January 13, 2011

A navigational essential

IN MY BOOK, the fixed steering compass is the most important navigational instrument on a boat. I know that GPS has tried to steal this title ever since it was invented, but I don’t think it has earned that honor yet.

The wonderful thing about a compass is that it points the way to go, day and night and in all weathers. GPS can’t point the way to go because it only takes snapshots of where you’ve been in the past, and uses that information to tell you what your course was a few moments ago, and presumably will be in the future, if you keep going straight.

The compass is a beautifully simple piece of equipment that needs no electrical power and has hardly anything to go wrong. It does need to be lit at night, I admit, and an electric bulb is a good way to do this, but they also used small kerosene lanterns on square-riggers, before Mr. Edison came along with his new-fangled light bulb.

Oh, and sometimes, after a lot of exposure to hot sunshine, a compass will develop bubbles. In the old days, when the damping fluid was alcohol, you used to top up the compass with gin, if there was any left after the skipper had been at the bottle. Nowadays they use a petroleum-based fluid that is about 10 times as expensive, but you can get away with using odor-free, water-clear kerosene if the bubbles aren’t too big.

With compasses, as with most other things in life, you get what you pay for. If you’re buying a new one, here are two simple tests that will give you an idea of its quality:

► The test for pivot friction: Use a small magnet or a piece of ferrous metal to deflect the compass about 5 degrees to one side, then quickly remove the magnet or metal.

The compass should return to the previous position exactly. Do a similar test from the other side.

► The damping test: Deflect the compass card again, but this time let the card pivot through about 30 degrees. When it returns, see how far it overshoots the original mark. A quality compass with proper damping has minimum overshoot and will regain its original position with quick authority — that is, without excessive hunting backward and forward. A cheap compass that hunts endlessly will drive a helmsman nuts in a seaway.

Incidentally, don’t think you can cure bad deviation by installing a new compass. The new compass will have exactly the same deviation as the old one because deviation is caused by external factors on the boat around it. And if deviation is more than 5 degrees on any heading, don’t hesitate to call in a professional compass adjuster.

Today’s Thought
Change as ye list, ye winds! my heart shall be
The faithful compass that still points to thee.
— John Gay, Sweet William’s Farewell to Black-Eyed Susan

Boaters’ Rules of Thumb, #146
The wires carrying direct current from your boat’s batteries to the compass light create magnetic fields that can affect the accuracy of the compass. The rule of thumb is to twist the two wires around each other within a few feet of the compass. In this manner, the magnetic fields cancel each other out.

Books I’d like to find in my library:

Mother and Child, by Polly Anderson
The Appointment, by Simeon Mundy
Ceaseless Fall, by Eileen Dover
Shattered Window, by Eva Brick
Front Row of the Stalls, by Seymour Legge
Droopy Drawers, by Lucie l’Astique

(Drop by every Monday, Wednesday, Friday, for a new Mainly about Boats column.)

1 comment:

Bill Haimes said...

John, you are a man after my own heart. Compass adjusters like me will all love you. By the way, the bubble is evidence of a leak. No compass should ever have a bubble.