fluffc
Member
Re: Hire a Compass Adjuster
Roughly. But not quite.
Roughly. But not quite.
Compass swinging...best method
- Thread starter starboard
- Start date
michael_w
Well-known member
Re: Hire a Compass Adjuster
This must be the only practical use of simultaneous equations; as various teachers have tried to intstill in me without success.
This must be the only practical use of simultaneous equations; as various teachers have tried to intstill in me without success.
thefatlady
Well-known member
Re: Hire a Compass Adjuster
Please elucidate.
Please elucidate.
Phoenix of Hamble
Active member
Re: Hire a Compass Adjuster
The deviation curve doesn't have to be a sine wave!
The deviation curve doesn't have to be a sine wave!
thefatlady
Well-known member
Re: Hire a Compass Adjuster
You may be right, but that's a bald statement with no back-up.
Please explain what can change non-sinusoidally as the compass/boat is rotated on the spot.
You may be right, but that's a bald statement with no back-up.
Please explain what can change non-sinusoidally as the compass/boat is rotated on the spot.
Robin
Well-known member
Re: Hire a Compass Adjuster
[ QUOTE ]
and remove all doubt, because he will correct the compass, re-swing and issue a signed deviation table with his firm's name on it, with residual deviation only.
[/ QUOTE ]
Just not the one I had to swing our last boat. We thought we had little deviation from our own basic checks but went for the real deal swing when two boats on the next moorings had theirs done and we could split his travelling cost. After the 1st attempt it was way out compared to the electronic swung autopilot, but he said that was 'faulty'. However we could confirm it out when we 'missed Cherbourg 2 days later by several miles and on a transit inside there proved it was way out. So we called him back and he agreed it was wrong, but it must be because we had moved something - we hadn't. The 2nd swing was better but still 'out' a bit compared to the pilot.
Now I then decided to do it myself so purchased a Davies Pelorus from West Marine, set this up on the mainhatch so that we could read sun shadow on it and compared this bearing with the sun's bearings on different headings, data for this coming via the GPS clock and a Casio loaded Astro program. We did it over several weeks so that we had calm conditions on the different headings on sunny days. The results were excellent and confirmed what I suspected that Mr Swinger's card was not at all good. Ours was not too different as it happened from the electronic swing, no more than 2 degs I think. We were later told Mr Swinger liked a lunchtime pint and advised to have him do a swing before rather than after lunch....
On our current boat I did a 'temporary' swing after we bought her in a very quiet tide area at slack water neaps using a differential dGPS with COG averaged over just 5 secs. We got a very passable curve which fairly closely agreed with the electronic swing we did. I'm ashamed to say that 5 years on I still haven't done the Pelorus swing but I have checked as and when transits were available and the curve I have is pretty good. The key with using the GPS is that the boat must be on a steady course at a reasonable speed to cancel out momentary errors, we did it at 5kts I think and bear in mind this was a land based dGPS with S/A off, ie after the yanks stopped degrading it.
A Pro swing is probably best especially if the guy adjusts most of the deviation out, but take care to have it done before lunch.... /forums/images/graemlins/smile.gif I did also buy a book on it and it can be adjusted yourself with care and patience, I forget what the book was called and it is on the boat.
[ QUOTE ]
and remove all doubt, because he will correct the compass, re-swing and issue a signed deviation table with his firm's name on it, with residual deviation only.
[/ QUOTE ]
Just not the one I had to swing our last boat. We thought we had little deviation from our own basic checks but went for the real deal swing when two boats on the next moorings had theirs done and we could split his travelling cost. After the 1st attempt it was way out compared to the electronic swung autopilot, but he said that was 'faulty'. However we could confirm it out when we 'missed Cherbourg 2 days later by several miles and on a transit inside there proved it was way out. So we called him back and he agreed it was wrong, but it must be because we had moved something - we hadn't. The 2nd swing was better but still 'out' a bit compared to the pilot.
Now I then decided to do it myself so purchased a Davies Pelorus from West Marine, set this up on the mainhatch so that we could read sun shadow on it and compared this bearing with the sun's bearings on different headings, data for this coming via the GPS clock and a Casio loaded Astro program. We did it over several weeks so that we had calm conditions on the different headings on sunny days. The results were excellent and confirmed what I suspected that Mr Swinger's card was not at all good. Ours was not too different as it happened from the electronic swing, no more than 2 degs I think. We were later told Mr Swinger liked a lunchtime pint and advised to have him do a swing before rather than after lunch....
