demagnatizing after welding

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 problems. can it happen , can it be undone ? thanks walter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
Dave Young
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 25 Oct 2003 23:49:09 GMT, snipped-for-privacy@aol.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.
Gary
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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...
Erik
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 learn mode. Randy
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 problems. can it happen , can it be undone ? thanks walter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
WGalcik wrote:

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.