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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 11-18-2007
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electrical shielding

I am installing a starting battery in the only available space near the house bank under the starboard setee. The SailComp fluxgate compass will be within 18 to 24 inches away. Will I have a problem and if so is there a way the shield the compass or battery? Is there such a thing as a shielded battery box?

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Old 11-18-2007
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I don't see a problem. Lead and lead sulphate is non-ferrous so the battery should have no effect. Just make sure to run the cables as far away from the compass as possible. The only time DC can induce a magnetic field is for a few micro (nano?) seconds during power on and off, and a starter doesn't get used for very long. During charging a small magnetic field will develop around the cables, but a very small one, and I seem to remember that mag effects decrease with the square of the distance.

I have a GPS/depth sounder mounted on my binnacle and even when the sounder is one it has no effects on the compass.

There should be no problem, but double-check everything after installation.
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Old 11-18-2007
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The problem is that the magnetic field is proportional to the current IIRC... and if you're charging the battery, the current may cause a problem. That said, I doubt that it will be a problem, since it would take a fairly significant magnetic field to affect something 18" away. In any case, you really do need to test it to see if there is a problem or not, both under power and sail.

The battery itself shouldn't present a problem...it is just the cabling that may be an issue. Normally, you could twist the positive and ground wires to create some shielding effects, but that really doesn't work with battery cables since they're to thick to twist tightly enough to cause the shielding effect IMHO.
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Old 11-18-2007
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Yep, agree with other posts.

Best check will be to sail with autopilot and see how she holds the course then do same under power and see if there is any difference. There should be no problem.

Having said that, my fluxgate is under the stern cabin bunk alongside one of the clothing drawers and when my wife put a belt with a ferrous buckle in the drawer, the autopilot had a mind of it's own so they're pretty sensitive.

Andre
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Old 11-19-2007
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I think there will be a significant problem based on the currents and fields from around the battery cables. I don't think th battery itself will be an issue. Three step process:
1. Do the installation
2. Take the boat out with auto pilot on and turn on some big current drains along with engine and alternator charging battery. Compare auto pilot course to compass course on various hedings and insure boat will hold courses.
3. If there ARE issues...try shielding the battery cables first and re-check. If there are still problems...you gotta move the Fluxgate.
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Old 11-20-2007
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You know, I do believe you can not shield magnetic fields anyway. The only way to do anything is with distance. Keep potential offenders away from sensitive spots such as the compass.

You can shield electric fields.
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Old 11-20-2007
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yes u can

If magnetic fields could not be sheilded magnetic dial indicator bases that can turn off and on would not exist.
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Old 11-23-2007
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I'm not sure what a magnetic dial indicator is.... but. If you can shield DC magnetic fields, then you are a billionare genius and don't yet know it. You have solved the energy crisis and can now make a perpetual moving machine.
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Old 11-23-2007
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Quote:
Cable shielding
Electromagnetic shielding
Twisted pair cables are often shielded in attempt to prevent electromagnetic interference. Because the shielding is made of metal, it may also serve as a ground. However, usually a shielded or a screened twisted pair cable has a special grounding wire added called a drain wire. This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. The shielding must be grounded for the shielding to work.


Shielded twisted pair (STP)
STP cabling includes metal shielding over each individual pair of copper wires. This type of shielding protects cable from external EMI (electromagnetic interferences). e.g. the 150 ohm shielded twisted pair cables defined by the IBM Cabling System specifications and used with token ring networks.


Screened shielded twisted pair (S/STP)
S/STP cabling, also known as Screened Fully shielded Twisted Pair (S/FTP),[1] is both individually shielded (like STP cabling) and also has an outer metal shielding covering the entire group of shielded copper pairs (like S/UTP). This type of cabling offers the best protection from interference from external sources.

Screened unshielded twisted pair (S/UTP)
S/UTP, also known as Fully shielded (or Foiled) Twisted Pair (FTP), is a screened UTP cable
Here ya go....
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Old 11-23-2007
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You can definitely shield magnetic fields:
http://www.coolmagnetman.com/magshield.htm
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