thanks, man. I understand what you are saying and appreciate the time that you and everyone else, Dave Shu (one of you guys should consider changing your name to Demitri), Keith, Mark, and everyone else who has contributed to this thread (that started out as a simple keel-fairing question that Art started, thanks Art) has put into this discussion.
For someone w/o an engineering background, it really helps to have this stuff explained clearly by people who have sailing backgrounds. Just dont no one tell me the foil should've been spec'ed 0014.... --tf > 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 > > > > >
