COMPASSES

[alan54]

Last year I had fitted to my steel ketch a new Ritchie magnetic compass on the aluminium binnacle. Also a new Ratheon ST7000 auto pilot mounted in front of the Ritchie. A new GPS was located in the cabin. All have compass headings but vary >15 degrees !
In my one sail so far (coastal Med) I have interpolated the two headings between my Ritchie and autopilot readings. I am assuming that the GPS cabin reading is actually the most accurate due to its satellite link with the autopilot compass reading next due to its location up the mizzen.
My question is should I fit the external magnets to the Ritchie and attempt to reduce the difference between it and the autopilot reading or not really worry too much with the discrepency ?

regards,
alan jackson Lady S

 

[AndrewB]

It is quite normal for magnetic and fluxgate compasses to have a deviation of 15 degrees or more when sited on a steel boat. It is imperative that you have the main steering compass professionally corrected, which will generally involve fitting internal magnets. This is not a DIY task.

Most fluxgate compasses controlling autopilots can be corrected to within acceptable error for that purpose by a built-in process that requires you to circle the yacht at constant speed. Check the handbook. If you have done that, yes, it is probably more accurate than the main compass, but still not sufficiently accurate as a proper steering compass.

The GPS 'compass' will be quite accurate, but relates to the direction travelled over the ground. By contrast your steering compass tells you the direction in which the yacht is pointing, which is not necessarily the same thing.

Until you can get your steering compass adjusted, you should rely on the GPS alone. Interpolating headings is not sensible. All magnetic compasses will be disturbed by the yacht's magnetic field, and the 'average' is no more reliable than any one of them.

It sounds like you might benefit by some navigation training, an RYA Day Skipper course would be good. (See http:\\www.rya.org.uk).

 

[Guest]

OK ..... first we have to find out which one is most correct, or more accurately what is the error in each ..... HOW ?

I am sure that there are two fixed marks somewhere in your harbour / area .... such as leading marks or channel posts ?
Take the true line through these two marks on the chart and make note. Now you should line up the boat exactly on the line that passes through the marks and read the compasses.

Now you have a choice ..... continue on blind, but only knowing on that one heading the apparent errors in that area ! OR .... you can do a full swing of the compass and arrive at your deviation.

OK from the chart you have the variation.
Now steer a series of courses past the leading marks and when they are on line, read the bearings from a compass that also you can use for steering ..... this way you build up a series of bearings giving the error against headings. Now compare the bearings against true from the chart and that is the accumulation of variation and deviation. Remove variation from this error to arrive at Deviation only by the old rule :

Compass Best Error West
Compass Least Error East

Plot on a piece of graph paper and draw the probable sine-wave type curve through the points to arrive at the Deviation curve.

If you change position / make significant repairs / add some items around the compass etc. you will alter the magnetic influence of the boat on the compasses and then you'll have to swing again.

Once you have the data for your steering compass - comparison of all others will give you their errors.

Hope it all works out !!!!!! Its easier than it reads !!!!

Nigel L

 

[trays]

What about hand held fluxgate compasses. Should I hold it in front of me and spin?

More seriously, is there a difference? Can one rely on a hand-held fluxgate compass?

Ray

Ray

 

[trev]

Fine in theory, but how do you take accurate bearings with a steering compass sited forward of the wheel ??
How do you take any accurate bearings with a steering compass anyway?

Trev

 

[trev]

Sorry Nigel - I didn't read it correctly and see now that you advocate the use of a 'third party' compass for the bearings, and compare this to the steering compass.
This should work, but an easier way is to take a bearing of a distant object as the vessel swings through 360 degs ie a bearing on each main heading point (N, NE,E,SE etc), then add up all the bearings and divide by 8. This will give you the magnetic bearing of the object (the deviations will have cancelled each other out on the swing). Another swing will allow you to plot the sine curve of deviation by comparing the compass bearings to the magnetic, and maybe allow you to reduce it on some headings by adjustment.

Trev

 

[Guest]

Sorry but I cannot agree universally there .... having been on craft that deviation has not been 'self-cancelling' around the 360. It assumes that deviation is 'balanced' and in fact is often not the case.

Another factor that can be used is the Pelorus ..... simple idea of steer the boat and read 'relative' bearing by Pelorus, making sure that helmsman shouts out heading when you call 'Mark'.

OK some are now going to say ..... Pelorus ?????? Take a large 360 deg. marked card and a pointer pinned in the centre, free to rotate .... now fix it on the caochroof where you can sight via the pointer and also have clear view of marks to take bearings from. Make sure that 0 / North is dead ahead parellel to the fore-aft line of the boat.
This was a piece of standard clutter on older Magnetic Compass vessels to allow bearings to be taken away from the main steering compass, but always within communicable distance from helm .....

Sorry but really there is no way around a proper swing and best is to use transits .......

Nigel L

 

[trev]

Nigel
A boat that will not ballance out the deviation in a full swing is suffering from a thing known as coeficient'A' which is impossible to correct, mainly due to the compass being sited off the centre line, producing a lobsided effect.
Most boats will produce an imperfect 'sine curve', more so when 'A' is present - aircraft carriers are the worst !



Trev

 

[Guest]

A & B Coefficients, vertical and horizontal components are NOT what the guy is worried about ..... he is wanting to use his compass and be reasonably sure of what he is reading. I didn't like Magnetic Compass theory when I was in Marine College and I still don't like it now !!!!!

No offense meant ...... just trying to be practical for NAC !

 

[ean_p]

why are you so concerned as to which way your boat points.......what really matters is which way is she is heading.......go GPS.....have fun.....

 

[AndrewB]

There are some perfectly good handheld fluxgate compasses. The Autohelm one is widely used.

However, like a conventional handheld compass, it is pretty well useless on a steel yacht as it is equally affected by varying deviation caused by the yacht's magnetic field.

 

[Guest]

Before I make the next comment I apologise for my answer ....

The comment - about .... why worry about which way the boat points ... use GPS - for .....

That shows a lack of thought and is not a reccomendation I hope many will take up. I hope that you never get caught out in : fog / mist / poor visibility and electrics / GPS failed .... 'cause yer silly little magnetic compass will still be showing boats heading ... but you'll be rummaging about for AA cells to get yer GPS going again - if you are lucky !!