Richard Kemmer wrote: > Glen, > I'm no engineer, but I do have preferences. One is for flexible > skin-on-frame boats. Built a baidarka in a workshop and absolutely love it. > Before "modern" sea kayaks became the rage, built Folbot kits and loved them > too. There's no experience like paddling a boat that lives with and works > with the sea. Know what you mean, having paddled hardshells and Folbots, one-on-one. Different esthetics -- sometimes I really like the flex, and sometimes I want a rigid shell under me. > Held my tongue through the previous discussion, because I know nothing about > building glass boats. Still, based on experience with skin-frame craft, > have a hard time understanding the preference for weight and stiffness. > Would think that a kayak, even without a frame, would be like a > skyscraper -- where appropriate flex actually adds strength to the design. I have been intrigued by the many thoughtful posts on the "stiff vs flexible" issue, and share Rick's interest in whether there is any *hard data* to show that one is really superior to the other. I suspect such data would be very difficult to acquire, though the efficiency of dolphins, porpoises, and the like may suggest the issue is worth exploring. I bet the US Navy has data, though I also bet a "flexible" submarine is an engineering impossibility, sans genetically "expanding" a whale and equipping it with a conning tower and living quarters midships (midwhale?). <vbg> Who got data? We all have beliefs on this issue. -- Dave Kruger Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
Dave wrote; (SNIP) > > I have been intrigued by the many thoughtful posts on the "stiff vs flexible" > issue, and share Rick's interest in whether there is any *hard data* to show > that one is really superior to the other. > > I suspect such data would be very difficult to acquire, though the efficiency > of dolphins, porpoises, and the like may suggest the issue is worth exploring. > > I bet the US Navy has data, though I also bet a "flexible" submarine is an > engineering impossibility, sans genetically "expanding" a whale and equipping > it with a conning tower and living quarters midships (midwhale?). <vbg> > > Who got data? We all have beliefs on this issue. Max Kramer developed a rubbery material called "Laminflo" that was supposed to reduce skin friction by up to 50% but I believe experiments to replicate his results have not always proven successful. This dates back to the early sixties ( I think) and I could not find the tech papers on it in my files. Not sure I ever even had them. In any case, I think enough research was done on dolphins etc. to establish that the skin contributed to their efficiency but they also have very efficient propulsion systems and I am not sure if that was factored into the original research. In any case, the idea hasn't caught on so maybe it doesn't really work as well as thought. I saw a sample of it and I doubt if a skin kayak has similar characteristics as this stuff sort of felt like skin over fat - soft but resilient. I believe George Dyson suggests that a seal skin kayak would have that feel. I have not felt one so can't say. I think Ralph Diaz posted an article by a fellow with good credentials suggesting that the skin boat would have a resistance reduction due to a similar effect to that of a golf ball (ridges replicating dimples - correct me if I got this wrong Ralph) . I doubt if this occurs in any significant way since golf ball dimples create turbulent flow at the surface and reduce the wake size. On a blunt object like a golf ball reducing wake size has a huge effect. On a streamlined body like a kayak there is very little wake to reduce so the added resistance due to turbulent flow would probably give an net negative effect. This idea (dimpled surface) has had many kicks at the cat and always shows up poorly in the tank. Do not confuse dimples with microgrooves which do have a positive effect under special circumstances. I think the most valid argument for flexible boats comes from the energy damping effects of flexibility in waves. Ships flex and it has been shown that the flex reduces resistance. It was well known in sailing days that flexible ships made better times. Harold Saunders in "Hydrodynamics of Ship Design" discusses this and I suspect it has validity for kayaks. Unfortunately the math just boggles my mind given what one would have to do to determine if the elasticity of a skin kayak has enough similarity to the elasticity of a steel ship not to mention the enormous variations between boats given the method of construction. Matt mentioned the difference in wetness between a flexible and rigid boat of the same design. I think this gives some hints at what happens. It would prove interesting to get to the bottom of this but tank testing costs big bucks. Unfortunately simply paddling boats and forming impressions doesn't cut it as science given the inconsistency of human perceptions. Anyway, if we actually "knew" what would skin and hard shell people have to argue about? :-) Cheers, John Winters Redwing Designs Web site address, http://home.ican.net/~735769 *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
