I really think that keel sections are not all that critical at our angles of
attack, in water, because we are in turbulent flow, semi-stalled, in effect,
when going to windward. If we had anything approaching full laminar flow on
the keel, I believe we would have zero leeway, among other effects. Also,
because we are heeled going upwind, we must consider whether we have a
healthy amount of spanwise flow. All this applies upwind. I believe we
probably get pretty good flow downwind, but that means any plausible keel
foil would do.
But I think the only way to be sure is to get a look at what the keel is
really doing.
It¹s interesting to remember that the Wright brothers had about as good a
theoretical adviser as you could get at the time, Octave Chanute, who passed
along all the data from Otto Lillienthal, who professed to have made
measurements of lift according to chord sections. But they learned they
could not trust those figures and so they built their own wind tunnel. When
they obtained results that differed widely from Lillienthal¹s, they learned
to trust their own data, as it was indeed correct.
--Demitri
On 3/18/08 6:52 PM, "DaveBreski" <[EMAIL PROTECTED]> wrote:
> Or then again maybe it¹s being over thought through
>
>
> Dave simple stupid
> Cape Cod, MA
>
>
>
> From: [email protected]
> [mailto:[EMAIL PROTECTED] On Behalf Of David Shaddock
> Sent: Tuesday, March 18, 2008 6:21 PM
> To: [email protected]
> Subject: RE: catalina27-talk: Keel Design
>
> Peter,
>
> That¹s something! What a job. Glad to hear the storiesand that you believe
> it¹s about 50/50 art/science.
>
> And, all too obviously, Abbot and von Doenhoff, open on the desk, beats the
> heck out of my memory, asked to dredge up details I haven¹t worried about for
> many years. So I was wrongthe stall is softer with more thickness carried
> aft. I¹m trying to figure out how to carry over flying knowledge (and stalls
> aloft) to sailing knowledge. If the stall is softerless abruptdoes that
> mean that we could retain productive keel effort through a wider angle if we
> carried more thickness aft? I mean, when a keel stalls, it¹s not like the
> boat shudders and nosedives out of the ocean You just start slipping a lot
> further to leeward. I¹ve been puzzling about this as I was driving to a
> client¹s today (five hours on the road)if we want to pinch higher to
> windward, we need a keel profile that generates lift with less angle of
> attack. I think that means we need a higher-lift profilelike Tim¹s putting on
> his i550. But the price we pay is a little higher dragwhich makes itself
> more noticeable at higher groundspeeds, which is also where we don¹t need as
> much lift from the keel profile because we¹re putting faster flow over the
> keel and getting more lift because of the faster speed. So, if I¹m thinking
> correctly (the audience scratches their chins skeptically here) Tim¹s going to
> have a profile that will allow higher pointing at lower speeds and benefit him
> around turns and in lighter winds, but possibly pay a price in top speed due
> to drag. This would be like depowering a mainsail by tightening the outhaul
> when winds pick upflattening the draft and moving it aftexcept with the keel
> you have to pick a shape and you¹re stuck with it. Am I thinking through this
> right?
>
> Dave the Cruiser Shaddock
>
>
> From: [email protected]
> [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED]
> Sent: Tuesday, March 18, 2008 4:29 PM
> To: [email protected]
> Subject: Re: catalina27-talk: Keel Design
>
>
> Dave
>
>
>
> All this brings back some fond, but often disturbing memories. Back in '86 I
> was working in a hydrodynamic model basis towing more 22foot white sailboat
> models than I ever want to see again. This was all in the run-up to Star's
> and Stripes getting ye-olde-mug back. We were servicing at least 4
> syndicates, each of which had multiple canoe bodies and literally dozens of
> appendages. They were sticking wings in truly bizarre places.
>
>
>
> For most design teams, the guys we delivered the results to were the VPP
> modelers, but the guys who came up with the ideas were the aero types who had
> been brought in to try and model the performance in their RANS codes and other
> tools. You've never seen a more dejected guy than one who's sure his large
> thickness reverse dihedral forward swept wing is the greatest thing since
> sliced bread.
>
>
>
> I've kept up some since then, and I'm still convinced it's equal parts art and
> science.
