Matt wrote: >> Dimples on a golf ball work by delaying separation of the airflow around the back of the ball so that it doesn't have as big of a "wake" retarding it. They are only effective in a certain range of Reynolds's numbers (a relationship between the speed the length effected and the density of the fluid). ...Kayaks operate in a different range than golf balls. This is exactly correct. The reduction in drag that can be gained by increasing the surface roughness, as in a golf ball, occurs over a very narrow range of Reynolds number. As I recall it is in the 600,000 to 1,000,000 range (about where a golf ball is at). It occurs in the "transition" range, where the flow transitions from laminar to turbulent. Kayak hulls are way beyond transition; 2 billion Rn (this is for a 16ft LWL, 6 ft/sec speed, in cold water). There would be no benefit to the dimpling of a kayak hull. >>Peter used the word "dead" to describe the rigor mortis of "stiff" kayaks compared to the squishy movement of a SOF in waves. wouldn't that make a partially inflated rubber ducky the ultimate in undeadness? C'mon Matt, you know better than this, I would not consider a "squishy" SOF a good design. Compare it to the stiffness of skis, too soft and they are "dead" and too stiff and they are "dead", but the right amount of flex for the type of terrain and speed you like and the ski is described as "lively". Same with the suspension on a car, too soft or too stiff for the conditions yields a poor ride. I always qualified this statement with "the right amount of flex" or "in a well designed SOF", you gave examples of the many poor ways to design a kayak. There are many ways to "kill" the performance of a kayak, too soft, too stiff, too heavy, too wide, poorly shaped, etc., etc. as you enumerated. Having the ability to flex gives the designer/builder one more feature to adjust and optimize. That option is not available in a hard shell design. Too much flex takes energy from the paddler and makes it hard to control, too stiff tends to knock the paddler around in rough conditions (also making it hard to control). On smooth water there would be little difference I suspect. Also, what is the "optimum" amount of flex would be subject to individual tastes, most commercial folders are too flexible for my tastes for example. It is even possible to install a truss adjustment and have the flex of the hull adjustable for different conditions/paddlers that could be adjusted from the cockpit. One option not possible with a hard shell. I have built a hard shell, and have used many others. And I have built and used a number of SOF kayaks. My personal observation is that I prefer the ease and economy of building a SOF compared to other construction methods (the last one I built for my daughter this year, a Greenland type, cost $43 and weights 20 lb., 15' 6" x 19"). There are many things to like about other construction methods, overall I just like SOF the best. Peter *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Didn't they work with micro textures on the hulls of the America's Cup boats a few years ago? I seem to recall it was supposed to decrease drag. I seem to recall they also had something similar a few years back for the Olympic swim team suits. ------------------------------------------------------------------------------- When logic and proportion Have fallen softly dead, Remember what the dormouse said: "Feed your head. Feed your head. Feed your head" -WhiteRabbit > This is exactly correct. The reduction in drag that can be gained by > increasing the surface roughness, as in a golf ball, occurs over a very > narrow range of Reynolds number. As I recall it is in the 600,000 to > 1,000,000 range (about where a golf ball is at). It occurs in the > "transition" range, where the flow transitions from laminar to turbulent. > Kayak hulls are way beyond transition; 2 billion Rn (this is for a 16ft > LWL, 6 ft/sec speed, in cold water). There would be no benefit to the > dimpling of a kayak hull. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 28 Sep 2004 at 23:58, WhiteRabbit wrote: > decrease drag. I seem to recall they also had something similar a few > years back for the Olympic swim team suits. Still in use. However, they seem to have more to do with psych wars between the swimmers than real hydrodynamic performance. The Thorpedo may or may not wear one and may or may not have a top. Ditto some of the others - some only cover the legs, while others wear only shorts. As long as every top swimmer keeps every other top swimmer quessing, they are more worried about performance than doing something about performance. In the long run, those suits may end up having been only a fad. Or not. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Peter wrote: <Snip>>>>Having the ability to flex gives the designer/builder one more feature to adjust and optimize. That option is not available in a hard shell design. Too much flex takes energy from the paddler and makes it hard to control, too stiff tends to knock the paddler around in rough conditions (also making it hard to control).