October 3, 2013

Navigating by barometer

NOT MANY OF US would define a barometer as a piece of navigational equipment. But two very experienced South African round-the-worlders, Barry and Patrick Cullen, introduced many skippers to the concept of barometer navigation during the first ocean yacht race from Cape Town to Rio de Janeiro in 1971.

The racing fleet had to skirt the South Atlantic High, a disk-shaped area of high pressure with wind strengths gradually increasing from zero in the middle to Force 4 at the outer edges.

The dilemma facing each navigator was whether to sail close to the center, thereby lessening the distance to be traveled, or to go farther north, the long way around the edge, thereby getting more wind.

The Cullens were sailing a famous 47-foot Colin Archer ketch called Sandefjord. They solved the problem by finding the wind they wanted and noting the barometer pressure. If the pressure dropped, they edged closer to the center of the high.  If the pressure rose, they headed out toward the edge. And so they automatically stayed almost equidistant from the moving center of the high-pressure area, carving a huge but efficient semi-circle across the South Atlantic.

Now this is, admittedly a comparatively crude method of rounding a high, but the Cullens stood no chance of taking home a trophy, so they were quite happy to know that they were probably doing the best that dear old Sandefjord was capable of, without all the stress and nail-biting that normally accompanies  electronic weather forecasting and optimum course-finding.

There are highs in all the big oceans that can be navigated the Sandefjord way. The only thing you have to watch out for is the natural variation of the barometer, the diurnal variation, which rises between 4 a.m. and 10 a.m., and also between 4 p.m. and 10 p.m.  The barometer falls between 10 a.m. and 4 p.m. and also between 10 p.m. and 4 a.m.

How much does it rise and fall?  Well the range of the diurnal variation varies according to latitude. It’s about 0.15 inch (5 millibars) at the equator and about zero at the poles.  And it occurs with great regularity, regardless of local weather patterns, although they may mask its presence, of course.

Incidentally, you need a brass-cased aneroid barometer for this business. A mercury barometer has no place on a small boat because the boat’s motion makes it “pump” up and down. But we now also have available the digital electronic barometer with a liquid crystal display screen that shows you a history of the changing pressure plus a current reading.  That’s very handy and a great safety feature — but there is still much to be said for the aneroid barometer. It’s very simple, extremely reliable, doesn’t need batteries, and doesn’t throw a fit if it accidentally gets wet.

Today’s Thought
There be triple ways to take, of the eagle or the snake,
Or the way of a man with a maid;
But the sweetest way to me is a ship’s upon the sea,
In the heel of the North-East Trade.
— Rudyard Kipling, The Long Trail.

A soldier applied for a weekend leave pass.
“What for?” asked the lieutenant.
“My wife’s going to have a baby.”
“Very well.  It’s good to see a man with family pride.”
On Monday morning the lieutenant asked the soldier whether the happy event had taken place.
“What happy event?” said the soldier.
“Did your wife have her baby?”
“Jeez, have a heart, lieutenant. Don’t you know it takes nine months?”

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

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