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Old 23-03-2015, 05:14   #1
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Cost of Compass Swinging

One recent discussion on here reminded me that I haven't swung my compass in more than 5 years. I haven't notice any inaccuracy, but I thought maybe it's time to do it again.

I was shocked to get a bid of 270 pounds (more than $400). Wow! What have others paid for this service recently?

Since I do have a deviation chart, maybe I'll just try to do it myself just to run a check, rather than laying out this kind of cash. Does anyone here swing his own compass?
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Old 23-03-2015, 05:29   #2
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pirate Re: Cost of Compass Swinging

Quote:
Originally Posted by Dockhead View Post
One recent discussion on here reminded me that I haven't swung my compass in more than 5 years. I haven't notice any inaccuracy, but I thought maybe it's time to do it again.

I was shocked to get a bid of 270 pounds (more than $400). Wow! What have others paid for this service recently?

Since I do have a deviation chart, maybe I'll just try to do it myself just to run a check, rather than laying out this kind of cash. Does anyone here swing his own compass?
Need a bouy situated with fixed bearings to at least 3 landmarks..
A friend in a dinghy and slack water.. with a quality hand bearing compass..
Tie up at the bouy, then tie the dinghy to your stern and using its engine work your way through the 4 Cardinals comparing as you go noting differences between Ships and Dinghy compass.. then do the Semi-Cardinals..
switch off dinghy engine and both head below and crack a coupla tubes and settle down for a long and beerfull discourse as you try and calculate the end result after + & - ing magnetic variation etc to your results.. and the differences off the fixed bearings..
Or.. be boring and use a clacultor...
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Old 23-03-2015, 05:53   #3
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Re: Cost of Compass Swinging

Yep, I do.

There a couple of ways but the way Boatie described is a good as any if you have suitable conitions in your locale.

Doing it in narrow river with a decent current and only using a couple of available transits can drive one mental - at least me it does.

You will really only be checking what you already have so you shouldn't have any discernable A error.

If you do have to make any adjustments and you have compensator screws, I would normally halve the NS error and remove that first and then halve the EW error and remove that.

Recheck using the same method as Boatie suggested and again halve any NS error and adjust and then do the EW error.

Keep going until you can't get better or until you are too drunk to care.

Assuming the former, than do a final swing every 30 or 45 degrees and draw up the deviation card from final results.

Of course using cash is easier!
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Old 23-03-2015, 06:04   #4
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Re: Cost of Compass Swinging

I was thinking about calibrating my compass not that long ago and the thought occurred that I could use my chartplotter, although a chart and gps should work, too. Something along the lines of plotting bearings to a landmark and then running the course. I can see maybe drift and set being a problem. Anyone tried this?

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Old 23-03-2015, 06:08   #5
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Re: Cost of Compass Swinging

My compass is electronic. I turn in a couple of circles and it sets itself.

Man- swinging compasses is a pain in the patouche and so time consuming. the one I set last I did comparing to the GPS.. seems ok
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Old 23-03-2015, 06:28   #6
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Re: Cost of Compass Swinging

I swing many a year, but in aircraft. Several ways to do it, including a "master compass", I normally use a "compass rose". Should be able to duplicate a compass rose by getting azimuths off a chart from an anchorage and then of course point the boat to that location and read the compass.
But if in slack water your gps will suffice, just of course know your deviation.
But unless you have changed something as in adding a magnetic field close to the compass, it should not have changed, you should be doing just a validation.
Easiest is the GPS / slack water way, and I think will tell you if you need to do anything else.
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Old 23-03-2015, 06:29   #7
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Re: Cost of Compass Swinging

Quote:
Originally Posted by Reefmagnet View Post
I was thinking about calibrating my compass not that long ago and the thought occurred that I could use my chartplotter, although a chart and gps should work, too. Something along the lines of plotting bearings to a landmark and then running the course. I can see maybe drift and set being a problem. Anyone tried this?

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This will work if you have no drift. Just check the ships head is pointing exactly to the landmark and while maintaing that, check to see if your COG remains poniting to the same landmark. If so, you don't have any drift or leeway. Compass heading should equal ships track (magnetic).

