Compass, service and swing

[ponapay]

I have recently had a new engine fitted to my boat and have also replaced a gas heater (mostly iron within 24 inches of the compass) and other items.

I tested the compass and found errors of up to 15 dgreees, clearly unacceptable.

Is it possible, in calm waters, to get a reasonably accurate swing using GPS headings. I tried this and obtained a reasonable sine curve of errors but am reluctant to rely on it entirely.

The boat is based abroad and I have been told that an adjuster always attends for the second week of May, regretably I cannot be there nor will the boat be in the water at that time.

I will only have coastal cruises (apart from one 185 mile passage) throughout this year. On the long passage I could easily use the GPS tracking system and my chart plotter to show the progress on track. I do have 2 GPS systems.

I know this is not correct but has anyone any ideas? Even as to the correction by me of the errors.

Ponapay

 

[Chris_Stannard]

Follow the following steps;

You will need a hand bearing compass, a small screw driver, preferably non magnetic, a willing assistant and a pad and pencil.
First make sure that, if the comapss was removed during the engine change, your lubbers line is correctly aligned to the ship's head. You will also need a fairly calm day.
Standing by the backstay you should find that your handbearing compass reads correctly. to make sure the handbearing compass is accurate check it on a transit.

Step 1. Steer North by the hand bearing compass, note the reading of the main compass. Repeat steering South. You should find both readings have any error in the same direction, either to the east or to the west. If they are not for the same amount the correction will be half way between. For example Steering North the compass reads 5 degrees West of North, steering South it reads 9 degrees West of South. The correction will be 7 degrees East.
Step 2. Steer North and correct the reading using the athwartships magnet. If you have not got a non magnetic screw driver, settle on North and then note the variation when you put the screw driver into the compass.
If you have had to make a large correction repeat steps 1 and 2
Step 3. Repeat Step 1 and 2 steering East and West. This time use the fore and aft corrector to take out the error.
Step 4. Check North South again to make sure the new correction has not upset what you have done already, it should not have. If it does repeat until you get the best answer.
Step 5. Swing the compass going right round reading every 10 degrees preferred (30 if you are running out of time). See what your deviation chart looks like now.
If it is reasonable, check the result against either a known transit or the sun at sunset. Remember true sunset is when the sun is a semi diameter above the horizon. Using the sun, if you can, is better than a transit. As the sun is so far away, any tidal effects are irrelevant.
A problem I have had in the past is that if the compass has been fitted for years the magnets may have corroded. If this is the case no matter what you do it will not work, you need a new compass.
This process is as set out in the Suunto compass manual, and I have used it in the past with success. If in doubt however take the first opportunity to get it done properly.
Good Luck

Chris Stannard

 

[tony_brighton]

if you have an autohelm with a fluxgate compass you could take a reading from that.

 

[jfkal]

Does not have to be necessarily accurate either. Happened with mine

 

[Guest]

Look at www.ritchienavigation.com/compensa1.htm

This gives a couple of methods of correction using transits or GPS

Rgds
Stan

 

[ponapay]

Many thanks for the useful hints and suggestions. I am looking forward to trying them out once we are back in he water.

ponapay

 

[andy_wilson]

Chris Stannard has the answer.

You should compensate to bring the error to within 5 degrees, then draw up a deviation curve.