The Cessnas and other aircraft sometimes use different foil shapes at the root and tip in order to make sure the inner part of the wing (closer to the fuselage) stalls first, making the aircraft dive and regain speed while still providing some control out at the wingtips to avoid a spin. This isn't an issue with sailboats.
But our keels can still stall--the keel provides windward lift if it doesn't stall, at the expense of some leeward slip. If the keel stalls, you lose the lift and you see a lot more leeward slip/skidding. There are so many NACA profiles that it's hard to imagine anyone using something that's NOT a NACA profile--they have tested and published results for hundreds of them, with some having only a tiny variation from others. But those tiny variations can make measurable differences, especially since we're operating our profile in a medium 800 times denser than air. I have a book I used for aircraft design purposes that's got everything they had published through about 1990. At any rate, selection of the ideal profile for a sailboat involves knowing the aspect ratio as well as the target speeds. 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. A bulb at the bottom of the keel offers two things--for one thing, it minimizes the tip vortex (which adds a great deal to drag), but mainly it helps provide a lot of mass at the extreme draft, which provides more righting moment. If the rules allow, you can carry more sail because of the extra righting, and you'll heel less which means more sail upright and working for you (although heeling may increase your waterline length on some hulls and raise your speed). If the rules don't allow added sail, you can take advantage of the increased righting moment by cutting weight out in other areas and you'll accelerate faster. The profile Tim has picked out for his sportboat is a good one; at the speeds he might be getting on a planning boat, he could probably have done well with a narrower profile, too, but this way he's covered for a wide range of conditions. A lot of this information is in Steve Killing's book on Yacht Design and also in Skene's Elements of Yacht Design--but the later publications of that are much more informed than the early ones). Dave Shaddock -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of David Shugarts Sent: Monday, March 17, 2008 10:52 AM To: [email protected] Subject: Re: catalina27-talk: Keel Fairing Hi, Tim-- I think your summation of it as "like a Chevy" is a pretty good analogy. To go back to the source, I have now heard Frank Butler answer a number of sophisticated questions with what sure sounded like naivete to me, so I have a hunch that our factory keel section was a "oh, whatever" decision at the time. Then these better keel sections would naturally be an improvement, but only because the bar was set so low. It would be interesting to hear from an expert here, because I just feel as though the designers of the cool toys are way beyond NACA foils. Or perhaps they really are more about the bulb than the keel section itself. For instance, if we could hang a heavy lead bulb on a carbon fiber keel, we would probably do it, and we might find that ANY keel foil would be fine for the purpose. BTW, this link: http://www.hanleyinnovations.com/glossary.html, shows a few cases where the NACA 0012 was used in aircraft, but it also shows that some venerable aircraft (e.g., the Cessna 150/152) had one foil at the wing root and another at the tip (in other words, more sophisticated). Notably, the B-17 Flying Fortress had it as the root foil (love that airplane!). Regards, Dave S. On 3/17/08 12:03 AM, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote: > I don't profess to have any knowledge whatsoever when it comes to fluid > dynamics, I have just been going on threads on SA and bits and pieces of > knowledge that I've read from different designers. > > I think that as far as high performance (e.g., sport boats, hulls that > will plane > off the wind) sailboats are concerned, a bulb on a keel foil is pretty > much the > name of the game. Certainly heavy displacement and cruising boats will > look toward other keel configurations. But the NACA foils offsets have pretty > much been determined to be the go-to configurations for fast keel struts > in the sportboat world. There are a few arguments over whether a 0011 section > might be faster than a 0012 seciton (with a resulting decrease in > strength/robustness > to loads, etc) for example, but the 0012 shape seems to be the chevy > pickup when > it comes to most foil sections below the waterline. > > These are fairly simple shapes. Pretty easy for an amateur to cut with a > hot wire, > or for a CNC machine to do it. (http://www.youtube.com/watch?v=q7uvq4RlhHM) > I can certainly imagine that areonautical designers would have the need to > come up > with more complex shapes for specialized, shape-specific demands, executed > at high speed with enormous G-force loads in the atmosphere, and new > materials and production techniques would allow for a huge amount of > variability when it comes to foil offsets these days. > > But these are just simple symmetrical foils shapes that you can order up > and get made pretty cheaply on-line...I just ordered a 54" piece of > spyderfoam cut to NACA0012 sections, > for about a hundred bux incl. delivery. It's a dream-world out there now > for home boat (or aircraft) builders! > > tf > > > > >> My ears perk up here. First, I confess ignorance. Are boat keels based on >> NACA foils, and do they apply to water, as opposed to air? Perhaps there >> was >> a series of NACA foils intended for water? I just never paid attention to >> that part of things, although I studied NACA airfoils for my own purposes >> many years ago. I vaguely recall a factor called Reynolds Number that >> would >> govern foils in various media, such as air and water. Can you elaborate? >> >> Regards, >> >> Dave S. >> >> PS--I was just a layman studying the foils at the time, but I went through >> them all pretty carefully. It seemed to me that they were kind of empircal >> in nature. I got the impression that the great virtue of a NACA foil, for >> an >> aircraft designer of the 1930s or 1940s, was that it was thoroughly tested >> and predictable. However, it seemed as though a lot of developments of >> later >> decades, such as the Clark-Y, not to mention variable sweeps and tapers, >> variable chords and foils in a given wing, etc., began to favor departures >> from the NACA foils (except when mere predictability was the goal, as in >> vertical stabilizer foils). So, although I later got into aviation writing >> and was constantly looking for NACA foils, I didn't find many in the wings >> of light aircraft. In my time, we saw NASA come out with the GAW-1, and I >> have always assumed that later, composite aircraft designers were free to >> work with an infinitely variable foil in mind. >> >> >> >> >> On 3/16/08 8:40 PM, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote: >> >>>>>> but they also value every advantage they can get. >>> >>> key words^, huh? >>> >>> nice explanation, Chris. >>> >>> So I guess Compu-Keel is still around? >>> >>> http://www.compukeel.com/ >>> >>> odd because you get NACA foil specs on-line for free...but I guess all >>> class >>> legal keels cant be derived from NACA sections. >>> >>> tf >>> >>> >>> >> >> >> >> > > >
