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Should hulls flex a bit?

I'd like the insight of the layup experts on this board. In a composite boat, is a little bit of hull flex a good thing? Or is absolute stiffness the desired goal in boat building?

I have four different fiberglass boats, all with varying degrees of stiffness. One boat surprised me with the amount of flex in the hull, but has shown itself to be extremely durable through several years of paddling. For example, I once sidesurfed it into a beach with mixed sand and rock, and the hull rippled over every rock. There was absolutely no damage I could see inside or out.

In contrast, I also have a heavier, stiffer boat. While I haven't tried to replicate the experiment above, I wonder if its stiffness would make it more prone to holing.

Any thoughts from the experienced fiberglassers?


  • depends on use
    For a river boat that might be expected to encounter unexpected obstacles, I think a bit of flex reduces the likelihood of cracking.

    For a flat water racing or exercise boat I would want the stiffest hull possible.
  • $64,000 question
    -- Last Updated: Jun-11-13 10:19 AM EST --

    We all want a boat that is 10 lbs, stiff, but not too stiff, and completely impenetrable to scratching, denting, bubbling, puncturing, gouging, etc.

    Generally on a flatwater boat, the stiffer the better. But stiffness is often a function of your pocketbook and weight. The three major fabric types and their endless combinations have benefits and draw backs.

    Ryan L.

  • Flex
    For trying to go fast (esp. on flat water), stiffer is better, as less energy is being expended to flex the hull, either vertically or horizontally. That energy is lost, because when the hull un-flexes, it does no useful work.

    In rough water, a little flex can help reduce jarring as well as noise. My SOF feels smoother and much quieter going through or over waves and wakes than my FG, kevlar/carbon and wood boats.

    Baidarka were often made with several slip joints along the keel line to promote flex. It is believed to be a way of keeping the boat quiet while paddling through waves while hunting; the energy lost to flex is traded for stealth.
  • Flex is sometimes good
    but in a glass boat I would check just how flexy it is..it may be some fibers are broken and need reinforcing. Are there soft spots?

    Some boats are designed to be flexible to ride over waves such as Pakcanoes that are an adaptation of SOF craft.

    Glass boats should be fairly stiff. It also may be that you don't have enough thwarts to prevent the hull from flexing.
  • I don't know the answer and am
    not an expert, but I would like to throw out an experience we had that made for a fun ride because of flex.
    We were on a fifteen day trip in the artic circle on the Noatak river and were using Ally pack canoes.
    They are a neat canoe for remote expeditions and come in a back pack they you put together out of aluminum struts and rubberized canvas.
    When we first assembled them, I had my doubts about them, but by the end of the trip I was sold on them.
    The Flex part was the fun part. Every time we would go through WW rapids, the whole canoe would flex either in the bow, middle or stern as we went over each wave.
    All we heard from other boats was a bunch of yee-hawing as we flexed over the wave trains.

    Jack L
  • For absolute performance, whether on
    flatwater or whitewater, stiffness is best. For durability, it's more complicated. In some areas of a ww boat that get hard-thumped most often (such as the stern when crossing ledge drops), some flex may reduce damage. But when a composite laminate is bent too sharply, delamination damage accumulates, whether obvious or not.

    Exterior cloths, such as glass and carbon, that stiffen the hull, may reduce distortion and show little local compression damage. But if glass or carbon let go, brittle propagation may be extensive. Inside Kevlar layers stiffen also, and may refuse to allow propagation of damage. But Kevlar may show some delamination when the laminate is repeatedly distorted.

    I have a super-stiff boat where the hull is S-glass outside and carbon inside. It has taken a lot of moderate whacks with only superficial chipping. I have an old, all Kevlar kayak that flexes easily, and the laminate has never cracked or been torn, but there is some delamination in the chines, which have been repeatedly flexed when the bottom was thumped upward. For long use and long life, the stiff boat would benefit from some Kevlar inside, and the flexi boat would be better with S-glass outside.

    Actual lab tests done long ago showed that SS/KK was the best performing combination. Carbon was not included, but probably an SC/KK laminate would be even better.
  • Options
    Only if performance is about efficiency and speed, maybe you don't want much flex. But, if you are doing Class II or higher whitewater and you're banging off rocks, you really need a boat that has some flex. The real advantage to plastic boats is their durability and ability to take extreme abuse without breaking.