On our current boat I did a 'temporary' swing after we bought her in a very quiet tide area at slack water neaps using a differential dGPS with COG averaged over just 5 secs. We got a very passable curve which fairly closely agreed with the electronic swing we did. I'm ashamed to say that 5 years on I still haven't done the Pelorus swing but I have checked as and when transits were available and the curve I have is pretty good. The key with using the GPS is that the boat must be on a steady course at a reasonable speed to cancel out momentary errors, we did it at 5kts I think and bear in mind this was a land based dGPS with S/A off, ie after the yanks stopped degrading it.
A Pro swing is probably best especially if the guy adjusts most of the deviation out, but take care to have it done before lunch.... /forums/images/graemlins/smile.gif I did also buy a book on it and it can be adjusted yourself with care and patience, I forget what the book was called and it is on the boat.
Phoenix of Hamble
Active member
Re: Hire a Compass Adjuster
I note the point from observation rather than analysis...
however, thinking it through...
Magnetic fields can be quite localised in nature, and change in 'effect' rapidly over short distances.... it is entirely possible that a field could have a noticeable impact on a the magnetic component (or pole) of a compass on 180 deg of its swing, but much less on the same magnetic component (or pole) on the other 180 deg (because its in a much less strong part of the field being perhaps 4" further away from the source of the field), hence the curve would not be symmetrical (or reverse symmetrical), and hence could not be sinusoidal... also the fields are in 3 dimensions, so it is also possible that this could be caused by a complex interaction of vertical and horizontal field components, and all stages in between.
Does that make sense? or am I just trying to justify a statement that I know to be true through empirical observation, in a wild and irrational manner!
I note the point from observation rather than analysis...
however, thinking it through...
Magnetic fields can be quite localised in nature, and change in 'effect' rapidly over short distances.... it is entirely possible that a field could have a noticeable impact on a the magnetic component (or pole) of a compass on 180 deg of its swing, but much less on the same magnetic component (or pole) on the other 180 deg (because its in a much less strong part of the field being perhaps 4" further away from the source of the field), hence the curve would not be symmetrical (or reverse symmetrical), and hence could not be sinusoidal... also the fields are in 3 dimensions, so it is also possible that this could be caused by a complex interaction of vertical and horizontal field components, and all stages in between.
Does that make sense? or am I just trying to justify a statement that I know to be true through empirical observation, in a wild and irrational manner!
thefatlady
Well-known member
Re: Hire a Compass Adjuster
I find it difficult to envisage a situation in which parts of the boat which affect deviation can change their position relative to the compass as the boat is rotated.
Many years ago, I put this to my Yachtmaster instructor (who was an Aldermaston physicist), who said I was right, but most people wouldn't understand. However, if it is right, why isn't it used routinely? I've never brought it up again, until now.
Trouble is, it's too much hassle to try to prove or disprove it experimentally.
I find it difficult to envisage a situation in which parts of the boat which affect deviation can change their position relative to the compass as the boat is rotated.
Many years ago, I put this to my Yachtmaster instructor (who was an Aldermaston physicist), who said I was right, but most people wouldn't understand. However, if it is right, why isn't it used routinely? I've never brought it up again, until now.
Trouble is, it's too much hassle to try to prove or disprove it experimentally.
Phoenix of Hamble
Active member
Re: Hire a Compass Adjuster
They won't change their position relative to the compass, but they will relative to the north pole of the compass which moves at the compass rotates....
If for example a spanner is sat next to the compass... as the north pole points at the spanner it could be say 2" away, but at south, it will be 6" away (for arguments sake)....
Now imagine that instead of a spanner it is an magnetic field that remains fixed in boat terms, but variable in position relative to the north pole, then the difference in field strengths could be quite significant between those two positions.... not linear, hence the deviation effect would be relatively much smaller on a south heading than a north heading..... so no sinusoid....
I'm no Aldermaston physicist, but have studied magnet theory as part of an electrical and electronic engineering degree, so familiar with Teslas and magnetic field theory....
If the field happened to be offest, then it could affect one side of the compass (ie when the north pole was at that side of the compass) much more than the other side... hance again, different effect between oppsing points, hence no sinusoid...
