Compass error

Compass Error.

We also swung the compass this afternoon. That's to check if the ship's compass might be out by a few degrees due to any metal influences onboard the boat.
Whatever degrees it might be out by, is referred to as compass deviation. Then you also have the difference between true compass and magnetic compass. That called compass variation.

You have to take both variation and deviation into account when calling for a compass course to be steered. The two together is called COMPASS ERROR.

It takes a while for the concepts to sink in, but I still recall Johnny Walker drilling it into our heads way back in 1978 at the old Port Elizabeth Technicon where he presented his navigation courses.
"VARIATION WEST, COMPASS BEST, etc" "VARIATION EAST, COMPASS LEAST". To this day, I draw a little diagram on all my charts. On the stormiest of nights, a glance at the diagram will ensure that all is well at the navigation table.

I was confident that the Banjo would have very little deviation, if any at all. I had carefully planned the positioning of the radio and instruments and all the cabling during the building process to ensure we are free of compass problems.
The abscence of an inboard motor and alternators etc. also helps. Many years ago I came to within meters of putting a boat on the rocks in thick fog off Hout Bay due to and inboard motor and alternator throwing out a compass.

There are three or four different ways to swing the compass. I placed a handbearing compass on the cabin top, on the centreline of the boat. Thats about as far away from anything as you will get. Then motored on the various headings and got Jamie to call the readings out so I could compare it with the ship's compass.

We had a big problem. The compass was out by 180 degrees!
"No Jamie, read the numbers on the inside circle, not the numbers on the outside circle!"
That solved the 180 problem.

Then we were out by 20 degrees!
"No Jamie, the little lines with no numbers next to the 90 means 100 degrees, not 80! Gee my boy, which way do you count at school!"

Then got that one waxed.

The compass is spot-on. Shoo! Anyway, at least so close that no one can steer on any of the deviation that might be there.

The course to St Helena is 310 my co-skipper tells me. Magnetic, or True? Ah dunno! He never told me.

I made a special effort to check 310 degrees.
It's fine.

(the picture shows us doing our thing in front of the house. That's Jamie on the cabin top, reading out the handbearing compass numbers for me.)