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Author Topic:   Compass Corrections
Guts posted 04-11-2002 02:20 AM ET (US)   Profile for Guts   Send Email to Guts  
Have any of you ever swung a compass or had it done? What is the cost? How long did it take? How is it done?
phatwhaler posted 04-11-2002 08:36 AM ET (US)     Profile for phatwhaler  Send Email to phatwhaler     
Back during my CG days we had to swing the compasses of our boats after each yard period, and after installation of any new electronics, etc. There are only two compass guys in the whole state of Florida! The procedure is fairly quick. The guy sets up his gear and makes you yell out headings so he can check your heading against his. If there's a difference, he adjusts the compass, or he mounts small magnets near the compass to get rid of any deviation. Usually he gets rid of almost all deviation which makes navigation a little easier. It takes about one hour per boat, at about $200 each. Seems a little expensive but since there are only two of the these guys in the whole state that's not too bad. I think they touch on this in Chapman's.

Phatwhaler out.

jimh posted 04-11-2002 09:21 AM ET (US)     Profile for jimh  Send Email to jimh     
A metal ship's magnetic compass mounted in a binnacle has many compensating magnets available for adjustments. Older copies of AMERICAN PRACTICAL NAVIGATOR by BOWDITCH devote over 50-pages to a terse explanation of the numerous errors and their correction.

The typical small boat compass mounted in a fiberglass boat has likely only two adjustments, if any at all.

Observation of compass error is easily done by recording the COMPASS BEARING (CB) of a fixed object whose TRUE BEARING (TB) is known with precision.

Rotate the ship's COMPASS HEADING (CH) at the desired interval (say 22.5-degrees) while maintaining the ship on station. (It is better to sight at a distant object because any error in the ship's station will have less effect the farther away the object being used for the bearing.) At each position note the COMPASS BEARING (CB).

The COMPASS ERROR (CE) is thus:

TB - CB = CE

Construct a table of CE versus CH.
Apply corrections. Your compass maker should have supplied instructions for how to make adjustments to his compass and what errors can be corrected.

An often overlooked problem is the failure to align the compass parrallel to the keel's fore-and-aft centerline.

phatwhaler posted 04-11-2002 10:10 AM ET (US)     Profile for phatwhaler  Send Email to phatwhaler     
O yeah, Good old Nathaniel Bowditch , I have a copy of his book next to my bed. 100% cure for insomnia.

Anyway, all of the Deviation tables I have seen break down to 15 degree increments. Seems to be the norm.

phatwhaler out.

jimp posted 04-11-2002 11:17 AM ET (US)     Profile for jimp  Send Email to jimp     
Guts -

Another option, depending on size of boat, is getting an electric or fluxgate compass. I got a Ritchie Mag-One from Skipper Marine at their annual Christmas sale a number of years ago. I installed it in my Revenge 22 (fit the same hole as the old compass with minor modifications). Hook it up to your electric system and you're all set. To compensate, make two slow 360 degree turns in the same direction... and the compass automatically compensates itself. Now my LORAN, GPS and compass all agree within 1 degree (majority of the time). Cost was under $200 on sale.


InHerNet posted 04-11-2002 02:39 PM ET (US)     Profile for InHerNet  Send Email to InHerNet     
When I went to Sea School for my captains license we spent quite some time on compass deviation and variation. Maybe this will help.

Two types of headings on a compass rose you will find on most charts. 1. True North 2. Magnetic North. The difference between True North and Magnetic North is called Variation.
The Variation is different based on geographical location.

Deviation is a individual problem with your magnetic compass. It could be in error due to many factors such as electrical wiring, metal objects near the compass, or even stray electrical currents from other electrical equipment onboard.

The term "Compass Error" refers to the total error between Variation and Deviation.

You can create your own chart and change it when ever you add equipment or any other change that would make you doubt your existing chart. If you have a GPS and compare the heading to your magnetic compass when making heading turns you will be able to establish your own Deviation table.

The correct Compass Error formula is as such. (you will need to have access to a Local Variation Table)

Variation will be given as degrees East or West of True.
Deviation will be given as degrees East or West of Magnetic.
To find total “Compass Error” you have to add the effect of Variation and Deviation. If both have the same sign, meaning both are East or both are West then they are added. If one is East and one is West, you take the difference of the figures and retain the East or West sign of the larger one.

Example: If Variation is 5 degrees East and Deviation is 2 degrees East the total compass error is 7 degrees East. If the Variation is 6 degrees West and Deviation is 3 degrees East the total compass error is 3 degrees West.

where2 posted 04-11-2002 07:21 PM ET (US)     Profile for where2  Send Email to where2     
If there's only 2 Compass guys in Florida, maybe I should just ditch my professional surveying career and do compasses... I'm all over that true north, magnetic north stuff already.
Guts posted 04-11-2002 07:28 PM ET (US)     Profile for Guts  Send Email to Guts     
Thanks guys! I'm takeing the boat to have it done on friday. but the sun has to be out, to do the adjustments.I'm praying for sun. if all goes right a fishing trip on sat.first one with the new plotter. I will post what I find out, and thanks agian.

lhg posted 04-11-2002 07:51 PM ET (US)     Profile for lhg    
Compensating a compass is much easier than mentioned above, particularly if you have a good GPS. If your GPS has Differential or WAAS enhancement, even better. I just re-compensated both of my compasses, and it's simple.