>
> PZ
>
> BTW - I have my Abbot and Von Doenhoff open on the desk, and it reminded me
> that one of the key things about the NACA shapes for keels is that you can get
> softer stall, at the expense of higher lift, if you use the modified thickness
> forms that carry more thickness farther aft (e.g. 0012-64 or 0012-65). That's
> a lot like what the orginal shape of the rudder is on the catalina.
>
> -----Original Message-----
> From: David Shaddock <[EMAIL PROTECTED]>
> To: [email protected]
> Sent: Mon, 17 Mar 2008 11:28 pm
> Subject: RE: catalina27-talk: Keel Design
>
> There are all kinds of different dragsform drag, lift drag, induced drag,
> interference drag, frontal drag, drag racing, life is a drag. You don¹t want
> your boat to end up being a drag queen, right? Although I¹m not saying that
> would be good for someone who wanted a boat to be a drag queen; it¹s a
> personal choice.
>
>
>
> Well, if you want your weight down lowas far from the center of gravity as
> you can get ityou have choicesa bulb, a wing that runs the length of the
> keel, a tapered or delta wing, or a Scheel keel, which isn¹t a racing animal
> (but it¹s what I¹m designing into my new boat, which is a cruiser). A bulb is
> good for just getting weight low because you¹re using a circular section,
> which encloses the most volume with the least perimeter, and that means less
> skin drag (less wetted surface). A wing has the benefit of stopping the
> vortices better than a bulb, but you trade off by having more surface area.
> There was a research study done on both sail vortices (a smoke trail hitting
> the leading edge of a Bermuda mainsail shows an amazing, long curly tip vortex
> dragging behind the tip of it) and keel vortices (injection of colored dye
> into the water with underwater photography). It turns out that at the bottom
> of a fin keel there¹s a vortex as the high-pressure water on the leeward side
> escapes to the low-pressure accelerated water on the windward side (opposite
> of the pressures on the sailwe¹re crabbing slightly and producing a 2-5
> degree angle of attack toward the weather mark and sliding leeward, right?but
> the serious drag is caused by the tip vortex on the aft end. It seems to me
> that the highly secret Australia II keel used a wing that started well aft on
> the fin keel and flared out and back from there, all to control the vortex
> while causing a minimal amount of skin drag. Can anyone confirm this?
>
>
>
> I remember being surprised at what a winged keel really was. When I first
> heard winged keel, it made me think back to what I had thought, in the early
> 70¹s before I knew better, would be a great design for a sailboattwo keels,
> flaring outward, so that when the boat heeled one would come up out of the
> water, reducing drag and increasing righting moment, and leaving the other
> keel basically vertical in the water for ideal lateral resistance. It¹s still
> a good idea in a way, and I guess there are some versions of it out there, but
> it is an idea that¹s ignorant of the drag of skin friction. But all through
> the Australia II¹s race, I pictured two keels underneath, and was really
> disappointed at the first drawing I saw. But it¹s been a long time back
>
>
>
> At any rate, there are actually ways to attempt to optimize a keel design,
> shape, size, aspect ratio, and so forth given the sailing parameters for a
> boat. It¹s still an art as much or more than a science when you get down to
> itwhich makes it much more interesting.
>
>
>
> By the wayTim will see a further benefit from the NACA0012 profile, if he¹s
> carrying 150 lbs of bulb underneath a carbon or glass skinned fina profile
> with 12% of its chord at max width will withstand side loads and stress much
> better than a thinner section like the NACA0006, which would only be half as
> thick.
>
>
>
> Dave Demitri Shaddock (Demitri Shugart¹s second cousin)
>
>
>
> From: [email protected]
> <mailto:[email protected]>
> [mailto:[email protected]
> <mailto:[EMAIL PROTECTED]> ] On Behalf Of michael mcvey
> Sent: Monday, March 17, 2008 8:16 PM
> To: [email protected] <mailto:[email protected]>
> Subject: RE: catalina27-talk: Keel Fairing
>
>
>
> What scares me is I understand what you are saying. so what is your thought on
> wings? not like a production wing keel but as in 12 meter wing keels as in and
> I hope I spell this right Kukabera or Austraila II.
