Everybody,
Back in the late 70's/early 80's, there was a lot of programming
work to optimize(symmetric and biased) shapes at specific Reynolds Numbers
(which is Non-Dimensional representation of the ratio between the forces due to
dynamic pressure and viscosity). These programs (Eppler and Leibeck were two of
the practitioners) developed sections that computed the point where the flow
would begin to separate, and induce a dramatic reversal of curvature that would
"trip" the flow into turbulence, adding energy and keeping the flow attached.
These shapes always had maximum depth further aft than the NACA sections (which
are usually at about ΒΌ chord). They do have a "sweet spot", and are less
efficient than the less radical shapes away from the design point.
Keith Sneddon (another engineering geek)
#4760, "Are We There Yet?"
________________________________
From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of
Sailor Chef
Sent: Monday, March 17, 2008 3:02 PM
To: [email protected]
Subject: Re: catalina27-talk: Keel Fairing
When I was in Houston (Galveston Bay) I had a Soverel 33 that by one design
standards had a highly modified keel. The former owner (a boat yard owner and
world class sailor) has a buddy that is a NASA engineer (also a world class
sailor). This guy did some "foil work" on the NASA computers. What he came up
with was a "laminar flow" design that incorporated a "drag bucket". The keel
shape was narrow and had the "high point" much further aft than the original
design. It also had a hollow area in the aft 1/4 of the cord (the "drag
bucket").
The end result was, coming out of a tack you had to keep the "nose down" 'till
the speed came up then, when you hit the "sweet spot", the boat would climb the
breeze on it's own. Down wind in medium air you could sail dead down (where
there is no angle of attack on the keel) with little or no (foil induced) drag.
The "groove" was about a 1/2 inch wide and when I was out, I knew it
immediately 'cause the boats from "B" class were passing me!
The A/C programs, when developing foils, not only take into account the wind
and sea state of the venue but also the salinity of the water (among many other
factors) as well.
Mark, Gratis (6115)
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----- Original Message -----
From: [EMAIL PROTECTED]
To: [email protected]
Sent: Monday, March 17, 2008 10:54 AM
Subject: Re: catalina27-talk: Keel Fairing
One might think, that because a keel is like an symmetrical wing,
which can only produce lift by altering the angle of attack, (as in a sport,
aerobatic plane) that the shape of the foil would be irrelevant, aside from
possibly reducing drag, which will make your boat faster.....But there is no
"high" or "low" lift with a symmetrical wing....just angle of attack.
Tommy,
One might think...
In a message dated 3/17/2008 11:30:33 A.M. Eastern Standard Time,
[EMAIL PROTECTED] writes:
For example, there is a concept called the lift/drag bucket--a
high-lift keel profile provides a lot of drag, but might be a
worthwhile
price to pay if you're trying to achieve the best VMG in light
air, because
at low speeds the drag doesn't hurt as much and adding lift
while minimizing
leeward slippage pays off. For higher speeds, a lower-lift
profile works
better because when the boat is moving faster through the
water, you'll get
a resultant increase in the actual lift windward and have less
drag to worry
about--but overall you'll see more leeward slippage.
________________________________
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