Has anyone had their compass adjusted for deviation on their boat? If so how much. Thank you, Richard (Duke)
Compass Adjustment
- Thread starter Duke13540
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Yes, three turns of the N-S screw
I don't recommend that you use my numbers however. You should take the compass out of the boat and "swing" it to remove all the "gross" errors. Instructions are on the internet. Then put the compass back in the boat and fire up the GPS. Set it for magnetic course display and insure that the magnetic variation is correct. A map compass rose is handy for this. Pick a location that has no/little tide. Then just go on as many different courses as you deem necessary (4 is the min and 360 serious overkill, 16 is a nice compromize) and note the compass reading. Make a table of actual magnetic course verses indicated compass course.
I don't recommend that you use my numbers however. You should take the compass out of the boat and "swing" it to remove all the "gross" errors. Instructions are on the internet. Then put the compass back in the boat and fire up the GPS. Set it for magnetic course display and insure that the magnetic variation is correct. A map compass rose is handy for this. Pick a location that has no/little tide. Then just go on as many different courses as you deem necessary (4 is the min and 360 serious overkill, 16 is a nice compromize) and note the compass reading. Make a table of actual magnetic course verses indicated compass course.
I pay about..
I pay about $90.00 to have my compass professionally swung. Charlie Cook owns Maine Compass Service and he is excellent. If you talk to a professional they will explain in very basic terms why adjusting based on the GPS won't get you a very accurate compass. I Maine, with the fog we get, I do take having my compass as accurate as possible seriously. It's one of the few things I'm NOT willing to attempt myself and I'll do most anything.... Charlie brings a huge magic "compass in a box" and we do many, many, many points of sail not just N,S,E &W and check and adjust and check and adjust and check and adjust. It takes about one hour to get it just right in very calm conditions.. For those of you located in Maine you'll want to use Maine Compass Service. Charlie Cook is the owner/operator and he's also a marine surveyor and general great guy who knows his stuff. Maine Compass Service: (207) 865-6645
I pay about $90.00 to have my compass professionally swung. Charlie Cook owns Maine Compass Service and he is excellent. If you talk to a professional they will explain in very basic terms why adjusting based on the GPS won't get you a very accurate compass. I Maine, with the fog we get, I do take having my compass as accurate as possible seriously. It's one of the few things I'm NOT willing to attempt myself and I'll do most anything.... Charlie brings a huge magic "compass in a box" and we do many, many, many points of sail not just N,S,E &W and check and adjust and check and adjust and check and adjust. It takes about one hour to get it just right in very calm conditions.. For those of you located in Maine you'll want to use Maine Compass Service. Charlie Cook is the owner/operator and he's also a marine surveyor and general great guy who knows his stuff. Maine Compass Service: (207) 865-6645
Engine Running???
It is possible that the compass will read differently with the engine running. For my purposes I don't need a super accurate compass. When I have checked my compass against the gps it has been very close. Closer than I can hold a course on a choppy day!!!!! I have also checked my boat's compass in reference to my binocular compass and they compare well. I head towards something on shore and take a reading with my binoculars and compare with the boat's compass. IMHO it is hard for a small boat captain to hold a compass course of less than +- 5 degrees without a lot of effort. Of course the traditional way is to find good landmarks on land and determine the bearings and then run them with your boat and check the compass. But again IMHO no one is going to run 20 miles or so without checking their position and now days that means a GPS. I do have a loran that works but I don't use it as it is not as accurate as a gps. It takes a stupid person to deliberately run in fog using only a compass for navigation.
It is possible that the compass will read differently with the engine running. For my purposes I don't need a super accurate compass. When I have checked my compass against the gps it has been very close. Closer than I can hold a course on a choppy day!!!!! I have also checked my boat's compass in reference to my binocular compass and they compare well. I head towards something on shore and take a reading with my binoculars and compare with the boat's compass. IMHO it is hard for a small boat captain to hold a compass course of less than +- 5 degrees without a lot of effort. Of course the traditional way is to find good landmarks on land and determine the bearings and then run them with your boat and check the compass. But again IMHO no one is going to run 20 miles or so without checking their position and now days that means a GPS. I do have a loran that works but I don't use it as it is not as accurate as a gps. It takes a stupid person to deliberately run in fog using only a compass for navigation.
