My compass is reading about 15 degrees different than my GPSs so I assume I need to calibrate my compass. I think there is an adjustment on the compass but before I get into it I thought I would ask if others have calibrated their compass lately.
Compass Calibration
- Thread starter Sailgunner
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If your compass is 15 degrees off on all course headings, the usual solution is to take that "deviation" into account when plotting your course.
As BillyK was hinting at, your GPS may be showing True North while your compass can only display Magnetic North. Or your GPS may not be adjusting for the magnetic "variation" in your local area. Check the data in the center of the compass rose on your charts to verify your variation. Don't forget to add or subtract the yearly change to arrive at the variation for 2011.
If the difference is only on specific courses, the compass is corrected by placing a small magnet in the needed location to compensate for things like the motor that may be pulling it off of magnetic north.
Finding and placing the magnets is the job of a professional. The activity is called "swinging the compass".
As BillyK was hinting at, your GPS may be showing True North while your compass can only display Magnetic North. Or your GPS may not be adjusting for the magnetic "variation" in your local area. Check the data in the center of the compass rose on your charts to verify your variation. Don't forget to add or subtract the yearly change to arrive at the variation for 2011.
If the difference is only on specific courses, the compass is corrected by placing a small magnet in the needed location to compensate for things like the motor that may be pulling it off of magnetic north.
Finding and placing the magnets is the job of a professional. The activity is called "swinging the compass".
Before anything else, I would check around thoroughly to be sure there is not a metallic or magnetic object close to the compass which is causing the deviation.
Swinging a compass is not that hard. There are only two screws to contend with. The hard part is getting on a known heading so you can compare it to what the compass reads. Unfortunately a GPS is not entirely useful for this due to current offset. While the boat may be pointed at (heading) 90 degrees it may not be traveling in that direction due to sideslip or current. You can us the GPS POSITION to start at and then locate landmarks in specific directions however. The best way is to find a fixed nav aid (dolphins not in shallow water) that you can locate N, S, E, and W landmarks from. Then motor to the nav aid and point at the landmark while adjusting the appropriate compass screw. If you can’t find one nav aid to work for everything then find another and do it over several cruises. Course if you are lazy (and don’t use the compass for “blind navigation”) you can just take advantage of nav aids and landmarks at your leisure.
In the final analysis you are going to have a deviation table anyway so if the compass is not calibrated it just means it has values in the deviation table that are larger.
Just make a table that has 0-360 by 5 degree increments and note the compass heading, GPS course (and any current if known), and the landmark direction. Record these in the log and compile / average them later to complete the deviation table.
In the final analysis you are going to have a deviation table anyway so if the compass is not calibrated it just means it has values in the deviation table that are larger.
Just make a table that has 0-360 by 5 degree increments and note the compass heading, GPS course (and any current if known), and the landmark direction. Record these in the log and compile / average them later to complete the deviation table.
The variation at Punta Gorda is about -5 degrees.. if you are pointed at mag north, a compass would read 355.. If it is reading 15 off in the same direction on all points, then the compass is turned off boat centerline.. If not, as others have said, there may be something ferrous or magnetic close by. Find that and move it.. after ya get deviation closer than 15, then it would be time to swing it.
Dixie beer from New Orleans was one of the last beers in steel cans..I used a little of that stuff because I didn't feel bad about sinking the cans in salt water.. knowing that they would rust away pretty fast... but I had to be careful about putting them close to a compass!
Things like binoculars or their clips or a case for glasses with a magnetic catch.. unshielded cockpit speakers in a center console.. Look carefully..
Dixie beer from New Orleans was one of the last beers in steel cans..I used a little of that stuff because I didn't feel bad about sinking the cans in salt water.. knowing that they would rust away pretty fast... but I had to be careful about putting them close to a compass!
Things like binoculars or their clips or a case for glasses with a magnetic catch.. unshielded cockpit speakers in a center console.. Look carefully..
In some cases the location of engine, autopilot motor, etc are not reloadable. So unless you move the compass you have to live with a large metal object nearby. Here in the Chesapeake Bay the variation is 11 degrees West (-11). The link shows variation in the US.
http://www.navfltsm.addr.com/isogonic.gif
http://www.navfltsm.addr.com/isogonic.gif
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