They won't change their position relative to the compass, but they will relative to the north pole of the compass which moves at the compass rotates....
If for example a spanner is sat next to the compass... as the north pole points at the spanner it could be say 2" away, but at south, it will be 6" away (for arguments sake)....
Now imagine that instead of a spanner it is an magnetic field that remains fixed in boat terms, but variable in position relative to the north pole, then the difference in field strengths could be quite significant between those two positions.... not linear, hence the deviation effect would be relatively much smaller on a south heading than a north heading..... so no sinusoid....
I'm no Aldermaston physicist, but have studied magnet theory as part of an electrical and electronic engineering degree, so familiar with Teslas and magnetic field theory....
If the field happened to be offest, then it could affect one side of the compass (ie when the north pole was at that side of the compass) much more than the other side... hance again, different effect between oppsing points, hence no sinusoid...
Phoenix of Hamble
Active member
Re: Hire a Compass Adjuster
PS.... i'm not wedded to this theory, so welcome a debate..... but think (in big inverted commas!) that i'm right at the moment..... happy to be proved wrong through superior arguments though....
PS.... i'm not wedded to this theory, so welcome a debate..... but think (in big inverted commas!) that i'm right at the moment..... happy to be proved wrong through superior arguments though....
thefatlady
Well-known member
Re: Hire a Compass Adjuster
What about the "South" pole - equal and opposite. I suspect that the magnet in a compass would be symmetrical about the centre of rotation, therefore the effective distance of the magnetic "anomaly" would not change and the output would still be approximately sinusoidal.
I've had a search and can only find anything meaningful relating to aircraft. This suggests that the external influences are indeed sinusoidal, but that there is significant induced magnetism on an aircraft. I suspect that makes it quite different to a boat. Allowing for the three orthogonal induced field directions, they suggest a five coefficient equation with two sine terms, two cosine terms and a constant. They use eight-point swinging, and determining the coefficients by Fourier analysis.
On a two-dimensional analysis, such as with a boat, the various permanent and induced components remain sinusoidal, but with phase and amplitude shifts. Question is, would the induced magnetism be of any consequence on a boat, given the accuracy requirement?
Anyways, the real way to find out would be to have a compass properly swung, which would also give two suitable points for deriving the sine wave. Then compare the two curves. Having said that, a sample of one would only have any meaning if there were to be a significant difference between the two curves.
What about the "South" pole - equal and opposite. I suspect that the magnet in a compass would be symmetrical about the centre of rotation, therefore the effective distance of the magnetic "anomaly" would not change and the output would still be approximately sinusoidal.
I've had a search and can only find anything meaningful relating to aircraft. This suggests that the external influences are indeed sinusoidal, but that there is significant induced magnetism on an aircraft. I suspect that makes it quite different to a boat. Allowing for the three orthogonal induced field directions, they suggest a five coefficient equation with two sine terms, two cosine terms and a constant. They use eight-point swinging, and determining the coefficients by Fourier analysis.
On a two-dimensional analysis, such as with a boat, the various permanent and induced components remain sinusoidal, but with phase and amplitude shifts. Question is, would the induced magnetism be of any consequence on a boat, given the accuracy requirement?
Anyways, the real way to find out would be to have a compass properly swung, which would also give two suitable points for deriving the sine wave. Then compare the two curves. Having said that, a sample of one would only have any meaning if there were to be a significant difference between the two curves.
peterb
New member
Sine waves
[ QUOTE ]
Someone correct me, but isn’t the compass error a sine wave as in
error = c + a sinb
where
c= offset
a = amplitude
b = bearing
In which case all that is needed is two calibrated points to give two simultaneous equations to calculate c and a.
[/ QUOTE ]
It's not quite that simple (but nearly).
Assuming that the compass errors are due to permanent magnetism in the boat, then, yes, the deviation curve will be an offset sinewave. But the phase of that sine-wave will be another variable, most easily taken into account by adding a cosine-wave:
error = a + b*sin(z) + c*cos(z)
where z is the heading and a, b and c are constants.
