Surprise! Surprise!

gus

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During a conversation recently with some 'experienced' boaters ( one in fact has sailed across the pond and the other has done the Med and back). Both were adamant that a hand-held compass does not suffer from any deviation at any time.
Perhaps this is something that is overlooked on courses and assumed that reference to deviation in a fixed compass would also apply to hand-held compasses without specific refernce being made. There may not be much chance of experiencing deviation up on the top of your big plastic power boat, but a whole different ball game on a steel boat.
It certainly would seem that something is amiss when such 'experienced' boaters hold such a view. Are there any others who do?
 

Mirelle

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Sloppy teaching, sloppy methods....

This is the dierect result of people being taught the quick and dirty way of checking the steering compass by taking a bearing with the h/b compass and lining the boat up on that. The two people you spoke to have forgotten the bit about "standing in a position where you are sure that the h/b compass is not affected by deviation".

The scary bit is that they clearly have no understanding whatsoever of the principles of magnetism or how their compasses work!

Bet they were Yacht Masters....

Incidentally a fellow in a chandlers tried to sell me an electronic fluxgate compass repeater unit as the only compass in the boat a couple of years ago; should I. in the interests of safety at sea hve enlightened him?
 
G

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Re: Sloppy teaching, sloppy methods....

As a Yachtmaster Examiner of some 26 years standing as well as a YM Instructor of both shore based theory and practical courses I am hurt by your remarks that "you bet they are yachtmasters!"

I have great fun in proposing that to check the steering compass against a h/b it is necessary to launch an inflatable dinghy, pick an exhibitionist member of the crew to remove all clothing (so obviating anything magnetic in pockets, zips etc ) and then get them c/w h/b compass to align the centre of the transom with the mast, read the bearing obtained and at the same time signalling to somebody else to read the steering compass. I don't get many volunteers, but the point is made.

Looking forward to examining you, one day!!!
 

AndrewB

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Fluxgate compasses

I've been suprised by people (including dealers) who say that fluxgate compasses don't suffer from deviation and/or variation. There is possibly some confusion with gyro compasses.

Only slightly less dangerous is the belief that by running the yacht round in a circle every so often, the deviation in a fluxgate compass will be eliminated. This method of adjustment often doesn't seem to give particularly good results. All compasses need to be checked against fixed bearings from time to time.

Of course, all this doesn't matter now we have GPS, which as everyone knows is perfect.
 

cynthia

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Is this why?

Is this why 'the captain' was not impressed by the storage of the toaster at the side of the cupboard containing the compass fluxgate for the auto-pilot? I did wonder why he should have been so baffled by the 5 degree variation!
 

Mirelle

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Indeed it is; well spotted!

It is easy enough to store magnetic objects under a common or garden steering compass and it is incredibly easy to put stuff next to a fluxgate because one so easily forgets what it is!

I plead guilty to having absent mindedly stored a couple of paint tins under my binnacle compass; my sister who pointed out the error of my ways, is probably still dining out on the story!
 
G

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Re: Fluxgate compasses

All fixed fluxgate compasses that I have used have had the magnetic head mounted as far away from magnetic interference as possible. Something that is rarely possible with a conventional steering compass.

They have then been "swung" using the designed method - all involve tunring in slow circles (between 1 and 3 depending on make).

Subsequent checks have always shown that the deviation becomes _constant_ which can be removed by adjustment of the head and repeating the process.
 
G

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Another story from the archives

He shouted at us. We were only a scratch crew, but no matter what we did he still shouted at us. Five days into our passage two of us got really fed up with him being downstairs most of the time struggling in the dark with his solar powered calculator and his mammoth thing called a GPS that ate batteries as if they were Windward Islands banana ... We found out by perseverence that he had a secret piece of paper that inverted the binnacle reading .... We also found a very large lump of iron attached to to an even longer piece of string .... Imagine our surprise when we discovered that if this circular piece of iron was placed next the ships medium wave radio and Radio 3 disappeared ....

Now every Captain has complete faith in the steering compass especiallially when it has a deviation chart dated less than 2 months ago ... Now imagine a scratch crew somewhere off St Kilda heading NW and not being too happy with either the continual shouting, the food (if that was what it was called) and a few other things beside. And then imagine their surpise when they (we ... erggh some ..) found out that the circular piece of iron attached to the long piece of string was called 'The Bilge Magnet'

The rest is left to you to work out how by both prudent and dilegent use of the latter piece of equipment we made Kirkwall when unser grrpenfuehrer Kapitan thought that we should be entering Reykjavik ...

He was even less pleased when the entire crew jumped ship and headed home.

But as we all said, "GPS's? Don't believe them" and we made sure that the bilge magnet was positioned suuch that north read south and that south read north.

As far as we know it took him another week to discover that he'd accidently put the bilge magnet above his pit in the aft 'state-room'

But that's life under a latterday Kapitan Bligh.

No apologies - As I said, it's from the archives.





Birgitta
 
G

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Story 1

Out in the Clyde one day two of us looked to the h/b compass to find our way in to Milport. Two different readings with the same compass?? My friend has glasses with a magnetic personality!

Story 2

My Centaur has the flux gate compass head mounted under the king post, way down low.

On the mooring, we had been using a large magnet to "fish" for the grill pan that had been lost over the side. Task finished, the magnet has been left on the cabin floor and of course the compass went mad.

Cheers

John Dillon
 
G

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Compasses becoming obsolete!

You're right about re-calibration. Should be done every 3000 miles N or S as the planet isn't a sphere and the magnetic lines of whatchamacallits arn't constant.
However the latest method which equels if not exceeds that of a gyro in accuracy is using 3 GPS receivers and a pitch and roll sensor. I'm sure you can work out how????
 

charles_reed

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Re: Fluxgate compasses

The only trouble with GPS accuracy is that the charts we use don't provide the same levels of accuracy - especially when you start trying to use Spanish charts.

The Autohelm 4000ST on my boat suddenly developed a bad case of the shakes, and nearly rammed a couple of powerboats in the entrance to Pwllheli.

Problem turned out to be the steel ring-pull on an aluminium beer can, about 100mm from the fluxgate compass under the saloon table.
 
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bob_tyler

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Re: Fluxgate compasses

Fluxgate compasses aren't meant to get pissed! Tell it to behave itself
 

peterb

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Re: Sloppy teaching, sloppy methods....

I'm not sure it's sloppy teaching. I suspect that it is more likely to be misunderstanding (which may, of course, be due to sloppy teaching).

Most of us realise that you can't make out a deviation table for a hand bearing-compass, because the deviating field will depend on exactly where you stand when taking the bearing. So, of course, students are taught not to use the yacht's deviation table when taking bearings. From there it's quite a small step to thinking that hand bearing-compasses have no deviation. I suspect that most courses will warn of deviation, particularly on steel boats, but it's still surprisingly easy to forget the warning when you usually sail a fibreglass boat.

Incidentally, I've also experienced deviation from spectacles. I dismantled them, and tested the individual parts. The deviation turned out to come from the tiny screws at the side of the frame. But then, opticians often use magnetic screwdrivers.
 

Mirelle

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A deviant spectacle....

So have I; as you say, they are probably assembled with magnetic screwdrivers, to pick up the tiny screws.
 
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