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.

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.

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.

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.