how to set up a new Compass

[Wahoo]

how do i go about setting up a new compass on the boat, how far/close to the electronics? does alloy affect the reading? also have been looking at Sestrel compass, any feed back on these? any other brands that others have/like

thanks

Daz

[Mindi]

there is a pretty good fun process called "boxing" the compass where you find two charted fixed points with a known bearing between them...ideally at cardinal points...and then a second pair at right angles ie (90 and 270)and then run between them comparing readings with the known bearing...then adjust outh half the error and switch to other direction...etc etc... unless you have the Melb port entrance piles from Gellibrand or similar its a bit hard to find suitable markers... look up "Boxing a Compass" on Google..

Maybe you can use your GPS to get true direction..? rather than known marks.

If you just want it "near enough" string a line bow to centre stern and set it parallel

As for electrics thats easy....turn all the electrics on and then slowly bring the compass towards them...when the needle is moved that's too close....typically around a foot..? maybe a bit less..?

[FNQCairns]

Although i have never done it by GPS I am guessing it's an easy way to go, zoom right in on your goto line putter along it at trolling speed and take the readings. I am thinking it should be done on the water to discount the car?

cheers fnq

[Noelm]

depends if you want to get it done properly, or just home handyman style, to do it correctly requires a Gyro Compass and a qualified person to do it for you, commonly called "swinging a compass" for trailer boats, normally a "close enough is good enough" is usually OK.

[Spaniard_King]

The most aurate way is to o do it on a proper compas bouy setup. Tese are known mooring bouys which tie up to and take compass measurements and adjust he compass as necesary.

Another way is to install the compass and ompare the heading when underway with the GPS and adjust as necessary.

As ar as distanes are concerned the general rule is 500mm but is hard to acheive at the best of times.

I check the compass away from the dash and then place in the desired position this will give you an idea if there is something affecting the compass.

[seatime]

http://www.compassadjuster.com.au/faq.htm

[Mindi]

Daz

I dont think you were looking for a pissing contest in compass knowledge but unless you have a ship you dont need a professional with a gyrocompass.

Put it somewhere that the instruments dont affect the needle when turned on..
Run in any direction (Say roughly North) on your GPS (it shows TRUE direction) not magnetic...
Your compass needs to show 12 deg LESS (it shows MAGNETIC direction)
Adjust as required by taking out half the error each time only...if it is within 2-3 degrees leave it alone.
Do the above in the other direction (going South)
Then do the above at approx East/West and West / East


or run buoy to buoy as said but you need a number of them and need to know the bearing from each to the other...GPS heaps easier...just take the digital heading number, no track watching needed.

That will be heaps good enough given the best helmsman can only steer to 2-3 degrees anyway.

It will then always show about 12 degrees less than the GPS because that is the Earth's magnetic variation here(difference between Magnetic Nth and True North)....as you change direction the boat and fittings can cause that difference to change and a professional would measure and plot that on a card for the navigator as part of the "boxing" or "swinging" process......but that deviation is irrelevant for our purposes. Hope that helps.

[Noelm]

that's why I said, that for trailer boats, the home handyman method is as good as any! never meant to imply that an expert is needed, unless it is for precise applications.

[FNQCairns]

Daz

I dont think you were looking for a pissing contest in compass knowledge but unless you have a ship you dont need a professional with a gyrocompass.

Put it somewhere that the instruments dont affect the needle when turned on..
Run in any direction (Say roughly North) on your GPS (it shows TRUE direction) not magnetic...
Your compass needs to show 12 deg LESS (it shows MAGNETIC direction)
Adjust as required by taking out half the error each time only...if it is within 2-3 degrees leave it alone.
Do the above in the other direction (going South)
Then do the above at approx East/West and West / East


or run buoy to buoy as said but you need a number of them and need to know the bearing from each to the other...GPS heaps easier...just take the digital heading number, no track watching needed.

That will be heaps good enough given the best helmsman can only steer to 2-3 degrees anyway.

It will then always show about 12 degrees less than the GPS because that is the Earth's magnetic variation here(difference between Magnetic Nth and True North)....as you change direction the boat and fittings can cause that difference to change and a professional would measure and plot that on a card for the navigator as part of the "boxing" or "swinging" process......but that deviation is irrelevant for our purposes. Hope that helps.

So in theory a person could set the GPS to run variable north and simply align with that at any bearing??? Like also does the GPS work out the exact variation for where it is?

cheers fnq

[Mindi]

So in theory a person could set the GPS to run variable north and simply align with that at any bearing??? Like also does the GPS work out the exact variation for where it is?

cheers fnq

No..GPS doesnt know about variation or anything magnetic...the GPS only knows the fixed True North Pole...(where Santa lives). ..the top of the world where the lines of longitude all meet and the base for the mapping system the GPS knows.....and when the GPS says you are travelling on "0" degrees you are pointing at this North Pole. For the sake of this argument your compass always points to the Magnetic North Pole off in Canada so if your GPS is saying "0" then your compass should be saying 348 degrees...because it is pointing a bit to the left......Mag Nth Pole is hundreds of kilometers away from the True Nth Pole and on the move... this difference between GPS (true) and Compass (magnetic) changes around the world..? here on Aust East Coast it is about 12 deg EAST as we look at it. We traditionally call it Magnetic Variation. (now officially called Magnetic Declination)

(For completeness sake Magnetic Deviation is the additional "error" introduced by the specific vessel.....but unless you are seriously navigating manually it doesnt matter to us.)