<<<<< I agree with most of what Peter says and even half agree with the above, but I would like to point out that flexing isn't the only way a kayak can avoid being knocked around in rough conditions and that any flex at all is absorbing some propulsive energy from the paddler. There are many aspects of a kayak's shape that can deflect the blows and forces it may be subject to in rough conditions. A rigid kayak doesn't have to feel like it is being knocked around when the conditions get rough. That depends on its design (and I think that is a very important aspect of a kayak's design). Peter, what formula did you use to calculate the Reynolds number of a kayak at about 2 billion? I don't recall the Reynolds number for a speeding golf ball but for a speeding sea kayak it is likely to be in the 7.5 million range (using the formula Re = (speed)*(LWL/1.2791)*10^5) and your speed and LWL figures (6 ft./sec. and 16 ft.). Note: 1.2791 is the Kinematic viscosity of salt water at 59 degrees F. (the temperature and salinity I believe towing tank results are corrected to) and 6 ft/sec. is about 3.5 knots. Even at 33 degrees F. the viscosity of 3.5% salt water would be 1.9323 so the Re would then be about 5 million according to my calculations. Can you tell me where I'm going wrong here? Speed in the formula is in units per second and LWL is in the same units of length as used for speed. I could roughly calculate the golf ball range from this if I knew the viscosity of air and the diameter of the ball (or took the time to look them up). Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Flex can also store energy as in a spring and all its practical applications such as the archery bow. Is it possible for energy stored in hull flex to be returned in a useful way? Can this be why some claim that their skin boats are faster over rough water? Steve Brown -----Original Message----- .......any flex at all is absorbing some propulsive energy from the paddler........ Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Steve Brown wrote: >>>>>>Flex can also store energy as in a spring and all its practical applications such as the archery bow. Is it possible for energy stored in hull flex to be returned in a useful way? Can this be why some claim that their skin boats are faster over rough water?<<<<<< It can work for paddles if they are springy enough so that the flex unloads while the paddle is still in the water at a more efficient angle than when it was flexed. However, even if the kayak was stiff enough (which skin on frame kayaks aren't) I can't see how flexing of the hull side to side during stroking could return energy in any useful direction to increase a paddled hull's efficiency. As I've already said, flexing up and down will allow a finer bow (above the level waterline) to lift over waves rather than plunge into them, although that same bow would be inadequate to the task if it were rigid. That vertical flex might possibly cut some extra drag caused by burying the bow but compared with a stiff kayak with adequate lift at the bow I doubt there is any significant advantage. Unless the side to side flex (that's robbing energy from the paddle strokes) could be reduced to that of a hard shell while the up and down flex was allowed to remain there would be a net loss. I'm not positive of all this. I'm certainly willing to listen to any arguments to the contrary, but I'll be very skeptical. There is a reason racing kayaks are made as stiff as possible for a given weight. Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I'm certainly in no position to make strong assertions or form convincing arguments on this subject. My questions were really intended as questions. There is a swimming stroke (can't remember the name) where the swimmer flexes their body in a traveling wave like motion that may not be dissimilar to vertical flexing. I have seen snakes swimming in a similar fashion excepting that the wave is turned on its side like side-to-side flexing. It seems unlikely to me that this flexing would be timed properly (is resonance the right word?) to return the energy in the form of forward thrust. On the other hand, if the paddler can sense this energy storage and return, there may be an unconscious tendency to paddle in a way that actually gets the benefit. Not an assertion, just wild out-loud thinking. Steve Brown ....... That vertical flex might possibly cut some extra drag caused by burying the bow but compared with a stiff kayak with adequate lift at the bow I doubt there is any significant advantage. Unless the side to side flex (that's robbing energy from the paddle strokes) could be reduced to that of a hard shell while the up and down flex was allowed to remain there would be a net loss. I'm not positive of all this. I'm certainly willing to listen to any arguments to the contrary, but I'll be very skeptical. There is a reason racing kayaks are made as stiff as possible for a given weight. Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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