On my return from Canada's Gulf Islands last week, I noted that the course I was on was not taking me toward the island it should have. After verifying the course I'd plotted on my chart, I got out my GPS and found that the course I was on per my binnacle compass was different from my GPS by twenty-three degrees! Yow. There was no mention on the chart of any magnetic anomalies in the area. I can only conclude my compass is seriously screwed up, at least on that bearing. I read in The Annapolis Book of Sailing, the procedures for verifying and correcting this problem, but I must admit to being a little intimidated. Has any one tried to correct their own compass deviations? How did it work out? Thanks for your help.
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A long time ago
I use to do it on an airplane I owned back in the 70's.. You'll need to find a compass rose that you can use to make the corrections. Of course, it won't be perfect in all directions.. The point is, it's not that hard to do. Be sure you didn't just have a piece of metal near your compass when the problem arose. Ron
I use to do it on an airplane I owned back in the 70's.. You'll need to find a compass rose that you can use to make the corrections. Of course, it won't be perfect in all directions.. The point is, it's not that hard to do. Be sure you didn't just have a piece of metal near your compass when the problem arose. Ron
First thing
First make sure there is nothing with ferris metal near the compass. I have often seen a handheld vhf or portable gps mounted or hanging from the binacle. Something like this could really set it off.
First make sure there is nothing with ferris metal near the compass. I have often seen a handheld vhf or portable gps mounted or hanging from the binacle. Something like this could really set it off.
no anomalies here
I have cruised the gulf islands for years and there are no (natural) anomalies anywhere in the area. Maybe the occasional iron whale swimming around (just kidding). 23 degrees is an awfull lot of deviation. Many things can cause deviations to a compass. Sometimes on the boat, sometimes off the boat, anything that carries a magnetic field such as a passing steel ship could have an effect. You can get your compass corrected. It is not something I would try to do myself as I understand there is considerable amount of skill and technique involved. Correcting one heading will affect other headings and so on. If you do it, make sure you check all the compass headings carefully before setting out on your next trip to avoid any more unexplained anomalies. In any case you should prepare a deviation table for your boat. Might save a lot of grief or more someday, especially if the trusty old GPS fails at the worst possible time. Regards, Doug
I have cruised the gulf islands for years and there are no (natural) anomalies anywhere in the area. Maybe the occasional iron whale swimming around (just kidding). 23 degrees is an awfull lot of deviation. Many things can cause deviations to a compass. Sometimes on the boat, sometimes off the boat, anything that carries a magnetic field such as a passing steel ship could have an effect. You can get your compass corrected. It is not something I would try to do myself as I understand there is considerable amount of skill and technique involved. Correcting one heading will affect other headings and so on. If you do it, make sure you check all the compass headings carefully before setting out on your next trip to avoid any more unexplained anomalies. In any case you should prepare a deviation table for your boat. Might save a lot of grief or more someday, especially if the trusty old GPS fails at the worst possible time. Regards, Doug
I have to ask....
The new charts I have seen do not include the magnetic readings on the compass rose. Here in Rochester the difference is about 11 degrees from true. Have you used this compass for navigation before? Ken Palmer, S/V Liberty
The new charts I have seen do not include the magnetic readings on the compass rose. Here in Rochester the difference is about 11 degrees from true. Have you used this compass for navigation before? Ken Palmer, S/V Liberty
How do you do it?
I have set the compass deviation on air craft. But that seems simple park it on a flta peice of the ramp that is already marked out and just not the deviation on the card next to the compass. I have never seen a place set up to do that for a boat though I am sure that it is done. One other thing you want to do it in at least four preferably eight different headings you may not have the same deviation going 090 as you would going 270 though you think it would.
I have set the compass deviation on air craft. But that seems simple park it on a flta peice of the ramp that is already marked out and just not the deviation on the card next to the compass. I have never seen a place set up to do that for a boat though I am sure that it is done. One other thing you want to do it in at least four preferably eight different headings you may not have the same deviation going 090 as you would going 270 though you think it would.
Check wiring
This spring I rewired our C27 completely, replacing some very dubious looking wire with a top quality shielded wire (shielded because the price was right). When I was finished I was amazed to find my compass was out by almost 20 degrees. I am assuming that it was calibrated for the EMF caused by the old wiring and that the use of sheilded wire radically reduced any EMF emissions. Is it possible a change in wiring or electrical components has caused the error you talk about?