So just remember...error EAST compass LEAST, (thats us) error WEST compass BEST (thats Europe etc)

so here you add 12 to your compass reading to get the same result on your GPS.....doesnt matter what direction you are going in your compass should always read about 12 deg less....(error EAST compass LEAST.)

Hope this is helping...its really interesting stuff which has just been made history by GPS... in practice I reckon the compass has been great off the coast to reassure you the Australia is THAT way...not the other way... but navigating seriously point to point is a bit like hard work

[seatime]

"GPS doesn't know about variation or anything magnetic" granted for non-plotters, not so for chartplotters.
Plotters don't have compasses inbuilt (magnetic or gyro), but they do have digital layers of information lifted from paper charts, which includes variation.
They won't have the ability to calculate annual changes in variation (neither can paper charts), so some quick arithmetic is still needed.

I run GPS chartplotters recievers in Magnetic Heading all the time. All of the chartplotters I've used can display True or Magnetic.

[Wahoo]

thanks all, did not think it was this much of a challange, so how do boat yards fit them, same as mindi's first post? or do they just put them on so it looks like they work? if im going to spend a few $$$ on a compass i might as well get it reading as good as i can, any thoughts on the Sestrel compass?

Daz

[Mindi]

"GPS doesn't know about variation or anything magnetic" granted for non-plotters, not so for chartplotters.
Plotters don't have compasses inbuilt (magnetic or gyro), but they do have digital layers of information lifted from paper charts, which includes variation.
They won't have the ability to calculate annual changes in variation (neither can paper charts), so some quick arithmetic is still needed.

I run GPS chartplotters recievers in Magnetic Heading all the time. All of the chartplotters I've used can display True or Magnetic.

I had a sneaky suspicion I was wrong there...thanks...so the map layers carry local variation info..? do you set that up at initialisation or are the preconfigured by region...I guess when you initialise it..? very smart.

[tunaticer]

Screw it to a steel base heheh.

[Mindi]

thanks all, did not think it was this much of a challange, so how do boat yards fit them, same as mindi's first post? or do they just put them on so it looks like they work? if im going to spend a few $$$ on a compass i might as well get it reading as good as i can, any thoughts on the Sestrel compass?

Daz


Havent had a Sestrel ...I have always had Suunto or Silva or the American one I cant remember....price would be a good guide..? they are British I think and may well be good.

Dont worry about all the boxing and swinging crap unless you are bored one day. Take some care to not put it where electrics can move the needle....and take some care to align it with the keel if mounting it flat. Then go fishing...if its a decent compass it will be more accurate than you can hold a heading.
If your GPS is set in True then just observe the 12 deg difference...if you can set your GPS to magnetic then observe they should be the same (see Gelsec post above)

[slyman]

Magnetic Deviation is the additional "error" introduced by the specific vessel.....but unless you are seriously navigating manually it doesnt matter to us


Magnetic Deviation is an error on a compass caused by the vessel's own magnetic field, which is produced through the presence of any ferrous metals, electrics, mobile phones. Magnetic Deviation can be up to 5 degrees or more, either east or west of the true reading, depending on the ship and its components. Some marine electronics will state in their specs what safe distance is needed for compasses to minimise deviation.

[petelaska]

Isn't the compass adjustment buoy in the bay used to swing or check compasses.
Pete

[seatime]

The compass adjustment buoy in Moreton Bay is set-up for small vessels to swing their compasses.
Bearings are supplied from the buoy towards prominent landmarks, e.g. Redcliffe Water Tower, Airport Control Tower, Peak of Big Sandhill Moreton is, etc.
This method can be replicated anywhere near the coast that has a beacon/buoy and a few good, easily visible landmarks. Lift off bearings from your local chart (MSQ charts have a Magnetic Compass Rose) line yourself up between the buoy and the mark, and check the difference between the known value and what your compass reads.

[black runner]

I have a basic hand held marine Garmin GPS72. It allows either true, magnetic, user or grid compass settings. By selecting magnetic the unit automatically sets the variation based on current location and date. The user setting allows the user to specify their own correction amount.

The variation is 14deg 50' or near enough to 15 deg in Hobart according to my charts and this is what the GPS automatically configures if set to magnetic.

Cheers

[Mindi]

The compass adjustment buoy in Moreton Bay is set-up for small vessels to swing their compasses.
Bearings are supplied from the buoy towards prominent landmarks, e.g. Redcliffe Water Tower, Airport Control Tower, Peak of Big Sandhill Moreton is, etc.
This method can be replicated anywhere near the coast that has a beacon/buoy and a few good, easily visible landmarks. Lift off bearings from your local chart (MSQ charts have a Magnetic Compass Rose) line yourself up between the buoy and the mark, and check the difference between the known value and what your compass reads.

There you go...but I bet you will do a better job just looking at the GPS heading than trying to steer a long course...heaps easier.