magenitic field

Distance or sheilding.

Cheers Trevor Jones

"Distance or sheilding."

Do you think that I should shield inside the motor cover with 1/4" steel mesh & ground it to something in the water?

For magnetic shielding you need mu-metal

Move the compass.

Gunner

"In my humble opinion, the petty carping levied against Bush by the Democrats proves again, it is better to have your eye plucked out by an eagle than to be nibbled to death by ducks." - Norman Liebmann

Relocate the fluxgate sensor as far away from the motor as possible. It will not be practical to sheild the field from the motor.

Most fluxgate compasses allow the sensor head to be remoted from the display.

Jim

Mumetal is excellent for shielding but pretty expensive and not too easy to come by.

The first and most important line of defence is DISTANCE ! Locate your flux gate sensing element as far away from the motor as possible. The interference field from the motor will not be circularly symmetric - some angles will be better than others. Try to locate the fluxgate along one of the better angles.

If this isn't good enough add the heaviest possible steel mesh forming a closed ring round the motor - a closed ring is more effective than single sheets or an open ring.

Heat the whole mesh assembly to red heat and allow to cool naturally. This gets rid of any unwanted magnetization in the mesh and leaves it in a magnetically soft state which is best for screening.

It doesn't make any magnetic difference whether the shield is grounded or not. Nevertheless, with electrical safety in mind, it's good practice to ground it anyway.

Don't expect to eliminate the problem completely. If the residual is still excessive, correction can be made with compensating magnets but you'll need a compensator unit and a bit of expert advice.

Jim

Can you verify that it is truly the magnetic field from the magneto magnet, or is it electrical interference from the spark? I have serious doubts that the magneto magnet in the flywheel is generating a wide enough magnetic field to affect the compass, even with it only 2 feet away. If it was truly magnetic, a shield of some type of soft steel, preferably "Mu Metal" would be needed around the flywheel area of the motor. But, I think it is so much more likely to be electromagnetic radiation from the spark plug wire. Here's a test that might show the difference. Start the motor, and get the autopilot responding to it, then short out the spark plug (that's probably how you shut the motor down).

If the autopilot continues to misbehave as the motor is winding down, then it is most likely magnetic. If the autopilot runs normally the very instant the spark is shorted, then it is clearly an electromagnetic (radio wave) problem. A resistor spark plug will help. Resistor spark plug wire would help, too, if you can figure out how to retrofit that to the magneto (won't be easy). Shielding the spark plug wire will also be a difficult thing to do, but might help. Placing aluminum foil all over the inside of the engine cover, and connecting it to a grounded point on the engine might be the easiest shielding technique. (I'm assuming the engine cover is plastic or fiberglass.)

I don't know if the motor will spin long enough when ignition is cut off, or whether the problem with the A/P is of the sort that it can be seen to come and go in this short a time. But, that's how you might be able to narrow the problem down to a particular type of field emanating from the motor.

Jon

The compass is installed inside the tiller pilot which I dont plan on moving.The last motor I had didnt bother the tillerpilot but that was solid state ignition. Thanx for everyones input.

The fluxgate compass works by sensing the earth's field. Anything that distorts that field will cause it to give an incorrect reading.

Any high-permeability shielding around the motor will by its nature similarly distort the earth's field.

A properly designed fluxgate will not be sensitive to high frequency rf noise caused by the ignition.

Jim

Shield in Netic-coNetic metal - it refuses E&M fields. Electro and Magnetic...

Or move it 10 feet forward, Or replace the points or caps on the motors.

Are the tops metal but rubber gaskets between it and the bottom ? - maybe it is an Antenna. Might have to put a metal wire link from the top cover of the motor to the bottom part - e.g. jump around the rubber gasket.

If you are a EE type - you might have a scope. If you do - then do this :

Take a pencil or a small plastic core and wrap 10 turns of a small diameter coil wire. Attach both ends to a BNC and then BNC (or other type of interface to scope) Then with a low scope setting - sweep the area - use multiple angles.

Use this to sniff around the motor as it runs in the water or a 55 gallon drum at home... :-)

You might be able to locate a screw that needs a cap to ground or a jumper wire....

Maybe the last time they were serviced, a jumper wire was removed.

Maybe it is just point cap time.

Martin

Sell the boat and buy a lathe

suggestions for

"Sell the boat and buy a lathe"

i have 4 lathes sometimes you gotta play

I'm having trouble figuring this out.

Is it that your four lathes are more like work, and the boat is 'play?'

Or is that you own four lathes because boats are too much work, you'd rather play with lathes?

I myself think the second is a better approximation. At least the lathes don't require the sort of maintance a boat needs!

Jim

There are two different fields the motor creates. One is a passive effect, due to ferrous materials in the motor. This distortion of the field is there whether engine is running or not. Secondly, the generator and ignition create magnetic fields. These are somewhat harder to compensate for, since they vary depending on draw of any electronic accessories.

Any decent compass has compensators designed to greatly reduce the effects of these fields/field distortions. However, the procedure is complicated and takes a fair amount of time Figure at least a half hour to hour. And, to 'swing' the compass, as they say when they do this compensation, one needs to have a pre-surveyed area where one can move the craft through the four cardinal compass headings repeatedly.

Do the instructions of our steering pilot mention compensators and the procedure to use them?

Wwj2110 wrote:

I agree. The magneto magnets will have passive paths that are designed to almost short circuit the flux in order to create a stronger spark. While not the major reason for this flux pathway, the result is considerable shielding of the magnets.

Does the motor have a generator to supply power to battery and external ancillary equipment? If not the biggest problem may just be the passive flux distortion due to ferrous metals in the engine.

J> Can you verify that it is truly the magnetic field from the magneto > magnet, or

On 07 Aug 2004 13:47:07 GMT, snipped-for-privacy@aol.com (Wwj2110) vaguely proposed a theory ......and in reply I say!:

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Use one of the lathes to turn the boat, instead of the autocompass?

***************************************************** It's not the milk and honey we hate. It's having it rammed down our throats.

On 06 Aug 2004 12:21:52 GMT, snipped-for-privacy@aol.com (Wwj2110) vaguely proposed a theory ......and in reply I say!:

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Watch the wrap.

The ONLY way of reducing the coupling factor is to place magnetic material around or sometimes beside the device to be protected. Early magnetic compasses on steel ships were protected by placing two large balls of soft iron on either side of the compass movement. As the ship moved through the earth?s magnetic field, it be-came magnetised, confusing the feeble magnet in the compass. Placing the iron balls adjacent to the compass concentrated the ships field in the iron, reducing the ship?s field around the compass movement, and hopefully leaving the compass in the undistorted magnetic field generated by the earth

Or move the sensor away form the motor.

***************************************************** It's not the milk and honey we hate. It's having it rammed down our throats.

"Does the motor have a generator to supply power to battery and external ancillary equipment? If not the biggest problem may just be the passive flux distortion due to ferrous metals in the engine. "

barebones pull start motor no charging system