In considering kayak stiffness, we probably need to discriminate based on the amount of flexing. Big, structural dynamic flex like having the bow bend upward to go over a wave produces different effects than soft skin flexing around a little tiny cell of turbulence. The "dolphin skin" drag reduction mechanism is close to being rationalized. When the right set of equations are examined for the transition between laminar and turbulent flow, the solution has a vibrational component. This vibration corresponds to little "ripples" in the flow around the hull. I imagine that they are on the order of several millimeters in length. Evidently the trick with "dolphin skin" is to tune the skin response in such a way that it "feels" the pressure oscillations and responds just down-stream of the excitation in such a way as to damp the fluid vibrations. Here, the dolphin has the advantage of evolutionary development. We are stuck with some of John Winters' complex mathematics --and the conversion of the model into useful, interactive material properties. Gross structural deformations are another problem. It it interesting that stiffening a skin boat hurt its behaviour in waves. bob phillips SE Michigan where the water surface is not flowing today. *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
735769 wrote: > I think Ralph Diaz posted an article by a fellow with good credentials > suggesting that the skin boat would have a resistance reduction due to a > similar effect to that of a golf ball (ridges replicating dimples - correct > me if I got this wrong Ralph) . I doubt if this occurs in any significant > way since golf ball dimples create turbulent flow at the surface and reduce > the wake size. On a blunt object like a golf ball reducing wake size has a > huge effect. On a streamlined body like a kayak there is very little wake to > reduce so the added resistance due to turbulent flow would probably give an > net negative effect. This idea (dimpled surface) has had many kicks at the > cat and always shows up poorly in the tank. Do not confuse dimples with > microgrooves which do have a positive effect under special circumstances. Actually the fellow raised the golf ball dimple effect but largely discounted it for different reasons than you have...because of the large surface of a kayak and relatively shallow height of "dimples" in a flexible skin. What he maintained lowered the critical Reynolds number of the drag crisis was the bellowing of a flexible skin boat (foldable or Northern indigeneous). The dynamic in and out motion of the skin sections between frame members is what has the effect on the Reynolds number. The rest of the explanation is complex. I have shared the article from Folding Kayaker with you, Matt and anyone else who asked for it. If some one has asked for the article and didn't get it, I apologize. I sent off a batch initially, then got waylaid and wound up sending another batch much later and may have lost a name or two. > > I think the most valid argument for flexible boats comes from the energy > damping effects of flexibility in waves. Ships flex and it has been shown > that the flex reduces resistance. It was well known in sailing days that > flexible ships made better times. Harold Saunders in "Hydrodynamics of Ship > Design" discusses this and I suspect it has validity for kayaks. > Unfortunately the math just boggles my mind given what one would have to do > to determine if the elasticity of a skin kayak has enough similarity to the > elasticity of a steel ship not to mention the enormous variations between > boats given the method of construction. Matt mentioned the difference in > wetness between a flexible and rigid boat of the same design. I think this > gives some hints at what happens. The fellow also hit on this, i.e. that the flexing and bellowing happens more readily in chaotic seas, probably giving skin and folding kayaks a bit of a speed advantage in such conditions for boats of equal length and width. > > It would prove interesting to get to the bottom of this but tank testing > costs big bucks. Unfortunately simply paddling boats and forming impressions > doesn't cut it as science given the inconsistency of human perceptions. > Anyway, if we actually "knew" what would skin and hard shell people have to > argue about? :-) The fellow has quite a bit of pull with the US Naval Observatory and tried to get them involved...no dice for reasons you mentioned I guess. best, ralph -- ----------------------------------------------------------------------- Ralph Diaz . . . Folding Kayaker newsletter PO Box 0754, New York, NY 10024 Tel: 212-724-5069; E-mail: rdiaz_at_ix.netcom.com "Where's your sea kayak?"----"It's in the bag." ----------------------------------------------------------------------- *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
