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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 02-21-2006
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rigging question

Greetings- I recently completed a rigging job. A few months afterward, the owner showed me that one of the wire sizes used is slightly magnetic, whereas the other size is not. Both reels of wire said 316 grade stainless. Obviously one was not. Would the more ferrous(magnetic) wire be stronger, but be more likely to rust(or corrode within the fittings)? The magnetism is very slight, but definitly present. The customer and I are unsure of what to do about it. Thanks for any enlightenment.
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Old 02-21-2006
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Stainless stell is not totally non-magnetic.
Magnetism can be induced in it.
This is not an issue that effects its strength or its resistance to corrosion.

Now that being said, one does wonder metalurgically if the wires are the same. Still there is no easy way to check this. A thicker wire will have a larger magnetic probability as it has more surface area.

All in all there is nothing to worry about.

Cheers!

Todd
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Old 02-21-2006
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Do you still have samples (off-cuts?) of the wires in question? You could have a chemical analysis done to determine whether or not both, one, or neither are really 316 SS. Scrap yards or metal recyclers might have a hand-held or bench-top XRF (x-ray fluorescence spectrometer) that might be able to tell you which alloy(s) your wire is.
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Old 02-21-2006
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induced magnetism

As RedCorvette suggests, magnetism can be induced. You can create a magnet easily by banging an iron rod with a hammer while it is aligned North/South. Students do this with a needle and stick it into a floating cork to make a primitive compass. Battleships magnetize their entire structure when they fire their guns repeatedly, and end up requiring degaussing. A shroud or stay could get magnetized from vibrating in the wind or from being "strummed" against by another line or halyard. Stainless is theoretically less magnetic than solid iron, but it still has iron in it. Even if you replaced the magnetized wire with a new, nonmagnetized section, who's to say the new piece would'nt become magnetized later, if that's what happened to the first one? For the owner, the best solution may be a good compass adjuster.
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Old 02-22-2006
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I'm no physicist or electrical engineer, but I do know a fair amount about magnets. I can only see two issues that could arise from magnetized wire and both are fairly theoretical.

1(and the more serious of the two): If the wire is in close proximity to the compass, it could produce false bearing readings, which could be dangerous for a off-shore cruiser especially in event of a gps failure. Same thing applies to a generator too close to a compass. However, if the magnetic source is only slight and outside of a few feet, it could well have no effect whatsoever. And as Paul said, a decent compass adjuster can clear this problem right up.

2: Magnets can form a basic current flow which over time could lead to electrolysis (the transfer of atoms from one metallic object to another via an electrolyte) If the boat is used in saltwater, seawater between the wire and the fittings could theoretically assist in this transfer (acting as the electrolyte), leading to degradation of the metal. Thing is, it would likely take years before this would occur to any noticeable degree, and the rigging would probably be way overdue for replacement by then anyway.

The only solutions to issue 1 (if it is indeed an issue on this particular boat) would be to relocate the compass or replace the wire. Issue 2 was presented only as general information and can (and should) be ignored.

As for the cause of the magnetism... In addition to the suggestions already put forth by Paul, the owner could have attempted to clean the wire with steel wool or a similar metallic abrasive, which can magnetize a rod, or wire. Another more likely cause could be a lightning strike in the vicinity of the boat (or striking the boat itself?) the wire could possibly have been magnetized that way.

One more thing to check for is an actual electrical current in the wire which could be caused by a bad ground contacting part of the attached metalwork. Any current running through a twisted wire can generate a small magnetic field. It may not be enough to feel the current in the wire (especially if it is DC current coming off the battery) but would be detectable with a multimeter. This is more likely in a steel hulled boat, but still could happen in a glass boat.

If any of this is incorrect I apologize, but as I have said, I'm no expert.

In any case, the spool of wire is likely not the culprit but can be easily tested with a bit of iron filings.

If you noticed a lot of "could have's" "theoretically's" and "likely's" in this post it is because, as RC said, there's probably nothing to worry about. Except of course the customer's peace of mind.

-James

Last edited by kazmeister; 02-22-2006 at 08:25 AM.
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Old 02-22-2006
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Another thought, there was a thread on cruiserforum.com about a halyard wire that had one strand rust, producing a candy-cane effect. It was determined that the whole reel of wire was defective, having had just one strand of the wrong alloy layed in when the wire was made up.

Regards,

Tim
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Old 02-22-2006
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Thanks for the feedback. The wire in question is the larger of the two sizes. Since they are the upper shrouds, the closest to the compass is 9 feet. It has had no effect on the compass thus far, but we will swing it anyway when we commission the new engine. There has been no cleaning(or inducement) of the wire so far(steel wool...), and all 5 wires from that spool are the same as far as magnetic properties. I'll get out the multimeter and check for any problems. All the rigging is bonded together, and the smaller wire exhibits no magnetic properties. Thanks again...
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Old 02-22-2006
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I'll look for a bad strand as well...
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