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?
Kayak Deck Gear Bags
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depends on use|
Posted by: pblanc on Jun-11-13 10:00 AM (EST)
For a river boat that might be expected to encounter unexpected obstacles, I think a bit of flex reduces the likelihood of cracking.
Posted by: radiomix on Jun-11-13 10:18 AM (EST)
Posted by: carldelo on Jun-11-13 11:17 AM (EST)
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.
Flex is sometimes good|
Posted by: kayamedic on Jun-11-13 11:27 AM (EST)
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?
I don't know the answer and am |
Posted by: jackl on Jun-11-13 11:36 AM (EST)
not an expert, but I would like to throw out an experience we had that made for a fun ride because of flex.
We have a Pakboat and the same|
Posted by: kayamedic on Jun-11-13 5:32 PM (EST)
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.
For absolute performance, whether on |
Posted by: g2d on Jun-11-13 2:46 PM (EST)
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.
Posted by: jimyaker on Jun-11-13 4:17 PM (EST)
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.
You aren't from these parts, are you? |
Posted by: g2d on Jun-11-13 5:30 PM (EST)
Posted by: jimyaker on Jun-11-13 8:55 PM (EST)
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.
Because you posted only to sound like |
Posted by: g2d on Jun-11-13 11:00 PM (EST)
you had something to say, and what you did say was totally irrelevant to the discussion.
I'll second the choice for plastic|
Posted by: tdaniel on Jun-11-13 8:08 PM (EST)
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******"
But this thread isn't about that. |
Posted by: g2d on Jun-12-13 3:15 PM (EST)
Of course plastic boats are popular in ww. I have some myself.
when you go fishin' ya never know|
Posted by: tdaniel on Jun-13-13 10:44 AM (EST)
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.
to g2d ... SCKK probably on paper ...|
Posted by: onnopaddle on Jun-15-13 12:03 AM (EST)
From arguments with Salty, I gather |
Posted by: g2d on Jun-15-13 4:04 PM (EST)
that most blows sustained by sea kayaks are like sharp hammer blows, and carbon or S-glass handle those pretty well.
Posted by: jhb8426 on Jun-11-13 5:10 PM (EST)
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 |
Posted by: seadart on Jun-11-13 5:26 PM (EST)
Flexible hulls for speed in heavy seas ..
I've "imagined" a similar scenario|
Posted by: Guideboatguy on Jun-11-13 6:18 PM (EST)
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 |
Posted by: seadart on Jun-11-13 7:17 PM (EST)
Unfortunately the whole article is pay for view ...
Of Course They Should Just Like a Fish|
Posted by: clydehedlund on Jun-11-13 7:34 PM (EST)
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."
fish dont float.|
Posted by: radiomix on Jun-11-13 7:54 PM (EST)
Only When Dead - They Surf Though|
Posted by: clydehedlund on Jun-11-13 8:09 PM (EST)
And that's what matters.
Boats are not fish|
Posted by: carldelo on Jun-11-13 11:48 PM (EST)
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.
Sea Mammal Maybe?|
Posted by: clydehedlund on Jun-12-13 12:47 AM (EST)
Especially kayaks covered with the flexible skins from them?
there is a difference|
Posted by: radiomix on Jun-12-13 9:47 AM (EST)
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.
Some Thought the Aleuts Had an Advantage|
Posted by: clydehedlund on Jun-12-13 6:23 PM (EST)
With their sea-mammal skin covered kayaks (SEA KAYAKER Magazine Fall 1987).
Paddle related to fish ... 'lifeless'|
Posted by: onnopaddle on Jun-15-13 12:08 AM (EST)
Only rigid fish is a dead one in your freezer ...
Sounds like sea kayaks, especially, |
Posted by: g2d on Jun-12-13 3:24 PM (EST)
might gain from having function-designed flexibility created by some genius.
The Genius were the Aleuts|
Posted by: clydehedlund on Jun-12-13 3:47 PM (EST)
According to a Professor Clauser|
Posted by: clydehedlund on Jun-14-13 11:42 PM (EST)
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?
Posted by: carldelo on Jun-15-13 12:11 AM (EST)
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.
You Bet, I've Lightened Up|
Posted by: clydehedlund on Jun-15-13 9:05 AM (EST)
Your response is very much appreciated, for I wanted to know your opinion. Figured this topic was right up your alley.
Book looks interesting|
Posted by: carldelo on Jun-15-13 11:30 AM (EST)
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.
Hopefully, You Can Perform Some|
Posted by: clydehedlund on Jun-15-13 4:41 PM (EST)
Of those "experimental methods" out on the water, so most of your summer is spent paddling instead of reviewing?
Skeletal ... I built my Tidelines with|
Posted by: onnopaddle on Jun-15-13 12:13 AM (EST)