I'm active duty Coast Guard and one of my jobs as the Group Engineering
Officer in Maine is the repair of our small boats. Before we weld on
any small boat we remove the compass for the exact reason you describe.
My bet is that the compass will have to be swung. The compass is not
magnetized. Not my forte' though. Either the current from the welder,
or the addition of the metal (or both) changed the characteristic of the
boat's compass. Not my forte'. We normally hire a compass guy to come
do it for us every year (we're required to swing the compass at least
annually), although I understand it's not a real complicated job.
On 25 Oct 2003 23:49:09 GMT, email@example.com (WGalcik) wrote:
Uh, compasses *are* magnetized. That's what makes them work.
Perhaps they meant that the *boat*, or part of it, has become
magnetized. That's certainly possible when DC welding medium
or high carbon steel (mild steel normally won't retain a remnant
field). Even the addition of a large piece of unmagnetized steel
to the boat's structure will change the way the compass reads.
The cure is to recalibrates (swing) the compass.
If the compass error is too large to correct, then you may have to
degauss the boat, or at least the parts of it that are most strongly
magnetized. You do that with a large coil through which you pass
*AC* current. The current is gradually reduced, or the coil is gradually
moved away from the object, to demagnetize it. What this does is
leave the magnetic domains in the metal disorganized so that there
is little or no net remnant field.
The Navy has huge degaussing coils through which they pass entire
ships. But you probably won't have access to them. A quick and dirty
way to degauss a section of the ship is to wrap as many turns of
welding cable around the part as feasible, set the welder to AC at its
highest 100% duty cycle current, stick a rod, then slowly (seconds)
turn the current down on the welder to minimum. This ought to work
well enough to allow them to successfully swing the compass.
I've heard (but don't know for sure) many large vessels have their own
hull degaussing 'belts'. The person who told me of them said he thought
they might protection from stray old magnetic mines.
Any nautical types here with first hand info? Always wondered if there
was anything to that story...
Leave the old compass and install a flux gate. I gave up on my steel boat
worrying about my compass with corrections balls and all the works. The
cost of a professional to swing and certify your compass is almost the price
of a new flux gate... certainly the price of a hand held.
My flux gate was well within a degree all around after doing a 360 in
i just finished adding a wave breaker ,( about 400 lbs of steel) on a steel
lobster boat. the compass is now way off. compass people said that it must
have become magnatized ? i have welded on plenty of boats before with no
can it happen , can it be undone ?
It is always best to remove the compass and any sensitive electronics
before welding on a steel boat but I don't think I have ever heard of a
smaller boat having to be degaused.
Adding 400 lb. of steel is definitely going to effect the compass in
some way. There is arare possibility that if you were electrically
welding the grounding ccurrent might have gotten close to the conpass
and changed the magnatized the area around the binicle but that would be
almost impossible when welding a wave breaker unless you placed the
ground clamp back near the stern. More likely adding the wave breaker
changed the overall pattern of magnetic flux through the hull.
At a minimum it will have to be reswung. Not complicated but better
done by a professional and should always be done when a significant
change is made to the arangement of steel on the boat.
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
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