But some of the error may be due to soft magnetism. Think of a long thin soft iron rod. When the rod is lined up with the earth's field then it will be magnetised, but because it's lined up it will not affect the direction of the field. When the rod is at right angles to the field then it won't be magnetised, so again will have no effect. But when the rod is at 45 degrees to the field it will be magnetised and will distort the field to cause deviation. The effect will be to give a sine-wave deviation, but having two cycles of sine-wave in one rotation of the boat. The resultant equation will be:
error = a + b*sin(z) + c*cos(z) + d*sin(2z) + e*cos(2z)
If I'm doing a compass swing, then I compare the main compass with a hand bearing compass (hbc). Bilbobaggins isn't quite right when he says that you don't know about magnetic influences on a boat. Take a bearing on a distant object while turning the boat through 360. If the bearing stays constant then <u>at that point on the boat</u> there is no deviation. If the bearing changes, then repeat from another point until a suitable point is found. I usually find that standing by the backstay gives a good starting point (on wooden or plastic boats anyway; probably not on steel boats). And you can't necessarily rely on the hbc; I check mine from an Ordnance Survey trig point, and it has a constant half-degree error, but I have found hbcs that have 5 degree errors.
You need to have a line to look along. Standing at the backstay and looking at the mast isn't really accurate enough. We set up a line by having a "backsight" in the cockpit (a mark on the coaming or on the sprayhood) and a "foresight" of a piece of red sticky tape on a guard wire. Just make sure that they are at the same distances from the boat centreline.
Most hbcs can be read (with care and in calm water) to half degrees, if not quarters. But most steering compasses are only calibrated every 5 degrees. To me it makes sense to align the boat so that it is exactly on one of the steering compass calibrations, then look at the hbc reading. In fact we find that it is very difficult to hold the boat exactly in one direction, so we have one person to look at the compass while the second looks along our line. We turn the boat slowly until the compass lines up exactly on the desired heading, the compass man says "Now" and the line man notes a distant object to which the line is pointing. Now the line man can use the hbc to get the bearing of the distant object; if the object is sufficiently distant then subsequent small changes in heading won't affect the hbc reading.
We take readings every 30 degrees, then do it again with the boat turning in the other direction. At each heading we take the average of the clockwise and anticlockwise readings.
You can, of course, just plot these figures to produce a deviation curve. But (being by training a mathematician) I analyse the figures (Fourier analysis) to get the best estimate for the five constants for the equation. Then I compare the figures from the equation with the measured figures. If the difference (on any single heading) is more than one degree we do the whole lot again. We end up with a deviation curve that we have real confidence in.
Of course, we don't do anything about heeling error. On a big ship, with a big ship binnacle, it is possible to correct for all the five elements, and to correct for heeling error. On a small boat compass it may be possible to correct the offset, and perhaps the permanent magnetism effect. But I've yet to see a small boat fitted with Kelvin spheres, those soft iron balls placed on either side of the compass to correct for the soft iron effect, or with the vertically moveable basket needed to overcome heeling error. We just accept it and hope it's not too big!
Edit: Fatlady got in first, while I was writing.
Yes, there can be a significant soft iron effect (quadrantal error). On one previous boat (a Sadler 34) we had two bulkhead compasses. They both had about the same maximum error (about 4 degrees, from memory), but one was almost entirely hard iron effect (one cycle) while the other was almost entirely soft iron (two cycles). Problem for the navigator, since he had to know which compass the helmsman was using before he could give the the course. On some courses the two compasses could differ by 7 degrees.
Incidentally, I did once take out a charter boat (maiden voyage) which turned out to have a deviation of more than 30 deg. We returned it with a deviation of less than 3 degrees.
[ QUOTE ]
Someone correct me, but isn’t the compass error a sine wave as in
error = c + a sinb
where
c= offset
a = amplitude
b = bearing
In which case all that is needed is two calibrated points to give two simultaneous equations to calculate c and a.
[/ QUOTE ]
It's not quite that simple (but nearly).
Assuming that the compass errors are due to permanent magnetism in the boat, then, yes, the deviation curve will be an offset sinewave. But the phase of that sine-wave will be another variable, most easily taken into account by adding a cosine-wave:
error = a + b*sin(z) + c*cos(z)
where z is the heading and a, b and c are constants.
