December 3, 2013

Know where your compass points

 BOY SCOUTS WILL TELL YOU that your compass points to magnetic north. When you start sailing a boat, you find out they’re lying. Boat compasses don’t point to magnetic north. They don’t point to true north, either. In fact, there’s hardly any telling where a boat compass might be pointing.

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 Thought
Though pleased to see the dolphins play,
I mind my compass and my way.
— Matthew Green, The Spleen

“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.)


BK said...

As an amateur sailor, I too have set aside this concern. But it still nags at me. Some older sailing navigation books recommend having your compass deviation calculated by a professional. Are there still people that do this? Or is there are way for the amateur do to this?

Thanks for the clarification.

biglilwave said...

Visit there's a DIY page the explains everything.

John Vigor said...

BK, yes, there certainly are professional compass adjusters, though they mainly work on larger commercial vessels. But it's pretty simple to make a deviation card for yourself, as Steve has pointed out.

John V.