Originally Posted by cal40john
....... Most the boats I've sailed the deviation is less than my course wandering, not to mention the 0.5 to 10 knot
currents making a hash of the direction you're going.
I once swung the compass on my (glass) yacht and made up a deviation card.. max deviation was about 4 degrees. As you say that is neither here nor there in the scheme of things where compass cards are typically only marked to 'half points' ie about 5 degrees.
Back to the OP.... working on a paper chart.... do your work in 'true'. If you lay off a course on the chart read it off in true degrees , apply variation and -if you must - deviation. Taking a bearing? Convert it back to true before applying to chart.
Further down... a question re steel
boats. Adjusting a compass on a steel
yacht? A good compass for a steel boat would have Lord Kelvin's Balls and maybe even a Flinder's Bar... on a smaller yacht compass maybe not but even the Plastimo
135(?) has the means for adjustment through those little holes in the binnacle. You should be able to adjust the compass down into reasonable levels on all headings but a 'Deviation Card' would be a must.
A copy of Alan Pickles' 'All about Marine
Compasses' ( ISBN 0 7316 3280 X )is a handy reference work.
Re variation worldwide.... off the top of my head
... on passage
from Fremantle to Bass Strait the variation will go from about 20W to 15E on what would be about a 15 day passage