But some of the error may be due to soft magnetism. Think of a long thin soft iron rod. When the rod is lined up with the earth's field then it will be magnetised, but because it's lined up it will not affect the direction of the field. When the rod is at right angles to the field then it won't be magnetised, so again will have no effect. But when the rod is at 45 degrees to the field it will be magnetised and will distort the field to cause deviation. The effect will be to give a sine-wave deviation, but having two cycles of sine-wave in one rotation of the boat. The resultant equation will be:
error = a + b*sin(z) + c*cos(z) + d*sin(2z) + e*cos(2z)
If I'm doing a compass swing, then I compare the main compass with a hand bearing compass (hbc). Bilbobaggins isn't quite right when he says that you don't know about magnetic influences on a boat. Take a bearing on a distant object while turning the boat through 360. If the bearing stays constant then <u>at that point on the boat</u> there is no deviation. If the bearing changes, then repeat from another point until a suitable point is found. I usually find that standing by the backstay gives a good starting point (on wooden or plastic boats anyway; probably not on steel boats). And you can't necessarily rely on the hbc; I check mine from an Ordnance Survey trig point, and it has a constant half-degree error, but I have found hbcs that have 5 degree errors.
You need to have a line to look along. Standing at the backstay and looking at the mast isn't really accurate enough. We set up a line by having a "backsight" in the cockpit (a mark on the coaming or on the sprayhood) and a "foresight" of a piece of red sticky tape on a guard wire. Just make sure that they are at the same distances from the boat centreline.
Most hbcs can be read (with care and in calm water) to half degrees, if not quarters. But most steering compasses are only calibrated every 5 degrees. To me it makes sense to align the boat so that it is exactly on one of the steering compass calibrations, then look at the hbc reading. In fact we find that it is very difficult to hold the boat exactly in one direction, so we have one person to look at the compass while the second looks along our line. We turn the boat slowly until the compass lines up exactly on the desired heading, the compass man says "Now" and the line man notes a distant object to which the line is pointing. Now the line man can use the hbc to get the bearing of the distant object; if the object is sufficiently distant then subsequent small changes in heading won't affect the hbc reading.
We take readings every 30 degrees, then do it again with the boat turning in the other direction. At each heading we take the average of the clockwise and anticlockwise readings.
You can, of course, just plot these figures to produce a deviation curve. But (being by training a mathematician) I analyse the figures (Fourier analysis) to get the best estimate for the five constants for the equation. Then I compare the figures from the equation with the measured figures. If the difference (on any single heading) is more than one degree we do the whole lot again. We end up with a deviation curve that we have real confidence in.
Of course, we don't do anything about heeling error. On a big ship, with a big ship binnacle, it is possible to correct for all the five elements, and to correct for heeling error. On a small boat compass it may be possible to correct the offset, and perhaps the permanent magnetism effect. But I've yet to see a small boat fitted with Kelvin spheres, those soft iron balls placed on either side of the compass to correct for the soft iron effect, or with the vertically moveable basket needed to overcome heeling error. We just accept it and hope it's not too big!
Edit: Fatlady got in first, while I was writing.
Yes, there can be a significant soft iron effect (quadrantal error). On one previous boat (a Sadler 34) we had two bulkhead compasses. They both had about the same maximum error (about 4 degrees, from memory), but one was almost entirely hard iron effect (one cycle) while the other was almost entirely soft iron (two cycles). Problem for the navigator, since he had to know which compass the helmsman was using before he could give the the course. On some courses the two compasses could differ by 7 degrees.
Incidentally, I did once take out a charter boat (maiden voyage) which turned out to have a deviation of more than 30 deg. We returned it with a deviation of less than 3 degrees.
bilbobaggins
N/A
Re: Hire a Compass Adjuster
It is generally accepted that one considers 5 'approximate' coefficients which taken together, describe the Total Deviation on any specific heading.
This is treated fairly fully in, of course, Bowditch para. 615
Thus, Total Deviation on heading x is given by.....
Devn = A + B.sinx + C.cosx + D.sin2x + E.cos2x
Edit: Didn't see 'peterb's post as it was on another page, but it seems we're largely in agreement - except for the incidence of Systematic Error ( 'A' Error ) in the use of an hbc. Also quite right about the standard process for compass correction i.r.o aircraft. Fourier analysis was/is used to assist with the reduction of residual errors to below specified mandatory limits e.g. 0.25 degree.
Standing out hunched over a Dumpy Level on a freezing Lincolnshire airfield, for several hours on a bitter January morn - while the aircraft ( warm inside ) shakes and vibrates in the wind - is not conducive to swift precision. But precision was required.