My new Ritchie Navigator compass came with easy instructions from Ritchie. If it's a good compass, like the Ritchie, it will have two compensating screws, one for North-South and one for East-West.

Basically, Ritchie tells you to first pick out an approximate North course, using a light house, marker, etc, and get directly south of it about a mile or two.
Exact N-S 0 degree accuracy is not necessary.
Then head straight for this marker at about 20 MPH, letting the GPS (set on Magnetic reading) settle in for a reading. After a few minutes you will be able to determine
how far off, in either direction, your compass is. Then stop the boat, and adjust the compass this amount. Then speed up again, the two readings should be the same.
If not, try again.

Then, do the same thing using an EAST heading, and using the OTHER adjusting screw. When done, your compass will be extremely accurate. You can check all of your work by selecting one of your waypoints, navigate to it, and both the GPS and compass reading should be the same.

You can even check to see if your compass is properly aligned on your boat. If everything (GPS and compass) corresponds heading north, but not South, your compass is not straight in the boat.

Ritchie says you can also do this without having GPS, but you have to use a chart to confirm the true course you are heading, and use the current variation.

Paying someone to do this is a waste of money these days.

jimh posted 04-11-2002 11:20 PM ET (US)     Profile for jimh  Send Email to jimh     
Another ad hoc technique for deducing your CE is to use navigation ranges. We did this for years with our sailboat's compass.

Whenever you are on a range, carefully line your boat's heading to be precisely on the range line. Note your Compass Heading (CH).
The True Bearing (TB) of the range will be noted on the chart. Apply the local Magnetic Variation (VAR) to find the range's Compass Bearing (CB). You can deduce the Compass Error (CE) from:


Each time we found ourselves on a range I would use the opportunity to verify the Compass Error. Over the years we had quite an accurate table constructed.

The sailboat's compass was more affected by changes in the heading because of the presense of several hundred pounds of iron (the YANMAR diesel engine) in close proximity ( 3 feet) to the compass.

On a fiberglass boat with an aluminum outboard engine, there should not be much deviation.

Don't mount a permanent magnet loudspeaker near the compass.

PMUCCIOLO posted 04-12-2002 01:25 AM ET (US)     Profile for PMUCCIOLO    

I saw the word "sailboat" in the above post...Say it isn't so!


InHerNet posted 04-12-2002 10:53 AM ET (US)     Profile for InHerNet  Send Email to InHerNet     
Creating your own compass deviation chart is the way to go if you travel with your vessel, or have the compass calibrated. I would still be checking the compass often. With a deviation chart you can use that chart wherever you go. I know that most small boats would not need to have such precise charting tool, since they very rarely travel far out to sea. Visual sighting is the most effective navigation tool.

When plotting a course to steer using a navigational chart on a True Course and converting it to your own Magnetic Compass heading other factors that would change your destination location would be Set and Drift calculations. As I can tell, Jimh has sailing experience and could expand on this. Most of the fun in sailing is to plot your course and make corrections based on currents and winds.

You never know if your electronics will go out!

Quite a few years back I purchased a 38' motor yacht in Key Largo FL, plotted my course back to Sarasota for a night trip after a few days of fishing in the keys. The boat was equipped with a Loran, radio frequency machine, radar, and autopilot. Without notice at about 1:00 am the Loran went out. Luckily I was only 40 miles off shore. I navigated by the radar and compass headings until I was off the coast of Fort Myers. After that trip I was convinced that relying on electrical equipment alone was not a wise thing to do. I would not plan a trip to the Islands without checking my electronic equipment against my compass headings and record often. You could find yourself in trouble quick and miss your destination. Keep good records when you travel far from shore.

simonmeridew posted 04-13-2002 07:39 AM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
How do you keep records of your trip? I guess I mean, record heading for a certain distance or time or RPM? Is it sort of a "ship's log" or what?
InHerNet posted 04-13-2002 08:44 PM ET (US)     Profile for InHerNet  Send Email to InHerNet     

Yes, it is a log. Record it in anyway or form that is legible to you and anybody else that is aboard with you that could relay important information. Here is a couple very good reasons for keeping such a log.

1.) You should keep yourself abreast of your location for any emergencies that may arise. The Coast Guard or Sea-Tow or anyone within radio distance will ask for your location and without a GPS or Loran working it would be wise to record your trip at time intervals or any change in course to give them your location. Again we are talking about offshore trips.
2.) Record headings on your compass. Record speed and time and you would be able to calculate distance. Record distance and speed you would be able to calculate time. (Any of the two will solve the third) This will help you find your way back or give you the ability to plot a new course from where you are.
3.) Record fuel consumption. What if your fuel gage breaks? If I use my log I could calculate what my fuel consumption is and judge my available distance.

