Depth sounder no good with engine running despite being isolated from boat electrics

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fredrussell

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As per the title. Nasa Target depth sounder gives false readings when engine is running. Following the advice of a helpful Nasa tech guy I made a stand-alone 12v battery pack to power the device and its still happening. Its fine while under sail. With engine on readings are all over the place.

I seem to recall that old car stereos would get interference from the wrong sort of spark plugs. Is something similar happening here?
 
fredrussell said:
I seem to recall that old car stereos would get interference from the wrong sort of spark plugs. Is something similar happening here?
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Assuming you have a diesel, about the only thing it might be is alternator interference, although I'm surprised that affects your sounder on an independent power supply. You can try putting a suppression capacitor on the alternator; loads of options, see http://www.ebay.co.uk/bhp/interference-suppressor for a few ideas.
 
Sorry, should have said - this is on a boat with an outboard. The outboard does have a charging circuit but I'm assuming its not an alternator as its only 6 amps I think.
 
fredrussell said:
Sorry, should have said - this is on a boat with an outboard. The outboard does have a charging circuit but I'm assuming its not an alternator as its only 6 amps I think.
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Maybe not an alternator like a car or inboard engine alternator but a coil that generates alternating current which is then rectified for battery charging.

My old Seafarer 3 echosounder is quite happy powered from the boat's electrical system when the outboard engine is running. That has coil and rectifier for battery charging.

OTOH my friend could not power his Seafarer 3 from the boat system when his diesel engine was running ! Fortunately it ran quite happily from an internal PP9 battery


What is your outboard and what spark plugs are you using. Maybe a change to suppressed plugs will help
 
Since we now know the engine is a petrol outboard and the OP has run the 'sounder from an isolated supply I would be fairly confident the interference is not coming via its supply line, so I would not worry about the alternator/battery charging arrangements. A far more likely source of interference is from the high tension ignition leads/spark plugs, as the OP has already suspected. Make sure the transducer cable is separated as far as possible from any wiring loom associated with the engine and if possible mount the transducer well clear of the engine, possibly not easy. Failing that you are into suppressor type spark plug caps and carbon HT leads, but perhaps these are standard original fit anyway.
Many years ago I had to make up screened HT leads on an old Albin petrol engine to counter similar problems.
Richard
 
MarcJ said:
Is the transducer anywhere near the outboard? Are you using a through hull mounting for the transducer or hanging it over the side?
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The transducer is quite close to outboard actually - sadly due to the size of boat its going to be difficult to mount it elsewhere though. I would estimate transducer is about two metres from engine.
 
DickieT said:
Since we now know the engine is a petrol outboard and the OP has run the 'sounder from an isolated supply I would be fairly confident the interference is not coming via its supply line, so I would not worry about the alternator/battery charging arrangements. A far more likely source of interference is from the high tension ignition leads/spark plugs, as the OP has already suspected. Make sure the transducer cable is separated as far as possible from any wiring loom associated with the engine and if possible mount the transducer well clear of the engine, possibly not easy. Failing that you are into suppressor type spark plug caps and carbon HT leads, but perhaps these are standard original fit anyway.
Many years ago I had to make up screened HT leads on an old Albin petrol engine to counter similar problems.
Richard
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Thanks Richard, I'll see what the spark plug type is - I kind of assumed that the suppressor type plugs would be the norm in marine engines due to this sort of thing, but its possible that non-suppressor plugs have been fitted during a service I guess.
 
fredrussell said:
The transducer is quite close to outboard actually - sadly due to the size of boat its going to be difficult to mount it elsewhere though. I would estimate transducer is about two metres from engine.
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Yes I wondered whether air bubbles/cavitation near the transducer was causing the problem. Sounds like the transducer is far enough away to discount that as a problem though...
 
I had exactly the same problem. It only dissapeared when I replaced the Target with a NASA Clipper depth sounder...
 
Even if the supply is isolated, it may be worth twisting the + and - wires together to minimise pickup. minimise the loop area which picks up magnetic field.
A capacitor of anything around 0.1 uF (microfarads) across the supply may help, close to the unit, value is not critical.
Transducer wiring is coaxial? Should be fairly immune to radiated interference.
But if there is a lot of spare cable, try coiling it in a figure 8 instead of a circle.
You could try a ferrite on the coaxial cable, you might find one on an old computer cable.
 
Remember that interference of this type is just a radio signal (without intelligence superimposed). So standard radio principles apply.

Signal are transferred by two mechanisms:
1. Direct connection. Usually the power supply, in this case that's been disconnected and an independent supply used.
2. Transmitted and received through air. The supply leads are now a receiving antenna, and might receive any strong local signal, and inject that into the depth sounder. To overcome that you need the leads to be NOT an antenna.
Options include:
a) shielding (using coaxial cable as the transducer is probably using) where the shield is earthed to the engine or boat system.
b)Shortening. The smaller an antenna is physically, the less efficient. So if you had long power supply leads, these could be an antenna. Make them as short as possible.
c) Filtering of some type. If your supply is DC (almost certainly), then a small capacitor across the leads will not short circuit dc, but will short ac (rf) if sized appropriately. Or, perhaps a choke coil so that the rf sees this as a high resistance, but dc passes.

I had what I thought to be interference once with a NASA system, but it turned out to be turbulence at the transducer. Replacing the in-hull with through the hull (despite my Rival's very thick hull) has worked a treat.
 
lw395 said:
Even if the supply is isolated, it may be worth twisting the + and - wires together to minimise pickup. minimise the loop area which picks up magnetic field.
A capacitor of anything around 0.1 uF (microfarads) across the supply may help, close to the unit, value is not critical.
Transducer wiring is coaxial? Should be fairly immune to radiated interference.
But if there is a lot of spare cable, try coiling it in a figure 8 instead of a circle.
You could try a ferrite on the coaxial cable, you might find one on an old computer cable.
Click to expand...

Yes, sound advise. Also MM5AHO ideas very worth trying. With short power cables coming purely from an independent battery and the extra filtering from chokes/capacitors together with the probably already screened transducer cable I would hope you would see a worthwhile improvement. Together with investigation of possible poor suppression from the ignition system itself I hope you find a solution.
Richard
 
Off the wall idea possibly, but what about screening as much of the inside of the engine cowling as possible with aluminium foil? If it works as a temporary fix, as all you are making is a faraday cage, you could do a better job with some form of metal gauze
 
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