Compass Deviation

[eric3a]

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[Night Sailor]

Some compasses are more sensitive than others, the biggest most expensive probably more sensitive, but it's important if you want accuracy to be aware of every possilbe thing that might influence it's freedom to swing to mag north. The helmsperson's or the crew's belt buckles, cameras, other electronic equipment, cockpit or navigation lighting on or off, flashlights, air horns, eye glasses, pocket knives, eating utensils, hand tools, etc. Whether engine is running or not. And what's stored directly below decks? Big tool box, dinghy engine?

Returning to a coastal inlet in thick fog or moonless night when your GPS has failed and you can't trust your compass to be accurate within a degree or two is not what I would want to experience.

[DaveB]

It all depends on location and how close you put your electronics and any other megnetic field to the compass.
I had a guy one time sitting next to the compass with a large steel Knife in the holster and threw the compass off 7-8 degrees.
Why I always carry two,second is away from everything and viewable from helm.
Those with AutoPilot and GPS will always see a diffrence.
In a Mac. there should't be much Variation if the compass is away from electronics or other means of Megnetic field.
Dave

You guys in the courses know this, but the deviation of Compass heading from True Heading is not a fixed thing - it isnt unusual to run into local magnetic variations where your compass can really be quite useless.

[Moe]

Before installing a compass, I was taught to zero it by affixing the compass to an accurately square cut piece of wood (putty works well) along a wood straightedge, aligning it to north, then ensuring it reads south when the square board is turned 180º, as well as east and west when turned 90º and 270º. This zeros the compensators, and makes it easier compensating for deviation once installed. This does have to be done away from metal, including nails/screws in wood decking or reinforcing wire in concrete.

[DaveB]

In Sailing,current,leeway,magnetic field has to much to differ on trying to keep a acurate heading, most experanced sailers will allow 5-7 degrees diffrence and if your useing a old chart the variation each year changes depending on latitude for Magnetic and true north.
I have had 25 degree diffrences 500 miles out of Beafort NC. Sailing to St. Thomas. There is a megnetic field out there that screws with your Compass and mind and depending were you are will varry to nothing to major.
I was brought up to always trust a compass, 40 yrs later I trust in all means of navigation and rely on all means you have aboard and always cross referance.
I still do dead reconing and use paper charts. But I do love my little Handheld GPS for Lat. and Long. and speed is great.
Dave
PS: I hope I will never have to use Celetial again, not to say it's bad just to damn hard to use in 40 ft. seas.

Before installing a compass, I was taught to zero it by affixing the compass to an accurately square cut piece of wood (putty works well) along a wood straightedge, aligning it to north, then ensuring it reads south when the square board is turned 180º, as well as east and west when turned 90º and 180º. This zeros the compensators, and makes it easier compensating for deviation once installed. This does have to be done away from metal, including nails/screws in wood decking or reinforcing wire in concrete.

[baldbaby2000]

It all depends on location and how close you put your electronics and any other megnetic field to the compass.
I had a guy one time sitting next to the compass with a large steel Knife in the holster and threw the compass off 7-8 degrees.

It's surprising what little it takes to mess up a compass. I was using a handheld compass and calibrating the heading systems on some SAAB340 turbo props. I was getting goofy readings and realized that my glasses were messing things up so I had to take them off!

[bastonjock]

so if the sighting of the compass away from electronics etc is very important,why has the dealer placed my compass on the outside of the cockpit,close to the fuse board on the inside of the cabin?

i have two gps and three compasses on my boat,so hopefully that will keep me covered

I intend to place the handheld US military compass that i have on the top of the sliding hatch and use it to work out a deviation chart for the boat.

inclineation is a new one to me

[Morimaro]

Dealers and most people place compass on either side of the companionway so it can be easily viewed from the steering position.
Putting it on the port side makes it nearer to the power distribution for conecting the compass lighting. The PDU (switchboard) has very little magnetic effect because it contains little magnetic material.
If a VHF is mounted in the same area it could affect the compass but swinging the compass would generate a variation correction card for the compass.
If you are going to rely on compass for offshore navigation then a current compass correction card would be a good idea and is a good backup as well for GPS.

