AM radio interference from drive shaft

Skipper

Skipper

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
Not really a question but comments welcome. More of an observation.
Sailing today with AM talk show on the stereo.
Have listened to this station for years offshore, very clear.
Today had pulsing interference. A cadence of scratchy noise from the speakers.
Tried portable radio no problem.
Decided interference was boat based.
Ran possibilities of pulsing noise through brain.
Realized I forgot to put trans in reverse instead of neutral. So I did it.
Noise stopped.
 
Ken Cross

Ken Cross

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Oct 24, 2010
2,405
Hunter 30 Everett, WA
Very odd, and my background is electronics. Can you make it happen again?

Ken
 
kloudie1

kloudie1

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Nov 6, 2006
10,209
Hunter 34 Mandeville Louisiana
Might be a bad connection and the slight wiggle of the engine from shaft turning is working the connection.. check the grounds to the engine..
 
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Mark Maulden

Mark Maulden

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Jan 25, 2011
2,439
S2 11.0A Anacortes, WA
Any good engineer will ask,"is it repeatable"? If it is, take the portable radio and move around the boat/engine spaces etc and see if it appears. Pulsing Interference on AM during the daytime hrs is usually a high energy discharge. Does it happen when the engine is running and in gear? Does the interference frequency vary with shaft speed in neutral? Gather all the info you can to characterize the issue..
 
C

Chief RA

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Nov 26, 2012
2,315
Catalina 250 Bodega Bay CA
Since the rotating shaft or motor would not likely replicate the radio frequencies I would have to side with Kloudie. It is more likely that it relates to DC power interfering with the radio so do check connections and especially grounds. Since it seems to react to which way the shaft is rotating I would check the motor and shaft mounts as well. Chief
 
Brian D

Brian D

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Feb 17, 2006
5,274
Lancer 27PS MCB Camp Pendleton KF6BL
Sounds like alternator noise to me. Cars are prone to alternator noise on AM broadcast band. Especially older cars. But it could be the bushings on the drive shaft.
 
C

Chief RA

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Nov 26, 2012
2,315
Catalina 250 Bodega Bay CA
Bushings will likely emit audio range freq.. Alternator sounds good but not in reverse? Not much to go on here. Chief
 
Skipper

Skipper

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
Engine off. Sailing.
Interference was a steady cadence of scratchy, going on for about 15 minutes.
After the noise was cancelled, I tried to repeat it at the time, by putting trans back in neutral.
It did not repeat. I found this odd, since the interference stopped exactly when I had previously shifted to reverse. Although I only tested it back to neutral for a few seconds.

Boat is bonded, all the through-hulls, chainplates, mast base, and engine block. Connections appear good. A little green on the through hulls but not what I'd consider a bad connection. I could be wrong.

It's probable that I've never been on that AM station sailing in neutral before. The routine is to shift to reverse after engine stop.

Shaft packing is operating correctly, trans is full with maintained fluid, coupler and cutlass are newer 3 years. Engine and driveline are smooth and in proper operating order.

Anyway, there's no problem here. Just an interesting topic for radio buffs :)
 
Brian D

Brian D

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Feb 17, 2006
5,274
Lancer 27PS MCB Camp Pendleton KF6BL
The reason I said alternator is because the engine shaft is spinning in neutral, is it not? I don't know, I am just assuming that the prop spins the shaft. It does on my outboard if I leave the motor in the water. Anyway, the shaft is spinning and this in turn causes the alternator to turn. Since the rotor is spinning against the stator plates, it creates a magnetic field. This causes a popping noise on AM radio bands. I know this is known info. So when Skipper engages the shaft, the spinning stops, yes?

But what gets me is he says on a portable radio this does not happen, is that correct? As ChiefRA mentioned, it could be the DC power wire. They could be acting like an antenna of sorts. SWAG here. LOL
 
Skipper

Skipper

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
Brian D said:
The reason I said alternator is because the engine shaft is spinning in neutral, is it not? I don't know, I am just assuming that the prop spins the shaft. It does on my outboard if I leave the motor in the water. Anyway, the shaft is spinning and this in turn causes the alternator to turn. Since the rotor is spinning against the stator plates, it creates a magnetic field. This causes a popping noise on AM radio bands. I know this is known info. So when Skipper engages the shaft, the spinning stops, yes?