MoonSailer, the running engine has the same magnetic effect
on your compass as the stopped engine. On another statement you made it takes a smart person to deliberately run in the fog with just a compass. One of the things you do is sail (or power) toward Bell buoys or whistle buoys so you can pick your way along. People still go out for a day sail without a GPS and suddenly the fog comes in. You can chose to sail smart or drift around, and I prefer to sail smart. I build my deviation table by using a far distant point like a mountain top. I steer many courses at either 5 or 10 degrees difference and taking bearings on the object. At a great distance any small angular error is negligible. Since the sum of the deviations is zero, I can add up all the bearings and divide by the number of observations to get the actual magnetic bearing of the object. Then with the little formula C D M, Going from C to M Easterly deviations are added. I then determine the deviation for each heading and put that in the table. Have fun Joe S
on your compass as the stopped engine. On another statement you made it takes a smart person to deliberately run in the fog with just a compass. One of the things you do is sail (or power) toward Bell buoys or whistle buoys so you can pick your way along. People still go out for a day sail without a GPS and suddenly the fog comes in. You can chose to sail smart or drift around, and I prefer to sail smart. I build my deviation table by using a far distant point like a mountain top. I steer many courses at either 5 or 10 degrees difference and taking bearings on the object. At a great distance any small angular error is negligible. Since the sum of the deviations is zero, I can add up all the bearings and divide by the number of observations to get the actual magnetic bearing of the object. Then with the little formula C D M, Going from C to M Easterly deviations are added. I then determine the deviation for each heading and put that in the table. Have fun Joe S
There is an electromagnetic field around current carrying conductors
for this reason it is wise to twist the pairs. But remember that magnetic field strength is inversely proportional to the cube of the distance separating the source and the affected devise.
for this reason it is wise to twist the pairs. But remember that magnetic field strength is inversely proportional to the cube of the distance separating the source and the affected devise.
Joseph that ain't exactly true
How bout an alternator putting out a magnetic field, or the voltage regulator. How bout an old atomic 4, with a point type ignition and a coil, making and breaking a couple thousand times a minute. Are you going to automatically eliminate these for whatever reason.
How bout an alternator putting out a magnetic field, or the voltage regulator. How bout an old atomic 4, with a point type ignition and a coil, making and breaking a couple thousand times a minute. Are you going to automatically eliminate these for whatever reason.
You don't adjust your compass for deviation
You swing your compass first, and then measure the deviation and you can only correct for it "manually" (mentally with a card written down with the deviations amounts for any given major compass heading) - that's what a deviation card is for; minimize the error first, but deviation will ALWAYS be there.
You swing your compass first, and then measure the deviation and you can only correct for it "manually" (mentally with a card written down with the deviations amounts for any given major compass heading) - that's what a deviation card is for; minimize the error first, but deviation will ALWAYS be there.
Gentlemen I beg to differ
The major affect on your boats magnetic field is the mass of iron in the engine. there are some currents caused by conductors, alternators, and ignitions, but they are miniscule and normally offsetting. Your magnetic speakers would have a greater affect than all of those, but if you have them reasonably far away from your compass they don't bother it. Stu, I don't follow your reply. What in my discussion are you referring to. I didn't say anything about correcting or adjusting a compass. I only said how I make a deviation card, and I never said the deviation wouldn't be there. I only gave a way to determine what it is on each heading. Please tell me what I'm missing because I don't know what you are talking about. If my deviation card shows large deviations, I would adjust my compass, but if the deviations are small, I would leave it alone. Have fun Joe S
The major affect on your boats magnetic field is the mass of iron in the engine. there are some currents caused by conductors, alternators, and ignitions, but they are miniscule and normally offsetting. Your magnetic speakers would have a greater affect than all of those, but if you have them reasonably far away from your compass they don't bother it. Stu, I don't follow your reply. What in my discussion are you referring to. I didn't say anything about correcting or adjusting a compass. I only said how I make a deviation card, and I never said the deviation wouldn't be there. I only gave a way to determine what it is on each heading. Please tell me what I'm missing because I don't know what you are talking about. If my deviation card shows large deviations, I would adjust my compass, but if the deviations are small, I would leave it alone. Have fun Joe S
Compass 101
Sources of error in pilotage are: math mistakes converting true true headings from the map to compass headings to steer. The compass having internal (not boat related) errors The interaction of the boat magnetic and compass magnetic field Compass malfunction (gimbles, fluid, etc) Forgetting the first and last one you can get the compass to read correctly when away from ALL magnetic fields except the earth's by swinging it. a pretty simple procedure. You cannot adjust the compass for the boat's magnetic field however. The only way to handle that is to record the errors on several headings and produce a deviation card. If you operate your boat in different modes such as engine off and engine on or with some large long term electrcal device (radar etc) you should really construct a card that has all the modes on it. As several have noted, it is possable to have a significant difference in the deviation for each mode. With all that said, I have never seen a deviation card on any boat I ever sailed on and the ones I have constructed have always had a deviation of less than +-3 degrees at any one spot. Since you would be an exceptional helmsman to steer that accuratly I'm thinking that the effort to create the card is really not worth it and you could just get by with spot checking against your GPS (after swinging the compass of course) from time to time to confirm that things are pretty close to correct.
