetic?

As I suspected it is not normally BUT that can be altered by the alloying agents used. Don't assume your tank alloy is entirely non magnetic.


Even so it may not take much magnetic force to alter the Auto pilot readings or interfere with them on certain angles..

Contact the A.P. mfgr. and ask them.

Some reading:
https://www.homestratosphere.com/is... contain manganese, iron, silicon, and copper.