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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