I have some questions on how you folks check and adjust your compass. Every navigation book I have read tells you to take the boat out on a known course such as a range or other landmark to check the compass. In this day and age of GPS wouldn't the GPS be a faster, more accurate approach? And regarding the adjustment screws. My Saturn compass has two screws (E/W and N/S) but I don't have instructions on when to use them and most books have only partial instructions along the lines of "use the adjustment screw to remove half the error". This doesn't tell you which screw is your first choice. Does anyone know a good web site with instuctions? Thanks Kevin
Compass checking
- Thread starter Kevin B
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Chapmans
While I have not tried to follow the directions yet the chapter in Chapmans Piloting seems to be a very good one on this. I read it and it actually made sense to me. As for figuring out the deviation for your compass on your boat they go over this and recommend a tool called a pelorus (sp?). With this you can point the boat in one direction and figure out the deviation for every 15 degrees of bearing. Apparently most chandlerys will sell an inexpensive plastic version. Chapmans is of course an exceptional all around book on every aspect of seamanship.
While I have not tried to follow the directions yet the chapter in Chapmans Piloting seems to be a very good one on this. I read it and it actually made sense to me. As for figuring out the deviation for your compass on your boat they go over this and recommend a tool called a pelorus (sp?). With this you can point the boat in one direction and figure out the deviation for every 15 degrees of bearing. Apparently most chandlerys will sell an inexpensive plastic version. Chapmans is of course an exceptional all around book on every aspect of seamanship.
GPS works sorta
Currents etc can mess up calibrations. Especially under sail when side slip is a factor. Of course even hard objects don't work when you have to compensate for a current or side slip. I don't use a compass for precise navigation and expect that it is off some. When there are waves and the boat is rocking plus or minus 5 degrees is good. When heading down wind my gps and compass usually agree within a few degrees so I enter the waypoint and when the gps indicated that I am tracking toward my waypoint I note the compass bearing and use that to to steer. Then once an hour or so I'll recheck the GPS. But most of the time I just look at the charts and use the depth sounder and visual references. If the wife takes the helm I give her a compass heading. Tom
Currents etc can mess up calibrations. Especially under sail when side slip is a factor. Of course even hard objects don't work when you have to compensate for a current or side slip. I don't use a compass for precise navigation and expect that it is off some. When there are waves and the boat is rocking plus or minus 5 degrees is good. When heading down wind my gps and compass usually agree within a few degrees so I enter the waypoint and when the gps indicated that I am tracking toward my waypoint I note the compass bearing and use that to to steer. Then once an hour or so I'll recheck the GPS. But most of the time I just look at the charts and use the depth sounder and visual references. If the wife takes the helm I give her a compass heading. Tom
Did mine with a GPS
I used a GPS to swing my compass. It will not work if there is a significant current. With my sails down on a light wind day, I went out and motored magnetic north (355 true)using the GPS at about 3k. One of the screws will cause a big change in the compass. Turn that one until the compass reads magnetic north. Then head magnetic south (175 true) and check the difference between the compass and magnetic south. Adjust the compass using the same screw so that one half of the error is removed. Do the same thing sailing east and west magnetic and make the adjustments using the other screw. You can go back and repeat the whole process one more time, but it usually does not make that much more difference. I then checked the error between the compass and the GPS at each of the four cardinal and the intermediate 45 degree headings to compute the deviation. I made up my own compass card, but I rarely use it. Make sure you have all the electrical equipment on which you would normally said with.
I used a GPS to swing my compass. It will not work if there is a significant current. With my sails down on a light wind day, I went out and motored magnetic north (355 true)using the GPS at about 3k. One of the screws will cause a big change in the compass. Turn that one until the compass reads magnetic north. Then head magnetic south (175 true) and check the difference between the compass and magnetic south. Adjust the compass using the same screw so that one half of the error is removed. Do the same thing sailing east and west magnetic and make the adjustments using the other screw. You can go back and repeat the whole process one more time, but it usually does not make that much more difference. I then checked the error between the compass and the GPS at each of the four cardinal and the intermediate 45 degree headings to compute the deviation. I made up my own compass card, but I rarely use it. Make sure you have all the electrical equipment on which you would normally said with.
Variation is different based on location.
So Kevin, go ahead and use Dan's method. It works well and is easy to do. Disregard the True figures given. They are good for Dan's area, but in your Toronto area, variation is going to be around 010°W plus the yearly changes. Do it on a calm day and you should get results. Tom is right about the 5° being good enough when the boat is rocking. Good luck
So Kevin, go ahead and use Dan's method. It works well and is easy to do. Disregard the True figures given. They are good for Dan's area, but in your Toronto area, variation is going to be around 010°W plus the yearly changes. Do it on a calm day and you should get results. Tom is right about the 5° being good enough when the boat is rocking. Good luck
- Jan 22, 2008
- 519
here is the 8 step procedure with named screws
This is from a pdf file, so I will not replicate the text here. Check the picture or print it out. Of course you can use your GPS instead of transit lines, just make sure to switch it to magnetic north, the follow the procedure as outlined.
This is from a pdf file, so I will not replicate the text here. Check the picture or print it out. Of course you can use your GPS instead of transit lines, just make sure to switch it to magnetic north, the follow the procedure as outlined.
Obvious but ...
I hate to state the obvious but be carefull with magnets!!!! Our handheld VHF has a speaker magnet that throws off the compass. There is a place on the pedestal for drinks and a handheld. On some headings it doesn't make much difference on others it will move the compass several degrees. Sometimes on some boats the engine will affect the compass when running. The compass is a great tool but a GPS provides so much more information that the compass's importance is much less than it was a few years ago. Tom
I hate to state the obvious but be carefull with magnets!!!! Our handheld VHF has a speaker magnet that throws off the compass. There is a place on the pedestal for drinks and a handheld. On some headings it doesn't make much difference on others it will move the compass several degrees. Sometimes on some boats the engine will affect the compass when running. The compass is a great tool but a GPS provides so much more information that the compass's importance is much less than it was a few years ago. Tom
Thanks for the info
and tips. Roger, your guide is exactly what I needed. If I get the time I plan to take the compass off the boat to calibrate it and after re-installing I will be curious to see what 280 lbs. of diesel engine and 2,400 lbs. of iron keel do to the readings. Kevin
and tips. Roger, your guide is exactly what I needed. If I get the time I plan to take the compass off the boat to calibrate it and after re-installing I will be curious to see what 280 lbs. of diesel engine and 2,400 lbs. of iron keel do to the readings. Kevin
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