A good way to check but a bit harder to adjust as it is way way easier to adjust on the NS & EW bearing and land marks aren't always so aligned. In fact I'm not sure if I could adjust on non cardinal bearings any more - if I ever could
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Old 23-03-2015, 09:12   #8
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Re: Cost of Compass Swinging

I like the concept of using the fluxgate compass to calibrate the pedestal compass, but my big question is...where does one find a non-magnetic screwdriver to adjust the compensators in the Ritchie?
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Old 23-03-2015, 09:17   #9
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Re: Cost of Compass Swinging

I use my GPS to swing the compass. There were some naysayers when I first proposed the idea years ago but time has shown it works.
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Old 23-03-2015, 09:20   #10
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Re: Cost of Compass Swinging

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Originally Posted by George DuBose View Post
I like the concept of using the fluxgate compass to calibrate the pedestal compass, but my big question is...where does one find a non-magnetic screwdriver to adjust the compensators in the Ritchie?
Take apart a cheap ball point pen, the tube that contains the ink is made from brass, bite the end of it and you have a non magnetic screwdriver. Or if you use a steel screwdriver you just have to let the compass settle down after you make an adjustment, just takes longer is all.
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Old 23-03-2015, 09:21   #11
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Re: Cost of Compass Swinging

For the classicists among us, the method that can be used anywhere you have some open water to work with will need nautical almanac, sight reduction tables, and a pelorus, to register the exact bearing to the sun in those headings where your compass may not work because of obstructions (cabin, dodger, etc.) This is especially useful, and perhaps the only way to do it if you're offshore, with no fixed reference points.
You'll want to get out early in the morning, or late in the afternoon, when the azimuth to the sun is changing very slowly. Using your almanac, and reduction tables, find the azimuths of the sun over twenty minutes to a half hour. Using your compass and pelorus, swing thru the cardinal directions, finding the difference between what your calculations tell you the azimuth to the sun should be at that time, and what your compass tells you it is. Do your addition/subtraction corrections for variation, and the remaining difference is your deviation. As suggested earlier, take out half the error using the compensation magnets built into the compass, or if you're really old fashioned, mounting small magnets around the compass to take out half the error, first on the N - S, then on the E - W. Repeat.
(OR
Compare to the GPS heading, using the same 'remove half the error' technique already described.)
File this under Arts of the Ancient Mariner
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Old 23-03-2015, 09:26   #12
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Re: Cost of Compass Swinging

Quote:
Originally Posted by capnjack.morton View Post
For the classicists among us, the method that can be used anywhere you have some open water to work with will need nautical almanac, sight reduction tables, and a pelorus, to register the exact bearing to the sun in those headings where your compass may not work because of obstructions (cabin, dodger, etc.) This is especially useful, and perhaps the only way to do it if you're offshore, with no fixed reference points.
You'll want to get out early in the morning, or late in the afternoon, when the azimuth to the sun is changing very slowly. Using your almanac, and reduction tables, find the azimuths of the sun over twenty minutes to a half hour. Using your compass and pelorus, swing thru the cardinal directions, finding the difference between what your calculations tell you the azimuth to the sun should be at that time, and what your compass tells you it is. Do your addition/subtraction corrections for variation, and the remaining difference is your deviation. As suggested earlier, take out half the error using the compensation magnets built into the compass, or if you're really old fashioned, mounting small magnets around the compass to take out half the error, first on the N - S, then on the E - W. Repeat.
(OR
Compare to the GPS heading, using the same 'remove half the error' technique already described.)
File this under Arts of the Ancient Mariner
F***k. Now $400 starts to sound not so bad


More seriously -- another good argument for a GPS compass, which gives you absolute heading with sub-degree precision. No problem swinging your compass if you have one of those on board, and they are not much more expensive than a pelorus, and vastly more useful.
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Old 23-03-2015, 09:56   #13
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Re: Cost of Compass Swinging

Quote:
Originally Posted by Dockhead View Post
One recent discussion on here reminded me that I haven't swung my compass in more than 5 years. I haven't notice any inaccuracy, but I thought maybe it's time to do it again.

I was shocked to get a bid of 270 pounds (more than $400). Wow! What have others paid for this service recently?

