Compass adjustment magnets-orientation?

[Kurrawong_Kid]

The compass adjustment magnets on my 27 year old Ritchie Compass had lost their magnetism and were so corroded that I could not see the painted colours. They were set at 90 degrees to each other towards the end of the adjusting rods. There are 2 adjusting rods crossing the base of the compass at right angles to each other. They have screwdriver slots so the rods can be turned to adjust the compass.

I've been supplied with new adjustment magnets and are ready to fit them.

Does anyone know how they should be orientated?

Thanks in anticipation.

 

[Searush]

Doesn't that entirely depend on the specific magnetic disturbance on your boat? I suspect you may need professional help to sort that out. How accurate is the compass without compensation?

 

[Deleted member 36384]

Fore and aft magnets: Blue, south seeking pole should face forward.
Athwartship magnets: Blue, south seeking pole should point to port.

The above orientation is for correction magnets that are mounted, in relation to the Compass, to port (fore and aft) and forward (Athwartship). With the two magnets orientated at right angles to one another the Blue, south seeking ends should form the right angle if the line through the magnets was to be extended.

Its easy to not use the magnets and establish a deviation chart, its much harder to establish the deviation and then try and correct it.

Source: Coastal Navigation, T.J. Williams, 1979, ISBN 0 900335 62 9, Thomas Reed Publications Limited

 

[Kurrawong_Kid]

Fore and aft magnets: Blue, south seeking pole should face forward.
Athwartship magnets: Blue, south seeking pole should point to port.

The above orientation is for correction magnets that are mounted, in relation to the Compass, to port (fore and aft) and forward (Athwartship). With the two magnets orientated at right angles to one another the Blue, south seeking ends should form the right angle if the line through the magnets was to be extended.

Its easy to not use the magnets and establish a deviation chart, its much harder to establish the deviation and then try and correct it.

Source: Coastal Navigation, T.J. Williams, 1979, ISBN 0 900335 62 9, Thomas Reed Publications Limited

Thanks very much. Now have to interpret information, but think I have got it.

 

[Billjratt]

Google the problem and you'll get a solution.
There are several compass-swinging hits, some of them aeronautical, which give a lucid explanation of the pocess.
A good practical method ( I think) involves a small board with a vertical rod in the middle.
A line is scribed across the board through the point where the rod is attached.The whole thing could float in a shallow ashet of water if necessary. Sunshine is reqiuired.
drive the boat North by the compass, turn the board so the shadow if the rod lies on the scribed line, turn the boat 180, (shadow on other half of line) take out 1/2 the error using the appropriate magnet. Repeat as neccessary to minimise error, then do East west.

 

[Kurrawong_Kid]

Doesn't that entirely depend on the specific magnetic disturbance on your boat? I suspect you may need professional help to sort that out. How accurate is the compass without compensation?

About 8 degrees out at some points. That's why I've investigated the correction rods and magnets. If I can't sort it out, I'll call in an adjuster, but I'd rather not spend the cash on this if at all possible since it is an interesting challenge.
Sorry to read about your tendon on another post. I did mine March last year. Just about right now! Keep up the excercises to get it moving again.

 

[pugwash60]

Suspect that you have a setup similar to that on an aircraft compass. As far as I recall, if that is the case, you will have pairs of magnets of similar strengths. You will need to fit them in pairs the opposite way around so that they are trying to 'stick' to each other. The screw will be set up to seperate them slightly so that you go from zero correction when they are together (they cancel each other out) to larger amounts of correction. You should have two sets and these should be at right angles to each other.
The ones orientated fore and aft are for EW corrections, the ones athwartships are for N S corrections. There is no correction for the intercardinal points with this type of system so you have to hope it is zero, (this would come be corrected by the delightfully named Kelvins Balls.) Aeroplanes don't bother with it either.
To correct your compass point the boat exactly at magnetic north then wiggle the correction screws a bit to get the compass to line up with North too, then point south and do it again but only take off half the difference this time, you should then have reduced the amount of error on North and South to a minimum and it'll be about the same on each. Do the same on E and W and that should be it. You can note any errors and make up a deviation card, it you want to do a rolls royce job you should also note the errors on the intercardinal points too and interpolate between your eight knowns to make the unknowns.
To practice before going out on the boat it can be helpful to 'swing' your wheelbarrow, this should give you the idea.
The main problem on small boats is to line up with magnetic North, but if you go offshore a bit you can find it from the chart with your gps and line up with a suitably placed church, tree headland etc. You want it to be a couple of miles away so that you can motor or sail in small circles without the bearing changing.
A slightly easier alternative is to remove all correction magnets and ignore the errors which are normally not that big anyway but do make sure the compass is well away from speakers which have big magnets inside them.
If you've just gone to the effort of getting the magnets, this system will probably not be that attractive.

 

[pugwash60]

Fore and aft magnets: Blue, south seeking pole should face forward.
Athwartship magnets: Blue, south seeking pole should point to port.

The above orientation is for correction magnets that are mounted, in relation to the Compass, to port (fore and aft) and forward (Athwartship). With the two magnets orientated at right angles to one another the Blue, south seeking ends should form the right angle if the line through the magnets was to be extended.

Its easy to not use the magnets and establish a deviation chart, its much harder to establish the deviation and then try and correct it.

Source: Coastal Navigation, T.J. Williams, 1979, ISBN 0 900335 62 9, Thomas Reed Publications Limited

I don't believe that it's possible to say from one boat what the magnet placement should be on another. The magnets correct for permanent (hard iron) magnetic errors on your particular boat. These will be different on every boat, as will the strength of the magnets and the distance of the magnets from the card and the strength of the magnetism on the needle of the compass itself. All of these affect the compass and are inter related.

If the screws on your adjusters are not separating the magnets then you need to establish the deviations before placing the magnets and use the magnets to remove the deviations. (There are three (at least) systems of magnet placement, on big compasses mounted on a dalek like binnacle, small single magnets are placed in holes under the compass bored horizontally fore and aft and athwartships in timber underneath the compass bowl, on smaller compasses the magnets are arranged in pairs so that in one orientation they cancel each other out and as adjustment is made then they cause corrections for NS and EW deviations. The other method is small magnets placed somewhere a little distance from the compass on a line fore and aft from the compass and on a line to port or starboard from the compass, this is the method used on medium sized boats which don't have the big binnacle but have a fairly large compass bowl.

 

[AndrewB]

Pugwash60:

Many thanks for this, I needed to be reminded as well as I'm swinging the compass this weekend. Nothing I'd googled was this clear.