scottb34300
New member
Hi, I'm taking delivery of a new boat in May and the commisioning charge does not cover swinging the compass! Can anyone tell me how to do it please?
Thank you,
Thank you,
Compass Swinging
- Thread starter scottb34300
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Howardnp
New member
Simplistically... in the boat travel up a known transit line, note the compass reading, get the true reading off your chart, the difference is the total error, adjust for variation and that gives you the deviation. Do it on a load of different headings and produce a deviation card.
The compass error can be reduced by fiddling with the adjustment magnets on the compass.
Probably best idea is to find someone local who does it as a job and let them loose on it, gets it done although at a cost...especially the magnet fiddling bit.
Someone on here is bound to know a local adjuster in the area you are going to keep the boat.
The compass error can be reduced by fiddling with the adjustment magnets on the compass.
Probably best idea is to find someone local who does it as a job and let them loose on it, gets it done although at a cost...especially the magnet fiddling bit.
Someone on here is bound to know a local adjuster in the area you are going to keep the boat.
snowleopard
Active member
here's a tip to save yourself an adjuster's fee:
if you have an autopilot it will have a function to generate its internal deviation table. i can only speak for autohelm but in that case you steer round very slowly in a circle. at the end of the process it reports the maximum deviation found. mine reported less than 1 degree.
if you are lucky you will get the same result, you can then treat the autopilot's compass as a datum and compare the heading on the autopilot with that read from the steering compass. if, like me, you find there is no detectable difference you can be confident that there isn't enough deviation to be worth bothering with it.
if on the other hand you find that these procedures don't give very small errors, you can produce a rough-and-ready deviation table by pointing the boat at various objects on shore and comparing them to the readings of a hand-bearing compass held well above deck level. i say rough because the accuracy of h/b cmpasses aren't always as good as you expect.
if you have an autopilot it will have a function to generate its internal deviation table. i can only speak for autohelm but in that case you steer round very slowly in a circle. at the end of the process it reports the maximum deviation found. mine reported less than 1 degree.
if you are lucky you will get the same result, you can then treat the autopilot's compass as a datum and compare the heading on the autopilot with that read from the steering compass. if, like me, you find there is no detectable difference you can be confident that there isn't enough deviation to be worth bothering with it.
if on the other hand you find that these procedures don't give very small errors, you can produce a rough-and-ready deviation table by pointing the boat at various objects on shore and comparing them to the readings of a hand-bearing compass held well above deck level. i say rough because the accuracy of h/b cmpasses aren't always as good as you expect.
iangrant
New member
Just had a thought - my GPS readout gives COG and heading I reckon that'd be fairly accurate?
(Head down awaiting hail of abuse from old seadogs with brass sextants, one leg and a patch over both eyes, slobbering: "youse needs a dinghy towed behoind in a force 7 wiv an 'and bearin' compass to do it proper loik aahhhhaaaa)
Ian
(Head down awaiting hail of abuse from old seadogs with brass sextants, one leg and a patch over both eyes, slobbering: "youse needs a dinghy towed behoind in a force 7 wiv an 'and bearin' compass to do it proper loik aahhhhaaaa)
Ian
chops_uk
New member
you could try towing an inflatable or other such (no iron / engine) and get someone (wearing nothing metal) to hold a hand bearing compass then compare your various headings.
iangrant
New member
See - told yer
Ian
Ian
ashanta
New member
I thought about doing this also but was unable to as my set up for tiller pilot was not as accurate as it should be. i.e. there is a slight error in the 90 degree angle. I therefore do not take much notice of the read out on the AP other than if i'm on my desired compass course I take note of the AP figure as a reference.
What I finally did instead was to stand clear of any metal and a reasonable distance from the engine with my hand held compass (which is accurate enough for me) and went through the full circle taking and comparing readings until I had sufficient to create my deviation card.
It has been accurate enough for me without having to pay for it to be swung.
