The puzzle that is leeway

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 almost all the time, except when they’re on 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

Tailpiece

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.”

Allowing for leeway

I SOMETIMES WONDER how many of the sailboat skippers I see blithely cruising among the San Juan Islands are making any allowance for leeway. If you don't make any allowance, there can be a surprising difference between your chosen arrival point and the actual place where you end up. And that's even without taking the tidal current into account.
  Leeway is sideways drift, of course, caused by the wind hitting the side of the boat and sails. All boats make leeway to some degree on all courses except two: when the wind is either dead ahead or dead astern.
  You can assume that a close-hauled sailboat will make between 3 and 5 degrees of leeway in a 7-knot breeze, and 8 degrees or even more in 20 knots. So, if you want to maintain a set course over the ground, to avoid reefs and rocks, you must head your boat toward the wind to offset leeway.
   It's not always easy to estimate how much leeway you're making, but you'll get a fair idea if you look aft along the centerline of the boat and compare that imaginary line with the line of the wake. If you have a hand bearing compass it will give you the difference in degrees, so you'll know how much correction to apply next time.
   Powerboats often make more leeway than sailboats because they present a greater area of topsides to the wind, compared with the amount of hull under water. But it doesn't affect their course over the ground so much because the sideways component is mostly a small percentage of the powerboat's forward speed.
   In any case, it's wise to be aware of leeway because it means your boat is not actually going in the same direction she's pointing at. It's not at all obvious, but she's actually crabbing along to leeward, and if you don't compensate by pointing higher, you might be in for a nasty surprise.

Today's Thought

To be conscious that you are ignorant is a great step to knowledge.
— Benjamin Disraeli, Sybil.

Tailpiece

Little shots of whisky

Little drops of gin
Make a lady wonder
Where the hell she’s bin.

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

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.

Tailpiece
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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