Hint - look at the chord of the fin. You can't compare the flow around a keel with and without a bulb. Totally different. And airplanes versus sailboat keels.Have to disagree on this one. The #5 example is certainly too fat. Vortex reduction fins can be quite small. Look at all the airplane examples and this, crappy, illustration from the 2003 AC:
That's interesting, but not too surprising. Gliders are really the wrong platform to see strong benefits from winglets -- they're already so optimized for low induced drag. Winglets rely on spanwise flow induced by the tip vortices to generate thrust. When your AR is greater than 30, the tip vortex is small, and you get very little AoA on the winglet, which means very little lift, which means very little thrust relative to their area... which means they're hard to justify.In aircraft, winglets have probably been more studied for sailplanes than anything else, as tip vortices (induced drag) are all important. Very well designed and modeled by a team of university students, you can get about 0.5% improvement in performance at some speeds, while sacrificing performance at other speeds. Not well designed - well, they can still look cool. On a Boeing, which flies at the same speed and conditions 95% of the time, you can tune them to do some good. I put the factory winglet kit on my sailplane, no measurable difference in performance before and after, I could circle a couple of knots slower so maybe they delay tip stall and spin departure a little. They do look cool on the ramp. Like PHRF you get a rating hit when you install them in handicapped competition. Like PHRF I don't think they make up for the rating hit.
Dangerous to compare a sailplane (AR = >30) to a boat keel (AR ~ 2). In a boat keel the whole thing is a tip vortex. For most of 'em, I think it is a place to put more lead, makes blocking a little easier, and ungrounding more difficult. The cool factor is only important at boat shows (admittedly important for the builder) since you can't see the wings at the dock.
As the boat heaves, the keel is pulled upward through the water, but the effect is lessened because the wave action also moves the water up. Also, the boat is moving forward. The combination means that the angle of attack on the wing can be large, at least briefly. I think it's a pretty common observation, noted above, that the wing is a greater handicap in waves, especially waves large enough to give the boat a pitching action. But still, the boats do get to the windward mark even if the deep keel boats get there first.It's like having the bottom of your keel molded to the shape of a Bruce Anchor... who wouldn't want that?!?
Yes, the whole whole boat moves with the wave but unless the water is also moving up and down equally, horizontal surface will do something good for once, and dampen the motion thru the water. As Zonker says, it's almost certainly not worth the added drag of that surface area in the first place.
I will take a well designed twin keel over a well designed wing keel every time.The twin keels are a great alternative either side of the English Channel for sitting down gracefully when the tides out without resorting to stilts, but they are not a match for performance for the same effective deeper single span (and sitting down can be a lot worse than graceful.)
Absent other constraints you are always better off with more span than winglets. The very first NACA papers on winglets state this, and it's been true every since.The original NASA technical report by Whitcomb which kicked off all the winglet fuss in the late 70's demonstrated a ~20% reduction in induced drag for a wing with an AR between 6 and 7, if memory serves. Modern airliners tend to have wing aspect ratios of around 7-9. So, compared to your average glider with an AR of approx. 30, an airliner has 3.75x more induced drag. Even on airliners, winglets only really make sense if you have a fixed span. The new Boeing 777-9X went with folding wing tips for a higher AR (AR = 11, apparently), rather than winglets -- it will be interesting to see if that becomes a trend.
Yes, wing loading matters. If you barely load your wing, there's less induced drag to minimize, and adding winglets is mostly adding parasitic drag. The rate at which induced drag increases with additional wing loading, though, depends on the aspect ratio. The Whitcomb paper found that:Probably bigger difference than AR is Cl. Induced drag is proportional to AR and Cl. Airliners fly at low Cl at cruise speeds, those speeds being about 4x what their stall speed has to be (so dynamic pressure 16x, and Cl & induced drag 1/16).
While you might get a 20% decrease in induced drag on an AR of 6, that will occur only at high Cl. At low Cl profile and parasitic drag dominate. The AoA of a sailboat keel is pretty close to its leeway angle, so likely Cl not very high.
100%. Airliners are relatively easy in this regard. Sailplanes are harder. Keels are harder still.It is much easier to optimize the winglets for one condition than two. Add to that the same design sailplane has to be competitive in weak eastern conditions and running at 75 knot (Cl about 0.4) vs strong western conditions running at 130 knots (Cl about 0.1). We fill them with water for strong conditions but still a big difference.
Actual tank tests on keel designs prove that it is very hard, sitting in an armchair, to guess what the performance will really be, especially at low AR.
I sailed with a keel much like that (winglets more aft) for years here on the US east coast, where the bottom is shallow and often mucky. The keel provided much less draft than the fin keel version, and since I wasn't racing, it was worth trading a bit of performance to increase range. When I would occasionally "find" the muddy bottom, it never stuck with suction as naysayers predicted. It's fun to pee on concepts that didn't take off, but most people posting here probably haven't sailed on a boat with a winged keel so don't know how they handle. (Okay, I admit I'm sailing a fin keel boat now.)FWIW, here's the keel on my Catalina 50. Draft is 5.6' / 1.7m, ballast is 13,250 lbs / 6,000 kg. Overall dry weight is 36,000 lbs / 16,330 kg. Full specs here:
Plenty of wetted area but she's a pretty stiff boat with a nice motion. Decent compromise for a Florida boat. Here in Seattle, a Max Prop and big asymmetric help recover some light air performance.