Compass advice please

[ChrisKaye]

Just received my new Plastimo offshore flushmount 105 ordered from the US where it was LESS than half the cost it is here (how can they do that ?? and it says its made in France on the box, so its travelled from France to the US and then to the UK !) to replace my knackered Ritchie compass.

Now the screw holes to fix to the helm are different but otherwise the compass is a great fit, so I plan to silicon the thing down dead centre to where the old one was.

Do I need to adjust the compass in any way i.e. deviation / compensation ?? or have it "swung" or anything like that before I get the silicon out and stick it down for good

 

[sarabande]

US and UK are both N of the magnetic equtor, so the angle of dip (0.5 N ^) will be the same (Zone 1). No need to adjust.

Deviation nd Variation is down to your maths.

 

[ChrisKaye]

"Deviation nd Variation is down to your maths"

Oh dear, I was never any good at sums

 

[sarabande]

well known fact that Sunseekers never have any deviation so only about 1 deg west variation at present.

Ignore all maths. You will only be one mile out after crossing the Channel. Near enough for moboing and 300 hp

 

[ChrisKaye]

"well known fact that Sunseekers never have any deviation"

Easy !

 

[nonitoo]

If fitting a new compass it is a good idea to get some sort of idea of the deviation (you have no idea how the compensating magnets are set).

An easy first go is on a set of leading lights, steam up them inward and note the heading, do the same thing outward. From the chart you will have the true heading on the leading lights, apply variation and, hey presto, you have two deviations. You can do this with any sort of transits but it is best if they are a decent distance apart.

With a smallish compass you will only be able to adjust for coefficients B (fore and aft magnetism) and C (thwartships magnetism).

For coefficients D (symmetric induction) & E (asymmetric induction) you need quadrantal compensators (balls to thee and me), usually only found on larger units.

Tom

(ps Coefficient A is installation error)

 

[ChrisKaye]

"With a smallish compass you will only be able to adjust for coefficients B (fore and aft magnetism) and C (thwartships magnetism).

For coefficients D (symmetric induction) & E (asymmetric induction) you need quadrantal compensators (balls to thee and me), usually only found on larger units"

Now you've made my brain all spudgey again !

 

[timbartlett]

Do I need to adjust the compass in any way i.e. deviation / compensation ?? or have it "swung" or anything like that before I get the silicon out and stick it down for good

You'd be wasting your time and/or money to have it swung before it is fixed down.

Just make sure that the lubber lines (the ones that mark the fore-and-aft line of the compass) are as perfectly lined up with the centreline of the boat as you can get them.
Then you should either swing it (or have it swung) for deviation.

Bear in mind that the deviation on most motor boats (even Sunseekers) is HUGE (can be several tens of degrees). But the better boatbuilders have their compasses professionally adjusted before they deliver them to customers. You can either do the same or (with some compasses) follow the DIY instructions that come in the box.

Swinging and adjusting can do nothing about Variation.

 

[hlb]

A compass is fairly good at getting you to america ish. Or to ireland. Though you might be up or down the coast a bit. Where as GPS has you there to the inch.

I'm not sugesting that folk dont learn basic navigation. Only that it's a bit iffy.

 

[aquapower]

depending on model a lot of compasses have to be removed from the dash to be adjusted, so dont permanent fix untill you are happy it is set ok, it is essential to have a new compass swung and get a deviation card as its acuracy out of the box will depend on what is close to it on the boat, any thing electronic or metal (ie chart plotter, wipers are usually close) can affect the accuracy.

 

[MapisM]

Just make sure that the lubber lines (the ones that mark the fore-and-aft line of the compass) are as perfectly lined up with the centreline of the boat

Do you mean lined up with the view from the helm position, and parallel to the boat centreline, surely?

 

[ChrisKaye]

Thanks everyone,
Just thought that seeing as it was for the N American market it might need adjustment to be used here.
The plan is to fix it same as previous ensuring the lubber line is same as well
Not reliant solely on compass but just wanted to make it as accurate as poss
Thanks again all

 

[nonitoo]

Now you've made my brain all spudgey again !

Sorry, I'll put my anorak away and put fresh elastoplast on my specs !!

I have managed to get my boats's deviation to a max of 5E-5W. Cannot steer to anything like that accuracy anyway in my boat so not a problem.

Tom

 

[timbartlett]

Do you mean lined up with the view from the helm position,

Not necessarily

and parallel to the boat centreline, surely?

Yes