Sources of error in pilotage are: math mistakes converting true true headings from the map to compass headings to steer. The compass having internal (not boat related) errors The interaction of the boat magnetic and compass magnetic field Compass malfunction (gimbles, fluid, etc) Forgetting the first and last one you can get the compass to read correctly when away from ALL magnetic fields except the earth's by swinging it. a pretty simple procedure. You cannot adjust the compass for the boat's magnetic field however. The only way to handle that is to record the errors on several headings and produce a deviation card. If you operate your boat in different modes such as engine off and engine on or with some large long term electrcal device (radar etc) you should really construct a card that has all the modes on it. As several have noted, it is possable to have a significant difference in the deviation for each mode. With all that said, I have never seen a deviation card on any boat I ever sailed on and the ones I have constructed have always had a deviation of less than +-3 degrees at any one spot. Since you would be an exceptional helmsman to steer that accuratly I'm thinking that the effort to create the card is really not worth it and you could just get by with spot checking against your GPS (after swinging the compass of course) from time to time to confirm that things are pretty close to correct.
I've actually tested it...
I've actually tested it two years ago, prior to a compass adjustment so I would know, every which way, on dry land, where I know the compass is NOT moving. Engine off and everything else off heading = 342 degrees Engine running everything else off = 342 degrees Engine running electronics on = 342 degrees Engine running electronics running & stereo cranked = 342 degrees Handhled VHF in holder next to compass on or off = 4 degree error Moved hand held VHF holder...... P.S. The compass that came on my 2005 Catalina 310 was off by 6 degrees to over 14 degrees depending on the point of sail... It's wise to know these errors and make a deviation card or get it as close as you can and still make a deviation card.
I've actually tested it two years ago, prior to a compass adjustment so I would know, every which way, on dry land, where I know the compass is NOT moving. Engine off and everything else off heading = 342 degrees Engine running everything else off = 342 degrees Engine running electronics on = 342 degrees Engine running electronics running & stereo cranked = 342 degrees Handhled VHF in holder next to compass on or off = 4 degree error Moved hand held VHF holder...... P.S. The compass that came on my 2005 Catalina 310 was off by 6 degrees to over 14 degrees depending on the point of sail... It's wise to know these errors and make a deviation card or get it as close as you can and still make a deviation card.
I think that Bill and I are on the same page
There is absolutely nothing wrong with having an accurate compass. Every bit of navigational information is valuble. Stupid might be too strong of a word...maybe silly??? But if you sail where there is often fog it is silly to not have a GPS with important waypoints in the memory!!!! We marked every dayboard on our entrance channel as a waypoint not because of fog but because it was diffcult to see the lighted dayboards at night due to shore lights. A compass was not useful as there were currents. Not strong currents but currents strong enough to push us out of the channel. Obviously if your business is fixing compasses you think everyone should have their compass serviced. But in my experience flying and sailing and orienteering a compass is very useful but unless you have other information it is worthless. You must know where you are to know which way to go to get somewhere. If you are out in the fog you are SOL unless you knew exactly where you were at before the fog set in. Not knowing your EXACT location makes the worlds most accurate compass useless as to knowing an EXACT bearing back to port. In everyday use most of us only know our approxiamte location give or take a few hundred feet. That means that we can only calculate our approximate bearing to where we want to go. For these purposes a compass does not have to be super accurate. Under most circumstances I can get most of my required navigational information with my binoculars and depth sounder. I haven't used my GPS in a couple of years. I am on a large lake with dayboards and a huge paper mill with tall smoke stacks. I look at my compass but useually just to find the wind direction to plan tacks. I also use the compass to tell my wife how to steer.
