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NJW3 09-07-2003 02:39 PM

Keel Lift
 
Hello out there.

I''m new to sailing and I''m confused. Maybe someone can give me a "dumbed-down" explanation of keel lift.

If I understand what I''ve read, when sailing on the wind the keel produces lift which offsets the heel of the boat caused by the leeward lift of the sails. The two opposing forces are what "squirt" the boat forward.

What I''m having trouble understanding is how a keel moving through the water, being symmetrically-shaped, would cause lower pressure against its leeward side. Since the front-to-back distance the water molecules must travel across both sides is equal, where does the lift come from?

I understand how the sail produces lift -- it''s wing-shaped and the lift comes from the lower pressure of the air having to move faster over the curved side (leeward). But I can''t grasp how the keel would do the same.

Thanks.

Jay Wardlaw

aflanigan 09-07-2003 03:52 PM

Keel Lift
 
Jay,

This is the kind of thing you can answer fairly easily using a search engine, but let me answer your question with another question: How do stunt planes fly upside down? Answer that, and you have answered your question about how keels can produce lift forces.

(Hint: can a symmetrical or even a non-cambered (flat) airfoil produce lift? What is the relevance of "angle of attack"?)

Allen Flanigan

Alexandria, VA

NJW3 09-07-2003 05:25 PM

Keel Lift
 
Allen,

Aha! Thanks, the angle of attack was the piece of the puzzle I wasn''t using. The upside-down stunt plane is a good visual. I guess blowing on the underside of a piece of paper, held horizontally, would be another to illustrate the flat airfoil.

You know, maybe there really are stupid questions.

Jay

aflanigan 09-07-2003 06:10 PM

Keel Lift
 
Date: Sep. 07 2003 10:05 PM
Author: aflanigan
Subject: Re:Keel Lift
Nonsense. Your question was an excellent one; it forced me to think. And it illustrates what a complex physical system a sailboat is.

Consider the fact that a sailboat moves forward because the force acting on the sail (lift plus drag) points at an angle of less than 180 degrees to the angle of the force acting on the keel:


\__

Generally, for airfoils like a sail or a keel, the angle of the force (lift plus drag) acting on the airfoil is roughly perpendicular to the angle of attack. Thus, as we pull the sail closer to the wind, and cross the sail over the center of the boat and push it to windward, we reach a point where the angle of attack of the sail to the wind and the angle of attack of the keel to the water are more or less parallel, and the angle between the forces acting on the sail and the keel is exactly 180 degrees. The boat stops moving forward. This is why you can''t keep pointing closer to the wind by simply pushing the boom of the mainsail to a higher angle relative to the boat''s centerline.

Of course, if we "backwind" the sail by pulling it even farther up to windward, the angle between these forces becomes greater than 180 degrees, and the boat starts sailing backward:

__
/


Dinghy sailers use this technique to back away from a starting line before a race, for example.

Allen Flanigan

Amateur physicist & sailor

NJW3 09-07-2003 07:04 PM

Keel Lift
 
Allen,

And that begins to make clearer to me how "heaving to" differs from being "in irons."

Thanks again.

Jay

Randolph Bertin 09-22-2003 09:05 AM

Keel Lift
 
The original post noted:
"If I understand what I''ve read, when sailing on the wind the keel produces lift which offsets the heel of the boat caused by the leeward lift of the sails. The two opposing forces are what "squirt" the boat forward."

While the two opposing forces (lift from sails and lift from keel or centerboard) push the boat forward, I thought that both forces tend to increase heel.

Are the forces acting against heel the weight of the keel and/or ballast (including crew) and the buouyancy of the hull?



aflanigan 09-22-2003 06:59 PM

Keel Lift
 
Randolph,

The short answer to your question is, yes.

Here''s a link to a page which shows a diagram (scroll down to near the bottom) which illustrates all of the torques acting on a boat about its roll axis. It''s much easier to illustrate it than to try and describe it in words. The horizontal component of force on the keel (lift keeping the boat from sideslipping) should be shown acting midway (more or less) down the keel, rather than at the central axis of the hull. But other than that, this is a good illustration of what is going on to determine a boat''s angle of heel and rolling motion.

http://www.lclark.edu/~gillman/physics%20of%20sailing.htm

Allen Flanigan
Alexandria, VA


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