Regards.
Peter.
What I finally did instead was to stand clear of any metal and a reasonable distance from the engine with my hand held compass (which is accurate enough for me) and went through the full circle taking and comparing readings until I had sufficient to create my deviation card.
It has been accurate enough for me without having to pay for it to be swung.
Regards.
Peter.
CaptainThames
New member
Tried the dinghy thing once - disaster unless deviation is huge.
GPS COG is that, as opposed to the direction in which the boat is pointing. Aguably, with little wind and no current or tide, this will not be far out and higher speed will reduce those influences further.
Transits is best and with a little planning, you can create a number from charted objects, remembering that you can do one heading towards and use the same transit heading away.
The trick is in making the adjustments to reduce the errors, although this is limited (but sufficient) on an average yacht compass.
GPS COG is that, as opposed to the direction in which the boat is pointing. Aguably, with little wind and no current or tide, this will not be far out and higher speed will reduce those influences further.
Transits is best and with a little planning, you can create a number from charted objects, remembering that you can do one heading towards and use the same transit heading away.
The trick is in making the adjustments to reduce the errors, although this is limited (but sufficient) on an average yacht compass.
Wiggo
New member
Sorry, but if doing it this way, how can you be sure that your ship's head is the same as the transit bearing? Even a slight leeway will have yuo going crabwise.
Theoretically, you should do it with the boat moored to a buoy in a known location or at anchor (assuming you have a GPS), then motor it round with the tender pulling the stern to get your bearings to known landmarks. Which is a pain in the backside, unless you know of some location conveniently surrounded by a couple of dozen or so obvious landmarks...
We did ours using the gyro comapss as a reference, cos that calibrates itself, mostly. AFAIK, they do it by measuring and comparing the rate of turn as measured by the gyro (actually more like a solid state accelerometer) with the readings from the fluxgate compass, which is why you have to turn very slowly through a couple of revolutions. Then you can set north by following a transit or using the COG from the GPS.
We did a deviation card last November, and it is all over the place, with 9* deviation at 310*C and only 3* at 330*C, and none at all on some bearings.
Theoretically, you should do it with the boat moored to a buoy in a known location or at anchor (assuming you have a GPS), then motor it round with the tender pulling the stern to get your bearings to known landmarks. Which is a pain in the backside, unless you know of some location conveniently surrounded by a couple of dozen or so obvious landmarks...
We did ours using the gyro comapss as a reference, cos that calibrates itself, mostly. AFAIK, they do it by measuring and comparing the rate of turn as measured by the gyro (actually more like a solid state accelerometer) with the readings from the fluxgate compass, which is why you have to turn very slowly through a couple of revolutions. Then you can set north by following a transit or using the COG from the GPS.
We did a deviation card last November, and it is all over the place, with 9* deviation at 310*C and only 3* at 330*C, and none at all on some bearings.
Vara
Active member
Quite a good way HERE
On sailing boat you don't need pole erected as fore stay, mast, back stay provide good transit.Make sure Lubber line on compass is aligned to boat(See Lemains post on this subject).
IMO using hand bearing compass is more than adequate to check heading compass, however if you can detect deviation more than 5 degrees probably best to call in professional!
Purists might add that you should have different deviation cards for engine running/not running,Electrics on/off or any combination of all three.
Using GPS to work out deviation unnecessarily complicates situation and introduces all sorts of other variables,maybe alright on Windermere on a windless day.
On sailing boat you don't need pole erected as fore stay, mast, back stay provide good transit.Make sure Lubber line on compass is aligned to boat(See Lemains post on this subject).
IMO using hand bearing compass is more than adequate to check heading compass, however if you can detect deviation more than 5 degrees probably best to call in professional!
Purists might add that you should have different deviation cards for engine running/not running,Electrics on/off or any combination of all three.
Using GPS to work out deviation unnecessarily complicates situation and introduces all sorts of other variables,maybe alright on Windermere on a windless day.
