Compass fitting.

Nos4r2 · 19 August 2006, 11:25

I've got a top mounted Ritchie compass here waiting to be fitted.I'm not sure how far it needs to be from the VHF-there isn't anywhere more than 30cm away from it that is feasable to fit it.

Suggestions please?

ian mcconnell · 19 August 2006, 13:15

Nos I use to be a flyer before a ribster, the compasses are calibrated to the individual plane via the 2 way adjustments which should be present on your compass, but you do need a second compass mounted away from any metallic object to calibrate from.

The easier option is switch all electrics on and hold the compass a couple of foot away from the vhf, as you move the compass closer you will see the deviation, place this just outside deviation range, but i suspect you will still be able to mount this reasonably close to the radio.

its about time i had some useful advise for you, its normally the other way around

Nos4r2 · 19 August 2006, 13:29

Cheers Ian!

It's only a baby compass though-one of these.

Would the adjustment be internal or would it even have any?

ian mcconnell · 19 August 2006, 14:06

Normally 2 obvious screws or poss to holes with posi adjusment for magnetic deviation.

perhaps it does not have it!! the vhf may pull it 15-20 degrees off depending on location to VHF. but blasting along at 20knts the compass normally moves about that back and forth anyway..

BogMonster · 19 August 2006, 14:35

Interesting - maybe this explains why the compass fitted to my boat is only slightly less useless than the speedometer

Does the VHF have an effect only when transmitting, or all the time (due to magnet in the speaker or whatever?)

IIRC radiation decreases as the cube of distance or something - there was a big discussion about it here recently when the first mobile phone masts were installed and one was next to a kiddies playground and all the mummies were up in arms about their children being microwaved

alystra · 19 August 2006, 14:45

My compass is surrounded by electronic things, see pic, and I was initially concerned about it. However, the compass is not deflected when any or all of the electronics are switched on. I suppose it might when I transmit on the VHF, but I hardly ever do that.

There's a little book I've found useful in the past called 'Compasses in small craft', by Cmndr C. A. Lund, ISBN 0 85174 237 8. I still need to use it to sort my compass, though it seems about right on most courses.

Cheers Tony

Nos4r2 · 19 August 2006, 15:13

aha! It DOES have adjustment! I thought the screw holes I could see were for dismantling the casing but then realised the casing clips together.You can see one of them on the picture.

Ok...

How do I adjust it? What screw does what?

There's the one in the pic and one 90 degrees anticlockwise to it on the casing.

wavelength · 19 August 2006, 17:35

Hi Nos
never had any need to adjust the compasses on our ribs. Vhf is a foot or so away and the mic is nearer but now they dont have a magnet and cone thing in them they dont seem to affect compass. I always plug new kit in, turn it on and wave it around the compass before fitting to see if there is any efect.
adjusting-just found instructions for last compass with the following advice- none of which we had to use!

Use chart to determine a north heading and steer boat on it of N doesnt line up use compensation rod on right hand side to adjust til N is right.
Now head to an east heading and if compensation is req then adjust using rod in front of compss until E lines up
Head South and if heading is not correctly shown adjust the side comp rod to take out half the error
Head west and use front comp rod to take out half of any error

Repeat the steps as above taking out half the error at each adjustment until req accuracy is achieved.
If you cant get it right you may need to move something or indeed the compass itself.
Use a non ferrous/non magnetic screwdiver to do the adjustments.

Steven your compass should work-consider if it is a compass weighted for the N Hemisphere. Cos the pull from the poles is downwards, as well as up as it were, the indicator of a compass may be weighted to compensate for the downward pull which will be on the other side where you are on the underneath bit of the world

.

Hightower · 19 August 2006, 17:42

I fitted mine right below my VHF and other gadgets. I took some time and trouble when first looking at fitting mine but found that none of the instruments seem to affect its accuracy. When I fitted a new steering wheel (supposedly Stainless Steel) when I rotated it my compass deviated by at least +/- 35 deg. Luck had it I had a De-Magnetizer at work. Anyway I don't think too many a skipper of RIB can reliably use their compass to any degree of accuracy anyway (not like a Yachtee), Being no more of a North, East, South, and West direction indicator so I recon that a +/- 15 deg accuracy is OK on a RIB.

