There are all kinds of different drags-form drag, lift drag, induced drag, interference drag, frontal drag, drag racing, life is a drag. You don't want your boat to end up being a drag queen, right? Although I'm not saying that would be good for someone who wanted a boat to be a drag queen; it's a personal choice.
Well, if you want your weight down low-as far from the center of gravity as you can get it-you have choices-a bulb, a wing that runs the length of the keel, a tapered or delta wing, or a Scheel keel, which isn't a racing animal (but it's what I'm designing into my new boat, which is a cruiser). A bulb is good for just getting weight low because you're 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 there's 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 sail-we're 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 70's before I knew better, would be a great design for a sailboat-two 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. It's still a good idea in a way, and I guess there are some versions of it out there, but it is an idea that's ignorant of the drag of skin friction. But all through the Australia II's race, I pictured two keels underneath, and was really disappointed at the first drawing I saw. But it's 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. It's still an art as much or more than a science when you get down to it-which makes it much more interesting. By the way-Tim will see a further benefit from the NACA0012 profile, if he's carrying 150 lbs of bulb underneath a carbon or glass skinned fin-a 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 Shugart's 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 > > > _____ Climb to the top of the charts! 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