Hi, other Dave S.-- Your hunch is correct; laminar flow in water disappears at a pretty low for the waterline lengths and foil lengths we're discussing. After I wrote my comments earlier, I went back through the thread and found the link to Bryon Anderson's excellent article, which explained everything I was trying to cover in a better fashion, with diagrams too. He mentions 5 knots as the approximate speed at which we lose laminar flow. That's oversimplifying, but it gives you an idea. But laminar flow has a lot to do with the NACA profile--some profiles intentionally move the maximum depth aft in order to maximize laminar flow; the idea is to keep the lift working for you as long as possible. My last plane was a Cessna 177RG, which had a laminar flow wing designed to keep the flow 'attached' for 70% of its surface or better at speeds of about 170 mph. It took a long time getting off the ground, but was incredibly efficient in the air. It was very different from my friend's 175 (we traded for a while so he could get his commercial license), which had a fat high-lift wing that got you off the runway in a third of the distance but only provided 2/3 the top speed for the same power and fuel burn.
Just because a flow goes turbulent, we're still interested in it and it can still perform some useful work. Besides, it's kind of a necessary evil; you can't just provide the part of the foil that gives you laminar flow and then remove the rest <grin>. The whole profile works together. Now, you mentioned vortex generators... They sell dimpled surface material for airplanes to put on the surfaces where the flow starts to detach, under the theory that the dimples (like the ones on golf balls, except standing proud of the surface) generate mini-vortices that help keep the flow (albeit turbulent) attached. Makes me wonder what this would do on the aft faces of a keel. I notice my BMW has raised bumps all over the edges and strut for the side-view mirrors, to cut wind noise by keeping the flow attached so it can't escape and whistle. But let's talk for a minute about porpoises. Capable of 25 knots underwater, and without a turbulent boundary layer... I guess they don't understand Reynolds numbers. Apparently they have a paper-thin outer skin with a thin spongy layer below that covering their real skin. One theory says they can detect turbulence and adjust their body shape to reduce it by controlling this soft layer. Dr. Kenneth Davidson studied them heavily and figured their speed was more due to their streamlined shape and the smooth, oily skin, but later research does suggest that the softness is apparently as important as the skin. But I wonder why my inflatable isn't quicker... Anyway, since I don't see a mechanism for the porpoises to control this fat layer, maybe it's a passive thing. I don't think I'm ready to plaster my keel with neoprene to try it out, but it makes me wonder if the flexible layer couldn't react to impending turbulence and change shape just enough to keep the flow attached. Dave Shaddock -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of David Shugarts Sent: Monday, March 17, 2008 12:14 PM To: [email protected] Subject: Re: catalina27-talk: Keel Fairing Hi, Dave-- That's all true, more or less, but what I have a strong hunch you will find is that these foils do not give us laminar flow at our speeds and angles of attack. In other words, going to windward, I believe you would find that we are nearly always in some form of turbulent flow, at some point in the keel section, unlike aircraft, where we do get laminar flow most of the time, over most of the wing. (BTW, the ratio of Reynolds numbers is 13:1, water versus air. Don't hold me to it, but I believe this is the practical consideration when modeling foils.) These days, there are inexpensive underwater cameras that could perhaps show us what our keels are doing. It isn't a fair comparison, but I get a good look at my rudder and it always looks like it's in some degree of turbulent flow going to windward. (I have the old rudder, which is an anachronism.) Regards, Dave S. PS-I am very familiar with the root versus tip design concept for beneficial stall behavior in aircraft, and we could throw in wing twist if we wanted to complete the picture. And let's not even get started with Whitcomb winglets, stall fences and stall strips, not to mention vortex generators. On 3/17/08 12:31 PM, "David Shaddock" <[EMAIL PROTECTED]> wrote: > 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 >>>> >>>> >>>> >>> >>> >>> >>> >> >> >>
