Compasses 20 degrees out

[dgadee]

YOur GPS will NOT give you a heading only a COG as it is not a compass

Some gps certainly give both cog and compass, so saying it will not do that is incorrect - my little handheld one does. You may be thinking of a chart plotter but I never said I was using one. One of the devices I was using was my android phone which certainly has a compass on it. One app I have is BC Racer which will give both cog and bearing on the same screen.

 

[Bilgediver]

Aged navigator? Heineken? That'd never work. Aged navigators run on higher octane fuel.

At least from Granton on Spey if not higher up in the Highlands

 

[Lady Campanula]

Some gps certainly give both cog and compass, so saying it will not do that is incorrect - my little handheld one does. You may be thinking of a chart plotter but I never said I was using one. One of the devices I was using was my android phone which certainly has a compass on it. One app I have is BC Racer which will give both cog and bearing on the same screen.

There are quite enough 'experts' here to argue the toss from positions of deep acquired knowledge and long experience.

I'm tempted to suggest that the OP just glue a couple of mobile phones to the bulkhead - one can get them in red and green. After all, a compass which senses the orientation of the earth's local magnetic field is 'so last century', and there are Apps for just about everything these days. One can understand the OP's nod towards traditional ways of navigating, but having two of them thar compass thingys in the same boat is surely too much of a good thing!

 

[Twister_Ken]

The proper way to swing a compass is with transits and known bearings lines like leading lights.

Exactly - except you can now make your very own bearing line by entering a conspic object as a GPS waypoint, to get a bearing to it. Then point the boat at the object. Beware ºC and ºT, obviously, and apply local ºV.

 

[fireball]

YOur GPS will NOT give you a heading only a COG as it is not a compass

Ah - but some GPS's will give you heading because they DO have a compass ...
Also - if you are motoring directly into or with the current - or there is no current then the Heading will more or less equal COG (allowing for magnetic variation) - and almost certainly not be out to the tune of 20°.

To the OP - take the compasses off - take them away from electrical influences and take a sight off a known point - you should be able to compare that to the true bearing off a chart/map - but do be aware of the magnetic variation in your area.

 

[dgadee]

OK. I am the OP. I will - when I have the time - take the compasses off and check them. I have also ordered compensator boxes, so will have a go with them and see what happens.

My original post was really saying, "Should I bother doing anything", since with digital methods of discovering course required, you just need to get the heading required from the gps and then decide what the relevant course on the compass to steer is.

I learned to sail with two compasses way back at the beginning of the 1980s and wouldn't really like to be without them - squinting over to the other side of the boat has no appeal. But that doesn't mean they have to be as accurate as they did 30 years ago (when mobile phones weighed 2Kg).

 

[VicS]

As above

but the first thing to do is to check that the compensation screws are set in their neutral positions.

I would work carefully and methodically through the instructions in the Plastimo manual http://www.plastimo.com/pdf/compass_inst(uk).pdf using your hand bearing compass to prepare initial deviation cards for each compass.

Then follow the instructions for adjusting the compensating screws and finally prepare two new deviation cards.

As said bearings on fixed marks or transits are the most accurate references but you also need a pelorus to determine the various other headings from them... You probably don't have a pelorus!


Keep us informed. The results will be very interesting

I have a funny feeling you are going to find something odd before all this is completed but it may all go to plan.

 

[Deleted member 36384]

...... take the compasses off and check them. I have also ordered compensator boxes, so will have a go with them and see what happens. ...... My original post was really saying, "Should I bother doing anything" .....

I would not take the Compasses off to check them in the first instance. I would make a simple Pelorus and rotate the yacht through 20 degree divisions against a far object. Rotate on the Pelorus angular divisions and then note the heading on one compass. Repeat for the other.

I have made a Pelorus as follows: -

1. Print off a full protractor or use a timing disc for setting up car / bike timing (available from Auto shops).
2. Fix the Pelorus to a board.
3. Make a batten out of Perspex (or a plastic school rule)
4. Drill a hole in the centre of the batten.
5. Attache to panel pins (or glued on matches) along the centre line of the batten. Scribe a line through the hole and between the panel pins.
6. Attache the batten to the centre of the Pelorus on the board such that it can pivot.
7. Tape it to the centre line of your yacht.

If you get a sinusoidal curve the compass is just out due to some boat effect. Anything else needs further investigation e.g. two Friday Compass's with the same manufacturing defect.

Is it worth knowing deviation? In my opinion, yes otherwise why bother with the compass. A contingency is only worth having if it is fully working and understood. Its not necessary to remove the deviation but it is better.

It may be that GPS with backup has a very low probability of failure and you may choose to accept that. If that is you belief, dont have a steering compass. If you think you may need them one day, for whatever reason, then know the deviation. It could be the difference between getting back to a man overboard or not, being on the rocks or clearing them.

Perhaps you can think of the compass like a fuse. No use until it blows and stops the boat from burning down; no use either with a jump wire to bridge a blown fuse!

 

[Wunja]

I had a 10 degree error on an old Plastimo compass. The web site said that compensation was not possible, so I ordered a new compass.
On removing the old compass a compensator box fell out!
I then realised that swapping the old cast iron Ovlov for a Beta of around half the weight may have accounted for the error.

Doh!​

 

[LittleSister]

Surely, if both compassess are out by the same number of degrees, whether its 2, 20, or 200 degrees, the chances of the problem being the compasses are very slim? Not quite as slim, but also unlikely, is having 20 degrees deviation (from magnetic forces on the boat) and it being the same in both compass positions.

My money is on the 'heading' reading the OP is comparing it to being wrong somehow. The boat is going to have to be taken out and the compass readings compared to transits, known (charted) bearings of objects, or to a hand bearing compass read as far as possible from any magnetic influence. Buying two compensating magnets in advance could save doing this twice, or might just be a waste of money.

 

[grumpy_o_g]

Maybe I'm missing something but surely if the compasses are just pointing 20 to the left then you haven't got the compass lubber line pointing directly forwards? If the compass is 20 degree out on every heading you simply physically rotate the compass in it's mounting?

 

[nimbusgb]

Maybe I'm missing something but surely if the compasses are just pointing 20 to the left then you haven't got the compass lubber line pointing directly forwards? If the compass is 20 degree out on every heading you simply physically rotate the compass in it's mounting?

Good point Although the OP probably wants the lubber line to be fore and aft.

 

[VicS]

Maybe I'm missing something but surely if the compasses are just pointing 20 to the left then you haven't got the compass lubber line pointing directly forwards? If the compass is 20 degree out on every heading you simply physically rotate the compass in it's mounting?

Can you do this with a Contest 101 ?

I asked in post #17 if the error was the same on all headings.
An error in fitting or even the unlikelyhood of both of the compasses being defective would result in a constant error.
A magnetic influence from within the boat will cause an error ( deviation) that varies with the heading.

 

[Deleted member 36384]

Maybe I'm missing something .... just pointing 20 to the left then you haven't got the compass lubber line pointing directly forwards? .....

Spectacles perhaps, a 20 degree offset on a lubber line would be very noticeable.

Is the bulkhead at an angle less than 90 degrees to the fore and aft line? That could cause the offset. If so you need a tapered mount to make the compass lubber line fore and aft.

 

[dgadee]

Bulkhead is not angled at all. The 101 has no moveable elements.

As to various headings, haven't been out on the boat for the past couple of weeks to test. Off abroad with work for next week. Will surely look further into this and report back.