Dave Kruger wrote: > I bet the US Navy has data, though I also bet a "flexible" submarine is an > engineering impossibility, sans genetically "expanding" a whale and equipping > it with a conning tower and living quarters midships (midwhale?). <vbg> You've probably seen John's response on this, however... About twenty years ago, there was an issue the journal of the International Association for Aeronautics and Astronautics that was devoted to alternative sail craft. In it was detailed information on sail powered, flexible submarines! The idea was to transport large quantities of bulk products - crude oil in particular. The submarine was a giant, supertanker sized, flexible tank that would be filled with oil. It would operate at a moderate depth and be connected to a sail by a _long_ cable. The sail was essentially a helium filled balloon in a wing shape. The sail would be way up in the air, where it would pick up very high winds. No it was not the April 1 edition! Mike *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
Interesting debate for these long winter days....If it's that hard to determine whether a flexible kayak is more efficient than a stiff one, how much difference could there be?? - Bill Hansen *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
Bhansen97_at_aol.com wrote: > Interesting debate for these long winter days....If it's that hard to > determine whether a flexible kayak is more efficient than a stiff one, how > much difference could there be?? Ahhh... but if there is a difference, what does the difference mean? (curve) I used to do marathon+ length cross-country ski races. The difference there between good wax and bad, between good technique and bad and between good equipment and bad was measured in how you survived the _last_ few kilometers. (Amie wants us to do one next year - 150km in three days in Greenland, camping out both intermediate nights) The same thing could be true for kayakers. The difference isn't in what it could do for the average weekend tripper on normal length trips, it's what it could do for the serious distance paddler doing looong days. My personal concern is with basic perfomance, not extreme. That's if there is a real difference and if you're talking about performance. Most of the discussion, however, has been around weight and stiffness and how it relates to strength, toughness, and survivability of the kayaks. That's perhaps a more tenuous connection. It does not necessarily follow that weight means strength or stiffness or durability or anything else. The parameters that govern can be complicated and subtle. We haven't been very good about distinguishing between the capacity and performance issues, and have mixed them up a tad, switching contexts without clarifying. We should try to be good in the future :-) Mike *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
In a message dated 1/16/00 8:02:42 PM !!!First Boot!!!, Bhansen97_at_aol.com writes: << Interesting debate for these long winter days....If it's that hard to determine whether a flexible kayak is more efficient than a stiff one, how much difference could there be?? - Bill Hansen >> This is usually my question with relation to speed. We are talking about boats with a low top end and the vast majority of paddlers without the ability to attain and/or sustain the boat's potential. Throw in variables for conditions; weather and paddler. Even if the weather were constant, the condition of the average paddler (particularly those of us over forty) is not constant. Bruce McC WEO www.wholeearthoutfitters.com *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
John Winter Wrote: >Ships flex and it has been shown that the flex reduces resistance. I what way reduces flex resistance? Only in waves, and in hogging and/or sagging action? Would a floppy (oil-canning) hull be faster in waves? Does more flex means less resistance? Are white water (downriver) racing boats flexible, or should they benefit if they where built flexible? >From my own experience I have noticed that a skin and frame boat was indeed not much less faster when paddling in waves. But I never experienced this in polyethylene boats. In those I do feel a performance loss, especially in maneuvering! I like rather stiff (and as light as usable) boats because they react better (quicker) to my paddling strokes. Dirk Barends the Netherlands *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
Dirk Barends wrote: > > John Winter Wrote: > >Ships flex and it has been shown that the flex reduces resistance. > > Does more flex means less resistance? > > Are white water (downriver) racing boats flexible, or should they benefit > if they where built flexible? > Andy Bridge, who's been building race boats for several years, says not. Here's the description of the three Dagger wildwater racing layups: Racing Ultra Foam core deck with a Spheretex hull. Our stiffest lay-up for the racer who wants the best. S-glass, Kevlar, Carbon, Foam and Epoxy construction. Standard Racing Foam core deck with a Kevlar hull. Stiff deck with a tough hull. Best overall lay-up for stiffness and impact resistance. S-glass, Kevlar, Roam and Epoxy construction. Training Foam core deck. Same stiff deck as the Standard Racing lay-up but with extra layers of Kevlar in the deck and hull. A very durable lay-up, especially good for beginners and occasional racers. S-glass, Kevlar, Foam and Epoxy construction. If John's right, have racers been paying extra for something they don't need? Steve *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
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