Hi, Dave--
Funny you should mention the twin keel idea. My first boat was a Tiburon,
built in France to be able to squat on the mud of river estuaries. It was a
serious dog. (A Sirius dog, I guess.)
Early in the season, it moved forward at a rate of about two knots under
full sail. As a novice, I made a poor choice of bottom paint that year
(1974). At the height of the season, when sea grapes were hanging from the
inner surfaces, I managed to sail sideways across a lobster pot that I had
made good. In other words, headway was less than leeway. I sailed into the
wide mouth of the Connecticut River and got a half-mile up river when she
stopped moving over the groundfor three hours, until the tide turned.
I went over the side several times during that season trying to get the hull
clean, but I couldn¹t really get in between the keels very well. Butin the
boatyard she didn¹t really need a cradle! Sold her the next fall.
--Demitri
On 3/17/08 11:28 PM, "David Shaddock" <[EMAIL PROTECTED]> wrote:
> 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] On Behalf Of michael mcvey
> Sent: Monday, March 17, 2008 8:16 PM
> To: [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]
>> > To: [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] On Behalf Of [EMAIL PROTECTED]
>> > Sent: Monday, March 17, 2008 1:45 PM
>> > To: [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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