Should hulls flex a bit?

Only When Dead - They Surf Though
And that’s what matters.

Okay
Yes, slalom boaters like the light weight and efficiency of composite boats, but the racers are almost the only guys in composites these days (okay, squirt boaters too) and somewhere in the Class III / Class IV range the racers go to plastic. But “performance” means different things to different groups of paddlers and yeah, composites are better for some things while plastic is better for others.



Don’t know why that is getting you stirred up.

Because you posted only to sound like
you had something to say, and what you did say was totally irrelevant to the discussion.



Who doesn’t already know that most ww paddlers just mosey on down the middle of a run and surf an occasional hole? Who doesn’t know already that poly boats are used by nearly everyone for the same reason that poly trash cans are used by nearly everyone?



The original poster is mainly a sea/touring type and wanted to know whether a rigid boat is best, or whether some flexibility is needed. It appears to me, and should to you also, that he is talking mainly about composite boats, and those who replied are talking about composite boats. I posted a fairly well reasoned discussion of rigidity and flexibility in whitewater boats.



You came in and said that most ww paddlers aren’t that concerned about performance, and that they use poly boats which are flexible and damage resistant.



So, how is that relevant to the original subject, or to what I said?



Whitewater boats are made like trash cans because most people aren’t up to occasional repairs, and aren’t able to read water well enough to avoid rocks. I have a 40+ year paddling career, nothing to brag about, but using composite, Royalex, and polyethylene boats. Neither I nor any other person good enough to keep a composite ww boat together for many years needs to hear anything from the unwashed masses.

Boats are not fish
Fish propel themselves by flexing their bodies. Flexing is active and is the source of the propulsion. There is some modification of the boundary layer by the compliant skin of the fish, the mechanism is not well understood.



Boats are propelled externally by a paddle. Here flexing of the hull is passive. It is a (small) energy sink - only important to people trying to go as fast as possible. Flex in the hull may be beneficial in multiple scenario, and Clyde appears to enjoy those benefits on the open ocean.



But a boat is not a fish.

Sea Mammal Maybe?
Especially kayaks covered with the flexible skins from them?

there is a difference
Between a frame that flexes and a skin that flexes. I know its not completely applicable, but an airplane wing is made to flex, but not supposed to dent.



A boat that flexed over waves and gave with wave action could be great, but the skin would still be better off rigid.



Ryan L.

But this thread isn’t about that.
Of course plastic boats are popular in ww. I have some myself.



But the discussion concerns flexibility versus rigidity and the effect on hull performance. Used for normal sized canoes or for sea kayaks, polyethylene is so flexible as to degrade hull performance, so those who can’t or won’t do repairs will have to put up with flexy hulls. And heavy hulls.



I’ve had ww poly boats that were too floppy to perform well, though not recently. The shorter and small a hull, the easier it is to make it rigid with poly. But they’re still all heavy, and heavy degrades performance.


Sounds like sea kayaks, especially,
might gain from having function-designed flexibility created by some genius.



In the past, though, if a kayak came out flexy, it usually meant somewhat degraded performance. The safest course is to design a fairly rigid hull, but who knows if that is best, and in what circumstances?



For paddles, a simpler universe, some flex seems to store and return energy at and following the catch. I don’t know if that has been proved scientifically, though. But most paddlers are convinced that a bit of flexibility feels good, and feeling good translates to an impression of going faster.

The Genius were the Aleuts

– Last Updated: Jun-14-13 11:29 PM EST –

That 26 year old "Forked Bows and Flexible Skin" article in SEA KAYAKER Magazine discusses a lot of the technical aspects regarding flexible skins.

