Judging Leeway - Sail Magazine

Judging Leeway

Any boat under way in a crosswind, whether it’s a rowboat crossing a lake or a powerful cruiser reaching along the coast, will be pushed sideways to some extent. The effect is called “leeway,” and even big ships are subject to it. Sometimes leeway is insignificant; often it is not.
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JudgingLeeway

Any boat under way in a crosswind, whether it’s a rowboat crossing a lake or a powerful cruiser reaching along the coast, will be pushed sideways to some extent. The effect is called “leeway,” and even big ships are subject to it. Sometimes leeway is insignificant; often it is not. If you don’t account for it when plotting a course, grief may be your reward.

If you can see your destination, you can check whether you’re being blown to leeward by lining up some fixed object near where you’re going with an object behind it. This effectively creates a range that works the same as those thoughtfully set up by the authorities to lead ships into harbor.

If there’s no significant current and your two range objects “open,” or move relative to each other, you are making leeway from your direct line. The solution is to steer “up” into the crosswind to close your markers and keep them lined up.

 Applying leeway to a course to steer: start from (A) the theoretical course. Protractor is turned into the wind by the expected leeway angle to find real course (B)

Applying leeway to a course to steer: start from (A) the theoretical course. Protractor is turned into the wind by the expected leeway angle to find real course (B)

You can see that the boat in the illustrations has to head up about 10 degrees in order to stay on the line. If she steers straight for her destination she drifts off course, the range opens, and she must steer firmly upwind to close it again.

When your destination is invisible because of thick weather or distance, you’ll have to plot a course. Estimating leeway is a part of the process. A small yacht close-hauled in a stiff breeze may slide to leeward by up to 10 degrees. As she bears away her leeway will diminish steadily as the wind draws further aft until, when running, there’s none at all.

 Applying leeway to a course you're already steering: turning protractor away from wind indicates actual course made good (C) if leeway has not been applied to the course to steer

Applying leeway to a course you're already steering: turning protractor away from wind indicates actual course made good (C) if leeway has not been applied to the course to steer

COMPENSATING FOR LEEWAY

It’s easy to get confused when numerically figuring leeway into a course-to-steer, so it pays to do the job graphically. Sketch a wind arrow somewhere on your chart, place your chart protractor on the course you want to sail, imagine your boat is sailing up the edge of the protractor and see which way to turn it to counteract her tendency to be blown downwind. Swivel it between 5 and 10 degrees toward the wind and read off the heading you’ll actually steer.

APPLYING LEEWAY

If you’ve already been steering a course and have not compensated for leeway, when the time comes to estimate your position you’ll need to know where on earth you are really tracking. The answer is to apply the same technique retrospectively. Sketch the wind arrow on the chart and lay your protractor on the course steered. Now swing it 10 degrees or so away from the wind, redraw the course and you’ve cracked the problem. 

Top Illustration (by Steve Sanford): Sailing on a beam reach, the boat above first falls to leeward, “opening” a pair of range marks on shore (left); the boat steers a high course to close them up again, but overcompensates, so that they are once again out of line (center); back on course, the marks line up correctly (center)

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