    Most WW folks are only mildly interested in hull speed -- maneuverability and durability are the performance characteristics they need most. Go to the Green Race and you can count the number of composite boats that go by. I bet it'll be close to zero -- real close!
  • For myself
    I prefer my boats not to flex. I had a Ranger Otter that I felt had too much flex/oil canning. I fixed it by adding 2 more thwarts, one behind the bow seat and one forward of the stern seat. That stiffened the hull considerably. Note - it comes w/only a single thwart in the middle of the boat.
  • Ancient Technology
    Flexible hulls for speed in heavy seas ..


    Several years ago at Stanford I saw a computer animation of flexible Inuit kayaks frames moving through wave trains and confused haystacks. The modeling suggested that the flex added shock absorption but also allowed for increased speed in rough seas. I do not recall the justification, but it sure looked cool.
  • You aren't from these parts, are you?
    -- Last Updated: Jun-11-13 5:37 PM EST --

    Slalom paddlers go through entire seasons with only small repairs, BECAUSE THEY AREN'T INCOMPETENT.

    I guess you had a good time as a spectator on the Green Race. Sure, those boats are plastic. So what? I've paddled ww since 1973 on boats made from all materials. I care about performance, even when I'm not racing, and you are paddling to an inferior standard.

    Why did you bother posting? Your post has no relevance to mine.

  • We have a Pakboat and the same
    ride as the Ally.. Flexes dramatically which in cold water may be most desirable. Speed on Arctic rivers probably does not matter as much. I have only done the Snake and the Yukon. We didn't have our Pakboat then. We might have stayed drier if we had..Sometimes the wave trains up there just don't end.
  • I've "imagined" a similar scenario
    I understand what people say about flex being less efficient, the part about it taking energy to cause the flexing to occur, and no useful work is done on the rebound. I think that applies really well to any kind of flex that occurs as a result of propulsive and reactive forces. On the other hand, in choppy waves, I am quite sure that I can feel the impact with the waves slow me down. It seems that IF flexing reduced the suddenness with which the boat is lifted over a small wave, the end result COULD reduce overall energy wastage. I'm not sure if it's actually possible, and it might only happen during perfect wave conditions, but it SEEMS possible. Of course, a better plan might be to use a boat that slices straight through the wave instead getting knocked skyward. Oh well.
  • Also from Scientific American
    Unfortunately the whole article is pay for view ...

    The Aleutian Kayak; April 2000; Scientific American Magazine; by Dyson, sidebar by Schlenoff; 8 Page(s)

    When Russians first reached the Aleutian Islands and the coast of Alaska in the 1700s, the waters were thick with small, swift, split-prowed boats to which the explorers gave the name "baidarka." Made of driftwood, lashed with baleen fiber and covered with translucent sea-mammal skin, these craft were entirely creatures of the sea. The Aleuts paddled the lightweight, flexible kayaks at great speeds in the treacherous waters of the area, hunting whale, otter, sea lions, seals and other marine creatures with hand-launched darts, spears and harpoons.

    Over time, however, the design of the baidarka was altered to suit the newcomers' needs. Certain forms of the craft-including a narrow, open-jawed, high-speed version-ceased to exist. Because the tradition of building these kayaks was largely unrecorded, a host of unanswered questions have arisen for contemporary scholars, kayakers and Aleuts. Just how fast were early baidarkas? Why the forked bow and the oddly truncated stern? Did Aleutian hunters have an intuitive understanding of design principles that continue to elude engineers and mathematicians to this day?
  • Of Course They Should Just Like a Fish
    Paddlers spend a ton of money for super stiff boats every day. But heck! Fishes come with flexible skins and of course they can firm em up as needed too. But out in the ocean, my old used and cheap oil caning spec. surfskis perform just as well as the new fangle high tech craft. The old spec. style surfskis were also designed to be puffed up like a balloon too. No breather tubes needed, for depending on trim preference, a couple of puffs or more (usually 6) of air was all that was needed to be blown into the hull. If too stiff, you let out some air. In fact, the Spectrum surfski I use, still has small decals on it to remind the paddler to "Always Inflate."

    Now in case anyone wants to read what George Dyson has to say about flexible skins, then go to SEA KAYAKER, Issue 14, Fall 1987, page 64.
  • fish dont float.
    Ryan L.
  • I'll second the choice for plastic
    on the creeks. I definitely don't paddle creeks as hard as the Green but still do my share of boat abuse. I got tired of patchin' glass boats 30 years ago, so I've been paddlin' plastic ever since. I'll gladly be on that list of "incompetent boaters" who believe plastic is better suited for creeks than glass. It beats bein' at the top of the list for bein' an "elitist a******"
  • Only When Dead - They Surf Though
    And that's what matters.
  • Options
    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?
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