This spring I rewired our C27 completely, replacing some very dubious looking wire with a top quality shielded wire (shielded because the price was right). When I was finished I was amazed to find my compass was out by almost 20 degrees. I am assuming that it was calibrated for the EMF caused by the old wiring and that the use of sheilded wire radically reduced any EMF emissions. Is it possible a change in wiring or electrical components has caused the error you talk about?
Compass Setting - Will This Work?
I once adjusted my compass by using a chart to determine an exact heading through a long but straight and narrow channel. I then ran my boat down the middle of the channel and noted the difference between the compass heading and correct chart heading and adjusted the compass for the difference. As an alternative, why couldn't you do the same thing with your GPS?
I once adjusted my compass by using a chart to determine an exact heading through a long but straight and narrow channel. I then ran my boat down the middle of the channel and noted the difference between the compass heading and correct chart heading and adjusted the compass for the difference. As an alternative, why couldn't you do the same thing with your GPS?
More on Compass Deviation
Thanks guys for your replies. There was nothing unusual lurking around the binnacle the day this occurred. However, the engine was running as there was no wind. This may have set up some sort of electromagnetic disturbance. Before I start tearing the compass apart, I plan to go back out this weekend and compare compass headings with GPS direction through a range of points on the compass rose (and yes my charts have both true and magnetic settings on them) both under sail and with the engine running. I'll let you know the outcome. Thanks again.
Thanks guys for your replies. There was nothing unusual lurking around the binnacle the day this occurred. However, the engine was running as there was no wind. This may have set up some sort of electromagnetic disturbance. Before I start tearing the compass apart, I plan to go back out this weekend and compare compass headings with GPS direction through a range of points on the compass rose (and yes my charts have both true and magnetic settings on them) both under sail and with the engine running. I'll let you know the outcome. Thanks again.
GPS and binnacle compass will never be the same
because the binnacle compass is pointing to magnetic north and the GPS is telling you the 'track' you are moving on, which has the influence of current and wind. Your boat might be pointing to a certain direction based on the binnacle compass, but you could actually be tracking a totally different heading, as read on your GPS. Your compass and GPS will have similar readings if you are moving where there is no current and calm conditions. Roc
because the binnacle compass is pointing to magnetic north and the GPS is telling you the 'track' you are moving on, which has the influence of current and wind. Your boat might be pointing to a certain direction based on the binnacle compass, but you could actually be tracking a totally different heading, as read on your GPS. Your compass and GPS will have similar readings if you are moving where there is no current and calm conditions. Roc
Hire a pro
They generally don't charge a whole lot (max $100 but I've heard closer to $50) to come swing a compass. Seems like a reasonable investment for the most important piece of nav equipment aboard - plus, you won't have to have it done again for years. Good luck! LaDonna
They generally don't charge a whole lot (max $100 but I've heard closer to $50) to come swing a compass. Seems like a reasonable investment for the most important piece of nav equipment aboard - plus, you won't have to have it done again for years. Good luck! LaDonna
Current?
Another obvious question - If you were doing 5 kt it would take a current of about 2 kt perpendicular to your course to put you that much off. I assume that you weren't experiencing that kind of current? I could be less if you were making some leeway in the same direction too. I sail mostly in the San Francisco Bay where this kind of deviation is commonplace due to the currents. Just some thoughts.
Another obvious question - If you were doing 5 kt it would take a current of about 2 kt perpendicular to your course to put you that much off. I assume that you weren't experiencing that kind of current? I could be less if you were making some leeway in the same direction too. I sail mostly in the San Francisco Bay where this kind of deviation is commonplace due to the currents. Just some thoughts.
GPS Course Magnetic or True?
With the magnetic deviation in the San Juans being very close to 21 degrees is it possible that the GPS was set for reading True vice Magnetic? If it was set for Magnetic and the error was that much I would look for something with a magnet, such as speakers or VHF, or even an engine gauge. I tested a hailer remote on my compass and it deflected the card several degrees even though it was six or seven feet away! Ferous items that are not magnetic will have to be much closer, in comparison to magnets, to have the same effect.
With the magnetic deviation in the San Juans being very close to 21 degrees is it possible that the GPS was set for reading True vice Magnetic? If it was set for Magnetic and the error was that much I would look for something with a magnet, such as speakers or VHF, or even an engine gauge. I tested a hailer remote on my compass and it deflected the card several degrees even though it was six or seven feet away! Ferous items that are not magnetic will have to be much closer, in comparison to magnets, to have the same effect.
