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  #11  
Old 01-11-2012
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The keel works just like any other foil. It generates lift from differences in "pressure". Even though the keel is the same profile on bother sides the angle of attack is enough cause a change in pressure. There is lots of info online about this.

Angle of attack is how airplanes can fly upside down.
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  #12  
Old 01-11-2012
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Quote:
Originally Posted by zz4gta:815566
The keel works just like any other foil. It generates lift from differences in "pressure". Even though the keel is the same profile on bother sides the angle of attack is enough cause a change in pressure. There is lots of info online about this.

Angle of attack is how airplanes can fly upside down.
Okay, perfect - this is exactly the kind of statement I don't understand. Unlike the model airplanes I flew as a kid (which flew upsidedown nicely) my boat's keel as far as I can tell has zero angle of attack when I'm beating or any other course with the rudder straight.

Or do we look at the beam-on component of apparent wind as if the boat is moving laterally and say the keel is "generating lift" by resisting skid, even though of course relative to the water the boat's motion is only forward? But that seems counterintuitive as a description of what's going on...
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Old 01-11-2012
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As I understand it, the initial 'sideslipping' creates the angle of attack... as the boat picks up speed this translates into a partially compsensating lift force to windward, reducing ultimate leeway.

Excessive pinching/trying to point too high can defeat this effect (the keel can 'stall').. which is one reason why rearranging the deck plan of some boats to improve sheeting angles won't necessarily improve VMG to windward.

From discussions with my son (racing a Melges 32) failure to attach that flow to the foil really hurts because these keels have minimal lateral plane to resist leeway otherwise...

Any true experts should feel free to correct this if necessary....
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Old 01-12-2012
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Correct, the leeway is what generates the lift with the fin keel on a P30. It is a symmetrical foil so the angle of attack (AOA) is zero if you are moving directly forward with no leeway.
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Old 01-12-2012
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The other postings very well describe the lift that is developed from a keel. the P30 keel is similar to the famous NACA010 keel shape and does develop lift ... if the correct angle of attack is held by very slight ~3° of rudder angle
Quote:
Originally Posted by zedboy View Post
Is all you mean that if the jib shape isn't good (because the forestay is sagging) the jib will cause excessive heel which makes the keel less effective at keeping you on track?
What I mean by correct forestay tension is twofold:
1. the jib is operating at near 'design' shape ... a loose/slack forestay will cause the draft to go aft, draft will increase (powered-up and slow), and the leech will 'hook up' to weather and no longer be parallel to the boat's centerline.

2. most important, the (curved or 'hollowed') luff shape (luff 'entry' shape) with correct forestay tension will be aerodynamically operating at optimum as close as possible to the boats centerline. If the luff sags off from of its 'expected' shape and position, the jib's angle of attack will not match the boat designers 10-12° tack to jib fairlead angle ... and the boat will no longer be optimized for sailing 'forward' but will increasingly want to sail at a lower angle ... and the keel increasingly 'slips' to leeward (and no longer provides optimum lift as the keel begins to 'stall' along the 'windward' side) as a result. Simple speak: (totally Ignoring the poor shape induced by a too slack forestay) ... Operating a jib with a loose forestay, when beating, is like repositioning the tack of the sail to leeward from the boats centerline.

The 'article' I referred to (in an earlier post) about matching forestay sag to luff shape should better explain the effect of this 'matching' of sail shape and forestay tension: MatchingLuffHollow.gif picture by svAquila - Photobucket ... this is not a 'sail trim' issue, this is a sail SHAPE issue - controlled by forestay tension. If you cant read the URL because of the magnification, PM me and I'll send a .pdf of this article.

Last edited by RichH; 01-12-2012 at 10:19 PM.
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