February 6, 2009

Leeway in sailboats

EVER WONDERED why it is that the magnetic courses shown by your compass and your GPS never seem to agree? It’s something that has puzzled most of us in the past, but there is a simple answer. It comes about because sailboats slip sideways through the water on almost all angles of sail except for a dead run.

What we’re talking about here is leeway. Now, leeway is really hard for ordinary mortals to detect. Even if there is land behind the forestay when you look ahead, it’s almost impossible to detect that the boat is slipping sideways while making forward progress.

So, although your compass shows the boat to be on a steady course of 090 degrees, your GPS might be showing your course to be 095 degrees. That’s because your compass shows your magnetic heading, not your course. And your GPS shows your course over the ground, not your magnetic heading.

In the absence of current, the difference between the two is leeway, the sideways slippage of the whole boat, which is at its maximum when you’re sailing against the wind and non-existent when you’re running dead before the wind.

Interestingly, your boat wouldn’t be able to sail to windward if leeway didn’t exist. The keel has to be angled slightly to one side or other of your direction of travel before it can provide “lift.” You can test this next time you’re motoring on the freeway by putting your hand out of the window. Note how it moves up or down as you move it from horizontal toward vertical. The same holds true for an airplane wing, of course. It has to be angled slightly as it moves through the air.

Many factors affect leeway but the rule of thumb is that a sailboat beating to windward will make between 3 and 5 degrees of leeway in a breeze of 7 knots. As the wind increases, so does leeway, until it reaches about 8 degrees in 20 knots.

Short deep keels, known as fin keels and shaped like airplane wings, are more efficient than old-fashioned, long, shallow keels at providing lift to windward. Fin keels depend on forward motion for their efficiency, however. When starting to sail from a standstill, a fin keel will often allow a boat to slide sideways, providing no lift at all until forward speed is gained. Full-keeled cruisers, with their larger surface area, are more resistant to being pushed sideways at very low speeds, and have other advantages in survival weather on the open ocean, but they are less efficient at sailing against the wind.

Unless your destination lies dead downwind, it’s usually wise to point up into the wind 5 degrees or so to compensate for leeway. Then you will achieve your planned course over the ground and your GPS will be very happy.

The oldtimers had an expression for it. They said a boat wasn’t going where she was looking. But she’ll go where you want her to if you understand and anticipate the effect of leeway.

Today’s Thought
Where lies the land to which the ship would go?
Far, far ahead, is all her seamen know.
And where the land she travels from? Away,
Far, far behind, is all they can say.

—Arthur Hugh Clough.

Neil Diamond was once well into his act in a Chicago theater when four women held up a poster reading: “Will you sleep with me tonight?”
Neil didn’t falter. “Ladies,” he announced, “I can perform only once a night — and this is it.”

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