Still More on Compass Deviation
Well today was the next big test. We got into an area with essentially no tidal currents and motored our way around the compass, comparing gps to compass readings. We experienced a consistent deviation of 15 to 20 degrees east of north, i.e. due north on compass was 20 degrees east of north on the gps. Yes this could have been "course made good" as opposed to bearing except the deviation was the same on the reciprocal course. We killed the engine and experienced the same deviation under sail. I think my compass is ready for some surgery. Should I give it some whiskey to kill the pain?
Well today was the next big test. We got into an area with essentially no tidal currents and motored our way around the compass, comparing gps to compass readings. We experienced a consistent deviation of 15 to 20 degrees east of north, i.e. due north on compass was 20 degrees east of north on the gps. Yes this could have been "course made good" as opposed to bearing except the deviation was the same on the reciprocal course. We killed the engine and experienced the same deviation under sail. I think my compass is ready for some surgery. Should I give it some whiskey to kill the pain?
Deviation
There is one sure way to get the deviation you are experiencing and that is to have the lubber line on the compass 15-20 degrees off the axis of the boat.
There is one sure way to get the deviation you are experiencing and that is to have the lubber line on the compass 15-20 degrees off the axis of the boat.
Deviation and Variation
1. Most GPS read True bearings, which will need to be corrected for magnetic deviation. That probably accounts for a good bit of your perceived compass error. 2. Swinging a compass is not all that difficult. Find the adjustment screws for N/S and for E/W. Using either an accurate compass or GPS as a check, or by using a range sighting you know is accurate, motor on a calm day dead N or S. Using the adjustment screw, remove HALF the error. Turn around and sail 180 the other direction. Remove HALF the error. Do the same E and W using that adjustment screw, each time removing HALF the error. When you are done, make a variation card that lists the error at key readings. No compass is perfectly accurate. If yours can't get within a few degrees on key headings, deep six it.
1. Most GPS read True bearings, which will need to be corrected for magnetic deviation. That probably accounts for a good bit of your perceived compass error. 2. Swinging a compass is not all that difficult. Find the adjustment screws for N/S and for E/W. Using either an accurate compass or GPS as a check, or by using a range sighting you know is accurate, motor on a calm day dead N or S. Using the adjustment screw, remove HALF the error. Turn around and sail 180 the other direction. Remove HALF the error. Do the same E and W using that adjustment screw, each time removing HALF the error. When you are done, make a variation card that lists the error at key readings. No compass is perfectly accurate. If yours can't get within a few degrees on key headings, deep six it.
Bad Compass ?
Gary, Ritchienavigation.com has an article on calibratng your compass. There are several methods one using a gps. Though I would be very careful about using a gps to swing a compass. The compass measures a magnetic heading directly. A gps computes a mag heading/course by measuring a true track between two fixes and adding a predetermined variation based on a math model or manual variation entry. A magnetic reading from a gps can be wrong based on a number of variables. Have you taken a hand-bearing compass to verify your ships compass is in error? Did this magnetic discrepancy suddenly show up or have you had it all along? Took a delivery ride on an Island Packet from Florida to Maryland and found that the compass had never been secured in its mount. You could simply turn the compass completely around. After lineing the compass up with the mid-line of the boat it was accurate within a degree. But we spent a day trying to determine the deviation before we found the problem. And the problem wasn't in the compass but in the mounting. Don't jump too fast at turning those compass adjustment screws. It could be any number of other problems. You can make it much worse instead of better. Good luck Les s/v Mutual Fun
Gary, Ritchienavigation.com has an article on calibratng your compass. There are several methods one using a gps. Though I would be very careful about using a gps to swing a compass. The compass measures a magnetic heading directly. A gps computes a mag heading/course by measuring a true track between two fixes and adding a predetermined variation based on a math model or manual variation entry. A magnetic reading from a gps can be wrong based on a number of variables. Have you taken a hand-bearing compass to verify your ships compass is in error? Did this magnetic discrepancy suddenly show up or have you had it all along? Took a delivery ride on an Island Packet from Florida to Maryland and found that the compass had never been secured in its mount. You could simply turn the compass completely around. After lineing the compass up with the mid-line of the boat it was accurate within a degree. But we spent a day trying to determine the deviation before we found the problem. And the problem wasn't in the compass but in the mounting. Don't jump too fast at turning those compass adjustment screws. It could be any number of other problems. You can make it much worse instead of better. Good luck Les s/v Mutual Fun
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