William_H
Well-known member
Hi Wiggo using transits. The transits give you a location for your boat so that you can then find what the heading to one of the marks is from a chart.
Now having located your boat by using the transits you must turn the boat toward the mark usually one of the transit marks to check that the compass corrected for variation reads correctly. Now the boat doesn't have to be moving and in fact you should check with engine on and off.
If you are located a long distance from the mark you are using then a little drift sideways will not matter so much. it is far more imoportant to keep the boat heading to the mark (and later away for reciprocal) than to try to keep it on the transit. just check reasonably quickly.
For checking compass I would use a hand bearing compass from a towed dinghy or try hanging out from the transom as far as poss. lean back from the stern rail in a harness or loop of rope. olewill
Now having located your boat by using the transits you must turn the boat toward the mark usually one of the transit marks to check that the compass corrected for variation reads correctly. Now the boat doesn't have to be moving and in fact you should check with engine on and off.
If you are located a long distance from the mark you are using then a little drift sideways will not matter so much. it is far more imoportant to keep the boat heading to the mark (and later away for reciprocal) than to try to keep it on the transit. just check reasonably quickly.
For checking compass I would use a hand bearing compass from a towed dinghy or try hanging out from the transom as far as poss. lean back from the stern rail in a harness or loop of rope. olewill
srm
Well-known member
My last boat (steel hull and deck) had a Sestrel Major compass mounted on a pedestal in cockpit with a shadow pin (vertical pin in centre of card). You need a late evening (or early morning) and low sun so shadow reaches edge of card. Motor in slow circle noting heading, bearing of shadow, and time (UTC to 10 seconds) every 20 degrees or so. Only takes ten or fifteen minutes.
Moor up and retire to cabin. Calculate reciprocal of shadow on each heading to get compass bearing of sun.
Calculate true bearing of sun (ABC tables or calculator formula plus almanac) for time of first and last readings. Apply variation to obtain magnetic bearing of sun for these two readings.
Draw graph with time on horizontal axis and magnetic bearing of sun on vertical axis. Plot first and last point and join with straight line (accurate enough for the short time interval). Use graph to obtain magnetic bearing of sun for each heading. Compare with compass bearing (reciprocal of shadow) to get deviation.
There will be some headings that you were not able to obtain a compass bearing of the sun due to obstructions but drawing a graph of your deviation curve should enable you to interpolate any gaps.
OK, I know it sounds complicated but have used all the other methods suggested as well as a pelorus on a transit and found sun shadow was by far the easiest and most accurate. The swing and readings could be obtained singlehanded and the whole process took about an hour.
Unfortunately, my current boat has a bulkhead mounted compass in a fixed wheel shelter so it is back to messing around with transits and / or a second compass.
Only problem will be getting the sun GHA and dec as no longer printed in most almanacs.
Moor up and retire to cabin. Calculate reciprocal of shadow on each heading to get compass bearing of sun.
Calculate true bearing of sun (ABC tables or calculator formula plus almanac) for time of first and last readings. Apply variation to obtain magnetic bearing of sun for these two readings.
Draw graph with time on horizontal axis and magnetic bearing of sun on vertical axis. Plot first and last point and join with straight line (accurate enough for the short time interval). Use graph to obtain magnetic bearing of sun for each heading. Compare with compass bearing (reciprocal of shadow) to get deviation.
There will be some headings that you were not able to obtain a compass bearing of the sun due to obstructions but drawing a graph of your deviation curve should enable you to interpolate any gaps.
OK, I know it sounds complicated but have used all the other methods suggested as well as a pelorus on a transit and found sun shadow was by far the easiest and most accurate. The swing and readings could be obtained singlehanded and the whole process took about an hour.
Unfortunately, my current boat has a bulkhead mounted compass in a fixed wheel shelter so it is back to messing around with transits and / or a second compass.