/forums/images/graemlins/smile.gif /forums/images/graemlins/crazy.gif
It is generally accepted that one considers 5 'approximate' coefficients which taken together, describe the Total Deviation on any specific heading.
This is treated fairly fully in, of course, Bowditch para. 615
Thus, Total Deviation on heading x is given by.....
Devn = A + B.sinx + C.cosx + D.sin2x + E.cos2x
Edit: Didn't see 'peterb's post as it was on another page, but it seems we're largely in agreement - except for the incidence of Systematic Error ( 'A' Error ) in the use of an hbc. Also quite right about the standard process for compass correction i.r.o aircraft. Fourier analysis was/is used to assist with the reduction of residual errors to below specified mandatory limits e.g. 0.25 degree.
Standing out hunched over a Dumpy Level on a freezing Lincolnshire airfield, for several hours on a bitter January morn - while the aircraft ( warm inside ) shakes and vibrates in the wind - is not conducive to swift precision. But precision was required.
/forums/images/graemlins/smile.gif /forums/images/graemlins/crazy.gif
Phoenix of Hamble
Active member
Re: Hire a Compass Adjuster
Thanks guys,
That really informative, and interesting. Something else i've learned from this forum!
Thanks guys,
That really informative, and interesting. Something else i've learned from this forum!
thefatlady
Well-known member
Re: Sine waves
I knew there had to be an expert somewhere here. Thanks for that very informative response.
Interesting that the soft iron effect can be so large. I would have expected it to be small compared to the graduations on a steering compass and the accuracy with which a course can be held, especially in lumpy seas.
I knew there had to be an expert somewhere here. Thanks for that very informative response.
Interesting that the soft iron effect can be so large. I would have expected it to be small compared to the graduations on a steering compass and the accuracy with which a course can be held, especially in lumpy seas.
pau1gray
New member
Re: Hire a Compass Adjuster
Just a quick Question.........
We regularly carry out GYRO calibrations (not mag compass) with our positioning systems and use DGPS bearing and compare these to the gyro headings (rough and ready calibrations!!) but I have been bittena few times with Convergance when using GPS as GPS works using Grid heading (as we use UTM's)
Would convergance also have to be accounted for if I'm using WGS Lat and Long as opposed to UTM's?
Just a quick Question.........
We regularly carry out GYRO calibrations (not mag compass) with our positioning systems and use DGPS bearing and compare these to the gyro headings (rough and ready calibrations!!) but I have been bittena few times with Convergance when using GPS as GPS works using Grid heading (as we use UTM's)
Would convergance also have to be accounted for if I'm using WGS Lat and Long as opposed to UTM's?
bilbobaggins
N/A
Re: Hire a Compass Adjuster
It might be a quick question, but it certainly ain't a quick answer.....
/forums/images/graemlins/wink.gif
It might be a quick question, but it certainly ain't a quick answer.....
/forums/images/graemlins/wink.gif
peterb
New member
Re: Sine waves
Things can be worse. We brought our boat back from Goteborg to Enkhuisen in September. For some reason the fluxgate compass differed significantly from the binnacle compass on the run down to Kiel, so we decided to give the fluxgate the 'two circle' treatment. To get calm water we did it at Rendsburg, in the area where the entrance to the Obereidersee opens out. We got a surprisingly high deviation reading (18 degrees), outside the normal range but only just. Did it again, with the same result so accepted it.
From Rendsburg we carried on to Cuxhaven without really using the compass. But once we left Cuxhaven it became very clear that the check had made the compass worse, not better. A rough check against the hand bearing compass showed a maximum deviation of nearly 20 degrees, with the curve showing the error to be almost entirely quadrantal (soft iron, two cycles). It got worse when we tried to make the autopilot follow a route.
If you plot the compass reading against the real heading you should (with a perfect compass) get a straight line. The deviation curve shows the difference between the actual compass readings and this straight line. If the deviation is very large then the line showing the actual readings begins to look like a staircase, rather than a straight line. On the 'risers' of the staircase the compass reading will change rapidly with heading changes, whilst on the 'treads' the compass will appear insensitive. Our desired course was in the middle of a tread. As the boat moved off the heading the autopilot saw little change in compass reading, and found difficulty in holding a straight course. In following a route, though, much of the information comes from the 'cross-track error', which was working normally. The result was that our course became a zig-zag, deviating fom the mean by about 15 degrees each way. Fortunately the wind was so light that we had to motor; with the main pulled hard in the 30 degree course changes at the end of each leg caused little difficulty, and the maximum cross-track error was only about a cable.
