The trouble is deviation. Boy Scouts don’t have deviation but boats do. Boats have engines, electronic instruments, and lumps of metal that attract the magnetic compass and cause it to deviate from magnetic north.
That wouldn’t be so bad if the amount of deviation were consistent, but it never is. Deviation changes with the heading of the boat. That makes the correction of deviation complicated. It involves swinging the compass and drawing up a deviation table for every course you could possibly sail.
Luckily, the maximum amount of deviation is usually small, say 5 degrees or so. That fact, combined with the bother of correcting it, makes most amateur sailors ignore deviation and set it to one side for attention later, as one does with a smelly old uncle at a family reunion.
However, if you’re on a long trip, you must apply the correction for deviation because, even if you’re out by only 5 degrees, you’ll be a whole mile off course for every 11.5 miles run.
Today’s ThoughtThough pleased to see the dolphins play,
I mind my compass and my way.
— Matthew Green, The Spleen
Tailpiece“Come on, Johnny, be a good boy now. Say ‘Aaah’ for the nice doctor. He wants to get his finger out of your mouth.”
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)