>
>
>> > From: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
>> > To: [email protected]
>> <mailto:[email protected]>
>> > Subject: RE: catalina27-talk: Keel Fairing
>> > Date: Mon, 17 Mar 2008 14:49:41 -0500
>> >
>> > Depends, Tim. I know that sounds like a copout answer--but generally, your
>> > question relates to how thickness of the section relates to stall--and the
>> > stall is affected by angle of attack more so than any other factor, but
>> also
>> > relates to the curvature and profile of the section. How thick it is,
>> > truly, is only a small part of the situation. That's why I said MAY stall
>> > earlier and WILL create more drag.
>> >
>> > Consider, for example, using a very narrow, high-aspect ratio keel. It's
>> > going to be very efficient and provide a lot of lift at higher speeds, but
>> > it really has to count on a certain angle of attack in order to generate
>> > lift. The 12-meter yachts, for example, when using a short keel section
>> > that afforded a high aspect ratio, couldn't be pinched and had to be sailed
>> > full and by. Because they lacked some of the lateral resistance, they slid
>> > to leeward more, but because they were built with fuller ends (higher
>> > prismatic coefficient--kinda like your i550) they held enough speed through
>> > the water to make a better velocity made good and got to the weather mark
>> > faster after all. They just weren't very forgiving of pinching.
>> >
>> > Your boat is going to be really light, and you've probably got a good keel
>> > section that's going to let you pinch a bit more without killing you on
>> > efficiency if you're a little above or below ideal speed. I went down to my
>> > library to try to find my NACA book so I could relate exactly what it said
>> > about the section you chose, but couldn't lay my hands on it quickly. I
>> > believe I recall, however, that I used it in the vertical fin of the plane
I
>> > built because it was pretty forgiving and less inclined to stall during
>> > aerobatics (which provide a wide variety of angles of attack for a vertical
>> > fin because of aggressive yaw during slips and so forth), but still
>> slippery
>> > enough that I could make time across country. I think offhand you've made a
>> > good choice.
>> >
>> > Dave Shaddock
>> >
>> >
>> > -----Original Message-----
>> > From: [email protected]
>> <mailto:[email protected]>
>> > [mailto:[email protected]
>> <mailto:[EMAIL PROTECTED]> ] On Behalf Of [EMAIL PROTECTED]
>> <mailto:[EMAIL PROTECTED]>
>> > Sent: Monday, March 17, 2008 1:45 PM
>> > To: [email protected]
>> <mailto:[email protected]>
>> > Subject: RE: catalina27-talk: Keel Fairing
>> >
>>>>> > >>>A fatter profile, all other things
>>>> > >>being equal, will generate more lift at a given angle of attack, but
may
>>>> > >>also stall earlier and will create more drag-partly due to increased
>> > wetted
>>>> > >>area, frontal profile, and the induced drag that comes with lift.<<<
>> >
>> >
>> > whoa whoa whoa.
>> >
>> > I THOUGHT the fatter sections would:
>> > - generate more lift thru a wider range of angles of attack - GOOD
>> > - create more drag- BAD
>> > - STALL LATER- GOOD
>> >
>> > whereas, thinner sections would:
>> > - generate less lift thru the same range of angles of attack - BAD
>> > - create less drag - GOOD
>> > - stall earlier - BAD
>> >
>> > as defined above, you are saying fatter sections:
>> > - generate more lift - GOOD
>> > - create more drag - BAD
>> > - stall earlier - BAD
>> >
>> > and thinner sections:
>> > - generate less lift - BAD
>> > - create less drag - GOOD
>> > - stall later - GOOD
>> >
>> > so it seems like, the way you have delineated the trade-offs, there is
>> > much less incentive to go with a thicker foil.
>> >
>> > Please understand, I'm not trying to argue here, I'm just trying to grasp
>> > the concepts!
>> >
>> > I also understand that if the boat is planing downhill at 15 kn the
>> > thicker section is
>> > going to be more forgiving!
>> >
>> > So I guess my question is, once again, dont thicker sections stall LATER?
>> >
>> > thanks,
>> > tf
>> >
>> >
>> >
>
>
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