What had happened? We suspect that at that point in the canal there is a 'local magnetic anomaly'; perhaps a pipe line or a power cable. We had done our two circles right above it, and had caused the compass to 'correct' a deviation which we left behind when we left Rendsburg.
In future I will try to do my swinging and corrections in an area which is unlikely to have such an anomaly. I've used Hamford Water (in the Walton Backwaters) with good effect in the past.
Things can be worse. We brought our boat back from Goteborg to Enkhuisen in September. For some reason the fluxgate compass differed significantly from the binnacle compass on the run down to Kiel, so we decided to give the fluxgate the 'two circle' treatment. To get calm water we did it at Rendsburg, in the area where the entrance to the Obereidersee opens out. We got a surprisingly high deviation reading (18 degrees), outside the normal range but only just. Did it again, with the same result so accepted it.
From Rendsburg we carried on to Cuxhaven without really using the compass. But once we left Cuxhaven it became very clear that the check had made the compass worse, not better. A rough check against the hand bearing compass showed a maximum deviation of nearly 20 degrees, with the curve showing the error to be almost entirely quadrantal (soft iron, two cycles). It got worse when we tried to make the autopilot follow a route.
If you plot the compass reading against the real heading you should (with a perfect compass) get a straight line. The deviation curve shows the difference between the actual compass readings and this straight line. If the deviation is very large then the line showing the actual readings begins to look like a staircase, rather than a straight line. On the 'risers' of the staircase the compass reading will change rapidly with heading changes, whilst on the 'treads' the compass will appear insensitive. Our desired course was in the middle of a tread. As the boat moved off the heading the autopilot saw little change in compass reading, and found difficulty in holding a straight course. In following a route, though, much of the information comes from the 'cross-track error', which was working normally. The result was that our course became a zig-zag, deviating fom the mean by about 15 degrees each way. Fortunately the wind was so light that we had to motor; with the main pulled hard in the 30 degree course changes at the end of each leg caused little difficulty, and the maximum cross-track error was only about a cable.
What had happened? We suspect that at that point in the canal there is a 'local magnetic anomaly'; perhaps a pipe line or a power cable. We had done our two circles right above it, and had caused the compass to 'correct' a deviation which we left behind when we left Rendsburg.
In future I will try to do my swinging and corrections in an area which is unlikely to have such an anomaly. I've used Hamford Water (in the Walton Backwaters) with good effect in the past.
peterb
New member
Re: Hire a Compass Adjuster
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We regularly carry out GYRO calibrations (not mag compass) with our positioning systems and use DGPS bearing and compare these to the gyro headings (rough and ready calibrations!!) but I have been bitten a few times with Convergance when using GPS as GPS works using Grid heading (as we use UTM's)
Would convergance also have to be accounted for if I'm using WGS Lat and Long as opposed to UTM's?
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How do you use DGPS? Surely GPS doesn't give you heading; it only gives course over the ground, and that can vary significantly from heading. Unless, of course, you have one of those GPS compasses that compares the signals at different points on the ship.
[ QUOTE ]
We regularly carry out GYRO calibrations (not mag compass) with our positioning systems and use DGPS bearing and compare these to the gyro headings (rough and ready calibrations!!) but I have been bitten a few times with Convergance when using GPS as GPS works using Grid heading (as we use UTM's)
Would convergance also have to be accounted for if I'm using WGS Lat and Long as opposed to UTM's?
[/ QUOTE ]
How do you use DGPS? Surely GPS doesn't give you heading; it only gives course over the ground, and that can vary significantly from heading. Unless, of course, you have one of those GPS compasses that compares the signals at different points on the ship.
thefatlady
Well-known member
Re: Sine waves
A lesson worth remembering. I have in the past done the "two circle" treatment on commisioning new equipment without problems but, on reflection, that was in an area which has underwater cables. It seems I was lucky and should be more careful in future.
Thanks again for your comprehensive response. It clearly took you some time.
A lesson worth remembering. I have in the past done the "two circle" treatment on commisioning new equipment without problems but, on reflection, that was in an area which has underwater cables. It seems I was lucky and should be more careful in future.
Thanks again for your comprehensive response. It clearly took you some time.