But what gets me is he says on a portable radio this does not happen, is that correct? As ChiefRA mentioned, it could be the DC power wire. They could be acting like an antenna of sorts. SWAG here. LOL
Click to expand
Inboard diesel. In neutral the shaft is spinning under sail but only to the trans. The motor is not turning, which means the alt also is not. This would be true on an outboard as well. Unless it's some fabulous hybrid that has the shaft spinning an alt to charge batts from water power. :)
 
Brian D

Brian D

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Feb 17, 2006
5,274
Lancer 27PS MCB Camp Pendleton KF6BL
Yeah, after I wrote all that I realized the shaft is not engaged with the pistons. So... no alternator. But I was impressed with myself for coming up with such a grand idea. LOL
:worship:
 
Skipper

Skipper

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
Regarding possible ground issue:
Batt 1 is grounded to the engine.
Batt 2 negative goes to the breaker panel. There is a negative jumper between the 2 batts in the battery bay.
No electrical issues on the boat.

Maybe it's a trans clutch or bearing going out, and I've miraculously discovered a test to check your transmission. :-D
Just tune your radio to AM 640 while sailing in neutral. This will provide a comprehensive analysis of your transmission condition.

Last, I do have a Shakespeare splitter that enables using the VHF antenna at the masthead for the stereo. Not sure if that matters or how it would react with a mast bonded to the drive shaft.
Seems the cable and ant would be isolated.
 
SG

SG

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Feb 11, 2017
1,670
J/Boat J/160 Annapolis
You can't replicate it? It was an single incident?
My initial reaction is that the radio has a bad ground that was going through the shaft.

The boat's "Ground" or neutral shouldn't be through the shaft (though it is grounded through that too?

I think if you could replicate the condition, there are some things you could try. Put a direct wire to the ground on the radio circuit and connect that to a bronze thruhull or other direct ground to the (Salty) seawater? I
 
Bill Roosa

Bill Roosa

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
From my pre -ham radio days when I was into CB (am modulation) it was very common to have rotating tires cause interference. They even have "static dust" that you put in the tire to stop it!! The world of electrons moving about is a funny one. This is mostly true because the electrons actually move backward to what you read in the text books. All the books have positive ions moving from positive to negative when it is actually negative electrons (negative charge) moving from negative to positive. With that said when you have a bad ground you get electrons moving into and out of most of the things that have coils; starter, alternator, solenoid.....just about every motor. this can make AM RF as the magnetic field collapses when the ground opens.
My bet is a loose connection somewhere that can be jiggled when the trany is being rotated by the prop.
Good luck.
 
Brian D

Brian D

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Feb 17, 2006
5,274
Lancer 27PS MCB Camp Pendleton KF6BL
Or, it could be the spirit of some RailRoad communicator using spark gap. :)
 
Skipper

Skipper

.
Oct 9, 2008
1,742
Bristol 29.9 Dana Point
Bill Roosa said:
From my pre -ham radio days when I was into CB (am modulation) it was very common to have rotating tires cause interference. They even have "static dust" that you put in the tire to stop it!! The world of electrons moving about is a funny one. This is mostly true because the electrons actually move backward to what you read in the text books. All the books have positive ions moving from positive to negative when it is actually negative electrons (negative charge) moving from negative to positive. With that said when you have a bad ground you get electrons moving into and out of most of the things that have coils; starter, alternator, solenoid.....just about every motor. this can make AM RF as the magnetic field collapses when the ground opens.
My bet is a loose connection somewhere that can be jiggled when the trany is being rotated by the prop.
Good luck.
Click to expand
Interesting.
Although the motor does not move when off, with the shaft turning in neutral. Rock steady.
The scratchy noise was a steady rhythm. Maybe 3 scratches per second. About 200 rpm.
I checked ground connections. Looked clean and secure.
I think it was RF being generated somehow. Actual interference, not a wiring issue. I read on Rockwell International that all rotating machinery makes "bearing current". This concept is beyond my pay grade. Just adding fuel :)
 
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