Cheers

Morris Rooms
26X99 hullaballoo, Solent UK

[baldbaby2000]

so if the sighting of the compass away from electronics etc is very important,why has the dealer placed my compass on the outside of the cockpit,close to the fuse board on the inside of the cabin?

Try turning lights and things on and off and see if your compass changes. The magnetic field from electronic current is proportional to the current so high current DC devices will mess it up more. In our SUV turning on the heater fan affects our compass. Also there's hard iron and soft iron distortions: magnetized metal and non-magnetized metal. A rule of thumb is to keep soft iron at least 2 of its largest dimension from the compass. Things like speaker magnets can screw up the reading. Even stainless steel if it's close enough can affect the readings. Brass or plastic screws are better but mine seems to work fine with SS.

Inclination is nomally not a factor because the compass is gimbaled.

I've been working for a long time on a system that calculates, among other things, winds aloft on aircraft. GPS Ground track and aircraft heading are required inputs. It's a tough problem trying to use magnetic heading because of all the inaccuracies especially up in Alaska and Canada. Ring laser gyro systems are the best as they don't use the earth's magnetic field but that would cost more than our boat with everything on it!

[Catigale]

If you keep your wires (supply and ground) close to each other the net current and stray field is zero which will minimise deviation.

This is roughly the concept of twisted pairs of wires.

Twisted pairs, deviates and moral compasses...I just cant pass this thread up ....

(Brits substitute 'Earth' for 'ground' above)

[K9Kampers]

Twisted pairs, deviates and moral compasses...I just cant pass this thread up ....

Hey Cat - How about a Golden Compass!...

[baldbaby2000]

If you keep your wires (supply and ground) close to each other the net current and stray field is zero which will minimise deviation.

This is roughly the concept of twisted pairs of wires.

Yea, good point. Twisted pair is the best but at the least keeping the positive and negative wires parallel helps too. It's important that the same current be flowing in the positive and negative for the fields to cancel. In other words it won't help much to parallel a positive from one piece of equipment with the negative from another.

[Moe]

Balanced lines do cancel out fields at some distance, and the closer the spacing of the wires, the closer that distance is. Looking at it the other way, the closer the object one desires not to be influenced by those fields, the closer the balanced lines must be. However, there is a point at which the spacing of the wires is still a significant size compared to the proximity of the object, and the object no longer sees the fields as balanced. A good example of that is pairs of wires in network cable. Twisting ensures the object, in this case, a wire of another pair, gets equal field from each of the balanced lines.

It may be overkill, but I do twist DC power wires that will be running near a compass and other electronic gear. It's pretty easy to do with one end of the pair secured and the other in a drill chuck turned slowly.

[baldbaby2000]

It may be overkill, but I do twist DC power wires that will be running near a compass and other electronic gear. It's pretty easy to do with one end of the pair secured and the other in a drill chuck turned slowly.

I don't think that's overkill Moe. That's the way I do it too; with a drill. You can twist both lamp cord or individual wires that way. It also can make the wiring neater and easier to follow in the case of individual plus and minus wires.

The twisting helps significantly over just parallel wires because it forces the direction of the magnetic field near the twisted pair to alternate direction every twist. This provides better cancellation of the fields as you get further from the wires.

[Chip Hindes]

In theory, twisted pair wire cancels out magnetic field.

In practice, the DC electrical distribution panel has a lot of electrical stuff going on which does not allow the wire pairs to be twisted.

My compass was originally dealer mounted on the left bulkhead quite close to the switch panel. Just switching the running lights on and off, which not coincidentally also controls the compass night light, causes my compass to deviate over 10 degrees. And yes, the compass night light wires are twisted. I wouldn't have believed it if I hadn't seen it. I have no idea how other electrical loads might affect the compass, but that alone is enough to seriously compromise it.

My plan is to move it to the opposite side where the only source of electrical current is the light in the head. Easy to leave that off when I need to use the compass.