There is absolutely nothing wrong with having an accurate compass. Every bit of navigational information is valuble. Stupid might be too strong of a word...maybe silly??? But if you sail where there is often fog it is silly to not have a GPS with important waypoints in the memory!!!! We marked every dayboard on our entrance channel as a waypoint not because of fog but because it was diffcult to see the lighted dayboards at night due to shore lights. A compass was not useful as there were currents. Not strong currents but currents strong enough to push us out of the channel. Obviously if your business is fixing compasses you think everyone should have their compass serviced. But in my experience flying and sailing and orienteering a compass is very useful but unless you have other information it is worthless. You must know where you are to know which way to go to get somewhere. If you are out in the fog you are SOL unless you knew exactly where you were at before the fog set in. Not knowing your EXACT location makes the worlds most accurate compass useless as to knowing an EXACT bearing back to port. In everyday use most of us only know our approxiamte location give or take a few hundred feet. That means that we can only calculate our approximate bearing to where we want to go. For these purposes a compass does not have to be super accurate. Under most circumstances I can get most of my required navigational information with my binoculars and depth sounder. I haven't used my GPS in a couple of years. I am on a large lake with dayboards and a huge paper mill with tall smoke stacks. I look at my compass but useually just to find the wind direction to plan tacks. I also use the compass to tell my wife how to steer.
OK but..
Moon, Where you sail and I sail are at the opposite ends of the spectrum. Only Newfoundland, Nova Scotia and New Brunswick tend to see more fog than Maine, so for me, having an accurate a compass as possible removes that much more variability. If I sailed where you did I would probably not give as much though to compass calibration. Neither your home waters, nor my home waters, are representative of the entire country so the folks in between our extremes need to make their own choices. Again, as I stated, my compass that came on a BRAND NEW BOAT was off between 6 and 14 degrees right out of the box! Six to fourteen degrees is a LOT of error to start thinking about calculating out when that fog bank begins looming on the horizon and you have all those pesky little other things to do too like the man the stop watch, radar, fog horn, other boats & lobster buoys not even to mention adding or subtracting 6-14 degrees of known error to the mix I use a GPS plotter, and have been since about 1998, but I NEVER rely solely on one means of navigation equipment. That would be "stupid", at least where I sail...
Moon, Where you sail and I sail are at the opposite ends of the spectrum. Only Newfoundland, Nova Scotia and New Brunswick tend to see more fog than Maine, so for me, having an accurate a compass as possible removes that much more variability. If I sailed where you did I would probably not give as much though to compass calibration. Neither your home waters, nor my home waters, are representative of the entire country so the folks in between our extremes need to make their own choices. Again, as I stated, my compass that came on a BRAND NEW BOAT was off between 6 and 14 degrees right out of the box! Six to fourteen degrees is a LOT of error to start thinking about calculating out when that fog bank begins looming on the horizon and you have all those pesky little other things to do too like the man the stop watch, radar, fog horn, other boats & lobster buoys not even to mention adding or subtracting 6-14 degrees of known error to the mix I use a GPS plotter, and have been since about 1998, but I NEVER rely solely on one means of navigation equipment. That would be "stupid", at least where I sail...
From Compass to a chart
If you are working from a chart to a compass you need to be very accurate. We had to use very thin lead in the Captains class or you could be off by a degree. If you are working with charts you need the table for deviation. Thanks for all the responses. Richard ( Duke)
If you are working from a chart to a compass you need to be very accurate. We had to use very thin lead in the Captains class or you could be off by a degree. If you are working with charts you need the table for deviation. Thanks for all the responses. Richard ( Duke)
Heck, use a very fine pin! 
That's really not the point (whoops, sorry about that!) Seriously, the issue is knowing where you are in relation to other objects. Sure, you can take all the classes you need to or want to, and the need to perform calculations gymnastics is truly laudable and important to know, or even more so, understand. But the practical reality of placing yourself on a chart with a fix is what is the whole purpose behind the navigation concepts, regardless of whether you use a needle or a carpenter's pencil. Have fun, and be safe out there!
That's really not the point (whoops, sorry about that!
) Seriously, the issue is knowing where you are in relation to other objects. Sure, you can take all the classes you need to or want to, and the need to perform calculations gymnastics is truly laudable and important to know, or even more so, understand. But the practical reality of placing yourself on a chart with a fix is what is the whole purpose behind the navigation concepts, regardless of whether you use a needle or a carpenter's pencil. Have fun, and be safe out there!
I once figured that my dividers make a mark
about the size of my boat on 80,000:1 charts. Looking at a charts catalog the scale ranges from 5,000 to 1,250,000. On the latter your pencil lines would be about a mile wide.
about the size of my boat on 80,000:1 charts. Looking at a charts catalog the scale ranges from 5,000 to 1,250,000. On the latter your pencil lines would be about a mile wide.
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