Since I do have a deviation chart, maybe I'll just try to do it myself just to run a check, rather than laying out this kind of cash. Does anyone here swing his own compass?
I had a professional compass swing many many years ago when a group of us at the YC got together to share the guy's travel costs and therefore reduce the fees. That man was Hamble based and my previously very usable ( seemingly deviation free) compass was way off on some headings after the swing, so much so I had him back to redo it whereupon I was accused of moving all sorts of metal stuff around on the boat which I hadn't. I was later advised never to have this guy do it 'after lunch' as he liked a tipple or three. I then found a day later when going into Cherbourg on the leading lights it was still way off. I resolved thereafter to do it myself.

I bought a plastic pelorus made IIRC by Davies and put an engraved line filled with black ink line on the pointer of this to act as a shadow pin. I mounted the pelorus ( accuracy fore/aft is essential) on the mainhatch garage. I then took bearings of the sun via the shadow line every time an opportunity arose that was at different ships headings, preferably in a fixed location like tied alongside ( but beware, well away from metal pilings etc) It took a week or two, waiting for opportunities, and sunny days, but I gradually got readings taken on enough headings around the compass to construct a deviation card and create a spreadsheet to give actual ( corrected) CTS on each heading for both magnetic and the then fluxgate compasses. I used time from my GPS and a Casio programmable calculator I had loaded with a celestial almanac program (a PBO magazine series back then gave the program details to put in the casio,with valid almanac data until around 2020 IIRC) This program would give the sun's actual bearings at specific times and dates to compare with those that I obtained on the pelorus. I was very happy with my own DIY swing and could check it fairly quickly every season if needed.

ON another boat however and needing a swing soon after it's purchase, I did a very fast swing by motoring in a tide free area on different headings and comparing GPS COG with compass heading. I did one 'swing' in a well sheltered bay around slack water and another check one in the inner harbour of Cherbourg (outside the marina between it and the inner breakwater, (nowadays too close to the French Navy base and they would most likely chase you away) I later did another confirmation check upriver of the Arzal dam in S Brittany between it and La Roche Bernard on the Vilaine. I did a similar check to calibrate log speed versus GPS SOG

Am I right in thinking there are tide free places in the Baltic you could take advantage of whilsyou are there?

MY DIY swings did show up significant errors on some headings, I remember on 'Easterly headings on one boat it could be as much as 5 degs out, not much over a mile but significant on a very long leg, like 5mls in every 60 mls of easting. I never adjusted the compasses with their inbuilt magnets like the original typsy adjuster did, but preferred to keep the compass in it's default neutrally adjusted state and use my prepared deviation cards for corrections.

BTW The fluxgate compasses previously swung by the circling method and auto correct instructions in the manuals always still had residual errors and I therefore had corrected headings for the autopilots to use printed out in a spreadsheet for those too.

Long winded but I hope understandable!

PS I hope I"M not slandering a currently working Hamble adjuster,but I tell it like it happened. albeit 25 years ago maybe now.
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Old 23-03-2015, 10:14   #14
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pirate Re: Cost of Compass Swinging

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Originally Posted by Dockhead View Post
F***k. Now $400 starts to sound not so bad


More seriously -- another good argument for a GPS compass, which gives you absolute heading with sub-degree precision. No problem swinging your compass if you have one of those on board, and they are not much more expensive than a pelorus, and vastly more useful.
It is handy to know tho'.. having been in situations where all elec's are stuffed.. knowing how off my steering compass was helped immensely in my DR across a few hundred miles of ocean..
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Old 23-03-2015, 10:24   #15
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Re: Cost of Compass Swinging

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Originally Posted by Dockhead View Post
F***k. Now $400 starts to sound not so bad


More seriously -- another good argument for a GPS compass, which gives you absolute heading with sub-degree precision. No problem swinging your compass if you have one of those on board, and they are not much more expensive than a pelorus, and vastly more useful.

my plastic Pelorus cost about $25 IIRC but you could easily make one from a compass rose cut from an old chart and pasted on a bit of board and with a sight pointer made out of some strips of clear perspex or similar. I made one but then found the commercial Davies one in a West store

Too late for this one on ebay Davis Compass Corrector Pelorus with Manual from Davis Instruments | eBay
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