Only problem will be getting the sun GHA and dec as no longer printed in most almanacs.
Lakesailor
New member
[ QUOTE ]
See - told yer
Ian
[/ QUOTE ]
Deviating slightly ( see what I did there?)
To be fair the other post was 2 minutes after your own, so you were probably both tapping away at once.
BUT I do agree that many forumites seem to put their point regardless of whether it's been already suggested, AND fail to acknowledge that they are reinforcing a point previously made. (Although this has not particularly happened in this thread )
See - told yer
Ian
[/ QUOTE ]
Deviating slightly ( see what I did there?)
To be fair the other post was 2 minutes after your own, so you were probably both tapping away at once.
BUT I do agree that many forumites seem to put their point regardless of whether it's been already suggested, AND fail to acknowledge that they are reinforcing a point previously made. (Although this has not particularly happened in this thread )
BrendanS
Well-known member
After years of posting and observing on ybw, the explanation is quite simple. Many people don't read all the posts in the thread before they respond. They probably read each post individually rather than open the thread up to read it whole - maybe they don't know how.
Others just don't read the question correctly (classic example was the request for advice on a laptop recently. It was stated in the text that a splashproof or secondhand laptop was not required, just an ordinary laptop. Immediately people responded with links to toughened laptops and secondhand laptops. People had seen the words without reading the context, and just waded in)
Others just don't read the question correctly (classic example was the request for advice on a laptop recently. It was stated in the text that a splashproof or secondhand laptop was not required, just an ordinary laptop. Immediately people responded with links to toughened laptops and secondhand laptops. People had seen the words without reading the context, and just waded in)
Vara
Active member
There is also the issue that was raised yesterday,of the posters who are so slow to compose a reply that they duplicate something with out realising.
Mea culpa
Mea culpa
chops_uk
New member
I solemnly promise you that it wasn't there when I posted!
[ QUOTE ]
1. (Head down awaiting hail of abuse from old seadogs with brass sextants ...
2. Many people don't read all the posts in the thread before they respond.
[/ QUOTE ]
Despite eyepatches, and my wooden leg knocking the desk, I thought I'd manage to offer an explanation - as I'd never wish to unintentially offend real sea dogs ;-)
An oft-occuring problem is that after opening a very active post, it takes some time to read it fully. A reply (typing it also taking time) may therefore easily be a few minutes after the last post, so, as happened here, we get an overlap with two people responding at the same time. this one was just a slightly more lovely coincidence than usual!
I can't be classified as an old sea dog - I'm under 30, but I do stand by my post - it's a very plesant job to do with a dinghy on a summers day in little wind, with calm water and a slack tide. caveat, caveat, caveat.
Actually, it's fun even if you don't get the job done properly.
[ QUOTE ]
1. (Head down awaiting hail of abuse from old seadogs with brass sextants ...
2. Many people don't read all the posts in the thread before they respond.
[/ QUOTE ]
Despite eyepatches, and my wooden leg knocking the desk, I thought I'd manage to offer an explanation - as I'd never wish to unintentially offend real sea dogs ;-)
An oft-occuring problem is that after opening a very active post, it takes some time to read it fully. A reply (typing it also taking time) may therefore easily be a few minutes after the last post, so, as happened here, we get an overlap with two people responding at the same time. this one was just a slightly more lovely coincidence than usual!
I can't be classified as an old sea dog - I'm under 30, but I do stand by my post - it's a very plesant job to do with a dinghy on a summers day in little wind, with calm water and a slack tide. caveat, caveat, caveat.
Actually, it's fun even if you don't get the job done properly.
J_Priedkalns
New member
Another way of checking your compass is to use a celestial navigation program with your computer to use the sun's shadow to check your compass deviation whilst sitting at your dock. The program results can be pre-printed beforehand for a particular day for each hour of the suns position and taking your local variation into consideration a deviation chart can be made. A lot of sailors consider celestial passe but it does have its uses.