Some Thought the Aleuts Had an Advantage
With their sea-mammal skin covered kayaks (SEA KAYAKER Magazine Fall 1987).

when you go fishin’ ya never know
what ya gonna catch. This message board is a little like that. All kinds of paddlers, different boats and paddling environments. The OP wanted composite advice on hull flex and performance. He got some of that and he also got an astute observation that one aspect of performance is durability as it relates to flexible materials like plastic. So durability may also be something to consider in a larger context than just ww or not. That’s up to the OP to decide, nothing wrong with that. That makes more sense than throwin’ canoes off the roof of a factory to market them as high performers, paddlin’ yaks made by trashcan manufacturers that had all the rigidity of a bleach bottle, or riverchasin’ in a brittle boat. Yes, even plastic has and is currently evolving. Rigidity, hull performance/speed, and durability are important considerations for all paddlers and boat makers, even when we’re talkin’ cheap poly. How about them chopped glass boats? Good and rigid but heavy as sin and “low performing” but cost effective. The real answer is always “it just depends.” Somebody way smarter than me figured that out.

According to a Professor Clauser
in that 26 year old article in Sea Kayaker Magazine I mentioned before: “There is a school of thought that believes that flexible skins make possible a delay in the transition from laminar to turbulent flow, and with this delay a consequent lowering of the hydrodynamic resistance.” So maybe we got it all wrong on demanding stiffer boats?

to g2d … SCKK probably on paper …

– Last Updated: Jun-15-13 12:09 AM EST –

But real world Characteristics of S closer to K and make a better marriage IMO ...

If I had to use those 4 I would go CSKK but Carbon is a waste here ...If deciding on carbon, One should go all carbon or not at all ... IMHO.

Edit to add .... I have no idea why my post is so far down and away from related

Paddle related to fish … 'lifeless’
Only rigid fish is a dead one in your freezer …



I feel the same way about isotropic paddle blades …


Again?
Clyde, you’ve mentioned the SK article five times at this point, lighten up, OK? I’ve read the SK article by Dyson, and also his (rare) monograph on which it’s based. There is not much definitive in the article, but a lot of intelligent speculation which can serve as a road map for future research.



Dyson quotes Clauser, who did important early boundary layer research - I know, because I reviewed it during my graduate research on turbulent boundary layers. Clauser’s comments are speculative at best, and refer to flow over elastic surfaces, which is maybe not exactly the same thing as flow over a flexing hull. There is still work to be done on deciding how to characterize where elastic surfaces cross over into flexible hulls.



I’ve just been asked to review a new book, “Boundary Layer Flow over Elastic Surfaces” a review of the state of the art. I’m hoping it will illuminate some of the important issues in considering this kind of flow - I’ll link to my review when done.

Skeletal … I built my Tidelines with
this view.

You Bet, I’ve Lightened Up
Your response is very much appreciated, for I wanted to know your opinion. Figured this topic was right up your alley.

Book looks interesting
I haven’t got into the book yet, but there are chapters on types of surfaces, experimental methods, modification of turbulence, drag reduction in swimming creatures, etc. Should be illuminating, but it’s 600 pages and pretty dense so will take a while.

From arguments with Salty, I gather
that most blows sustained by sea kayaks are like sharp hammer blows, and carbon or S-glass handle those pretty well.



Inside Kevlar layers are useful for ww boats that sustain relatively slow, deep distortions, such as getting wrapped against a rock or tree by the current.



Top end slalom racing boat hulls are usually high in carbon for stiffness. But if one buys the same boat laid up for lots of practice sessions and longer life, then Kevlar appears, mainly in the inside of the boat. These practice boat layups are described with words like “Flexi”.



As I understand it, Kevlar is about as strong under tension as carbon, but greatly inferior in compression strength. Kevlar in a layup seems to cut down on propagation of cracks. After long searching on the internet, I found an apparently knowledgable source stating that resin adherence to Kevlar is maybe 15% lower than it is to the average for other cloths. Maybe that relates to suggestions that an S/K/S/K layup might be superior in long term resistance to repeated flexing. Interleaving “fresh” glass layers seems to be an old strategy for when trying to keep hard-to-stick fibers in a layup.



Then there was the report that Mad River was obtaining Kevlar with a coating promoting adhesion. Quite possibly Kevlar would do better if such a coating could be used.

Hopefully, You Can Perform Some
Of those “experimental methods” out on the water, so most of your summer is spent paddling instead of reviewing?