Keeping records when offshore could keep you alive. I don’t mean to complicate this topic about calibrating a magnetic compass, It just made me think of charting and plotting a course and the need to have a good reading. If all of my boating was near shore the exactness of my compass within a few degrees would not really matter to me personally. I would head out and return the other side of 180 degrees until I spotted a land mark. Hopefully a sign bearing Tavern.

I would recommend to anyone that ventures offshore to keep a log. I also keep a bottom log for fishing holes, hard bottom, coral, ledges and record the weather and various other facts that would help me in the future determine what to fish for. It is just a good habit to get into. Best of luck to you.

Guts posted 04-14-2002 12:27 PM ET (US)     Profile for Guts  Send Email to Guts     
I would like to thank all of you for your replys!
I took the boat to Baker Marine Instruments & Repair.(Brian Osterberg)and his son Anthony. this a busnes that has been pased down from son to son.
now to swing the compass; it is a lot like phatwhaler takes two people. one at the helm and one to set the instrument.and can only be done on a sunny day at certin times of the day. I ask Brian if I gould take a pic. for the group that I post in. he said that he rather I did not, so I can only tell you what he did. this has to be done with a sun dile? thats the best word I could use to describe it.and can not be done at noon it has to be done befor or after the sun reaches the top (noon)so to let the sun dile read the marks. now with anthony on the bow with the instrument and his dad at the helm we headed south anthony would call out 1/2 1/2 1/4/and so on till anthony said mark. Brian would take note of this and go west then north then the compass came out and the adjustments were made.then we did it all over again and made the last adj. were made. then one final swing chacking every 45*
the bigest problem I had in my montauk was the raido and the mic. I useing a FN201 thats a flush mount. but this did not work so we had to get a binnacle mount. moveing tiis small amount did the looks like the one on page 923 west marine catalog
(binnacle-mount flat card.navigator.fn201.
Brian told me that the chart plotter and the sounder did not have that much affect as the magnets it the raido and the mic. this was what was the cause of the compass deviation and all the other things in side the console.
also the fluxgate compass for the autopilot didn't help.just to much stuf to have a flush mount compass. one more thing I was told not to mount the compass he wanted to do this.
the cost? well the cost is not important to me as I do take this boat well off shore. some times 60 miles one way, chaseing tuna in good wether. I did go fishing on sat. had one over limit small to med. fish nothing big but still a good day.and thanks again for the help!!!
jimh posted 04-14-2002 02:25 PM ET (US)     Profile for jimh  Send Email to jimh     
The technique you refer to being used by your compass correctors is a "sun compass".

By picking a particular time, generally around noon, and using a sun compass, the shadow of the indicating needle can be used as an indicator of a relatively steady bearing. If you know the time the sun will be on your meridian, its shadow will point due north. If a fixed compass rose is laid out with a small post in the center, the shadow of the post will point to bearings on the compass rose as the position of the boat rotates around. Aligning the compass rose so 000-degree points precisely at the bow, and performing the procedure within a few minutes of your local noon (sun on your meridian) permits the sun compass to act as a compass.

Guts posted 04-14-2002 04:09 PM ET (US)     Profile for Guts  Send Email to Guts     
Yes there was a time chart that they had with a clock/watch in the lid of the box that they carryed it in. but they could not do this at noon becouse the sun could not cast a shadow to read.I think thats why they only do this in time frames. thanks jimh for the name of the instrument.(sun compass)
Draftmanswife posted 04-17-2002 09:51 PM ET (US)     Profile for Draftmanswife  Send Email to Draftmanswife     
Here's my experience with adjusting compasses.....We're in the gulf one day, about 10 miles out, fishing...Grandpa Bill says...well...there's that screw driver I've been looking a console hole next to the compass....he pulls the screw driver out, and the compass goes to we got home? only God knows...
triblet posted 04-18-2002 10:22 AM ET (US)     Profile for triblet  Send Email to triblet     
The sun will not be overhead at noon execpt in
the tropics, and there only for part of the year.
So a sun compass will work at noon most places
all of the time, and some of the time in the


lhg posted 04-18-2002 03:25 PM ET (US)     Profile for lhg    
Go to for official instructions on how to compensate a compass. If anybody knows how to do this, they would. It's very easy to do.
InHerNet posted 04-18-2002 04:43 PM ET (US)     Profile for InHerNet  Send Email to InHerNet     
LGH- Good link on compenstion. I thought this was an interesting note on the bottom of their page that talks about a deviation card. How many boaters even have a deviation card or even use it?

"To assure accuracy on all headings, check for deviation every thirty degrees and record any deviation on a deviation card. We recommend checking at the start of each boating season for changes in deviation.
If you feel that the deviation on your boat is of an unusual nature, the services of a professional compass adjuster will be a wise investment."

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