What do other people think?

BogMonster · 19 August 2006, 18:23

Quote:

Originally Posted by wavelength

Steven your compass should work-consider if it is a compass weighted for the N Hemisphere. Cos the pull from the poles is downwards, as well as up as it were, the indicator of a compass may be weighted to compensate for the downward pull which will be on the other side where you are on the underneath bit of the world

.

So what you are saying Dave is that it will work when my boat is upside down?

It works, it just doesn't seem very accurate/consistent compared to the GPS heading, haven't really checked but probably out by 15-20 deg?

I don't think I will get lost anyway, offshore navigation is not on my list of goals to achieve, heading due east from Port William the next stop is Chile 13,000 miles away, and going the other way all we've got is the Argies and even that is about 300 miles, so I think I'll stay in sight of land for now

wavelength · 19 August 2006, 20:14

Quote:

So what you are saying Dave is that it will work when my boat is upside down?

Don't really know now I've read it again

Maybe that bottle of red has something to do with it now! But let me know if you try it

Dont know why there is all this problem with compasses - we run down the leading lights in and out every trip and they are bob on, and our stock courses we use for nav excercises all work fine with the compass. Being an old(ish) git, before even Decca was available for little boats we ran to our fishing marks on a compass and watch in all sorts of visibility. Mind you it was worth it then cos there was something left in the sea to catch

!

Nos4r2 · 19 August 2006, 20:20

Cheers Dave-those compensation instructions are most useful.

Do the speakers not have magnets in anymore on vhf then? New one on me!

Hightower · 19 August 2006, 23:07

Quote:

Originally Posted by Nos4r2

Do the speakers not have magnets in anymore on vhf then? New one on me!

Yes course they do. They are sheilded by another magnet that sits in the base of a can that in-tern is glued over the speaker coil magnet.

All clever stuff!

Nos4r2 · 19 August 2006, 23:47

I did wonder-I was trying to work out if it was possible for them to be using a pair of electromagnets working against each other instead of one fixed and one coil but I guess it'd be too costly.

ian mcconnell · 20 August 2006, 10:02

Nos everybodys getting very technical, BUT are you likely to do a channel crossing or navigate 20miles or more out to sea.

JUST TO CONFUSE THE ISSUE THERE ARE 3 HEADINGS FOR NORTH BEING TRUE / MAGNETIC AND GRID, SO WHEN YOU ARE FOLLOWING THE COMPASS WHICH IS MAGNETIC AND PLOTTING ON TO A CHART (WHICH IS GRID) YOU ARE ALWAYS GOING TO HAVE A DEVIATION, FOR CENTRAL SOUTH COAST THIS IS 3 DEG WEST, WHICH IS 300M'S AT 1 MILE.

So i guess if your not travelling at 50 knots plus over 20 miles or more out to sea with no visual references DONT WORRY. buy a GPS

Nos4r2 · 20 August 2006, 10:13

No, it's there for emergencies. I've got a GPS onboard but were it to decide not to work at the wrong time then I could be right in it.

I was a yottie for years so I guess thats why I was so worried about deviation. Never used one before on a RIB-with 20 deg variance while moving anyway it's good enough for finding NSEW.

wavelength · 20 August 2006, 21:31

Dont know where the grid came from?? Are you talking O.S. there
but variation here at the moment is around 4 degrees west.
Our leading lights here are 156 degrees (two big lighthouses in transit)- anything on the chart is in degrees true, it has to be cos magnetic varies-hence variation. Working on "variation west compass best and variation east compass least" that gives us 156 plus 4 = magnetic course of 160. And it is- bang on with all three compasses.
The only other set of numbers you could have would be "degrees compass" as opposed to just magnetic which you would get after taking into account the compass error (deviation) from a deviation card and I've never had need of one on a rib or any small boat for that matter. Just on yachtmaster practical and theory on bigger boats!

ian mcconnell · 21 August 2006, 08:19

thanks for correcting me wavelength. im afraid im still talking to flying terms i did not know sea charts did not read from grid.

this forum is designed to educate, i have been educated.

Blackpool has 4 degrees, southampton has approx 3 degrees variation.