# Why is stiff better?

For boats and paddles, that is.

I’ve heard that flexible boat hulls might absorb energy. But if the typical paddler generates only 3 lbs of force, is it still true? If you placed a 3 lb weight directly on the most flexible part of the hull and it doesn’t deflect, how could any propulsive force be absorbed? And if only wave energy is absorbed, isn’t that desirable? Granted, a softer hull may initially change shape slightly when placed in the water with the weight of a paddler, but as long as it maintains an efficient shape and doesn’t flatten or bow, it seems like there shouldn’t be any loss of efficiency.

As for paddles, again it takes a lot of force to deflect a paddle. But even for racers who opt for the stiffest paddles, why wouldn’t a slightly flexible paddle perhaps store and release some energy like a spring while still in the water. If you watch sprint boats race, the bows surge upward, wasting energy, during the most forceful part of the stroke. I’m just theorizing that nothing would be lost by storing and releasing some of that energy so as to spread it out over the entire stroke.

It isn’t necessarily so
As long as the hull is not so soft as to deflect from normal loads (oil canning), a degree of flexibility will help it absorb impacts better.

Really stiff paddles tend to be hard on the joints. A small amount of flex can make a big difference.

backwards reasoning

– Last Updated: Oct-30-06 12:45 PM EST –

If you make a paddle or kayak out of less material it'll have to be stiffer and higher tensile material or it'll break more easily.

Therefore "stiffer is better",,,and as long as it's better why not make up a few more reasons like "it won't absorb energy while paddling".

I think it's one of those things that's correct in an isolated context and used in others to pad out justification for spending an extra \$1200 for a composite boat or \$2000 in carbon/kevlar besides "it's lighter".

But that doesn't mean I'll paddle faster in a "stiff" composite hull compared to a flexible skin kayak.

Assuming
Assuming all of the other variables are equal (e.g., hull shape, hydrophobic properties of the material, weight, paddlers, paddles, etc.) it actually does mean that a stiffer hull will be faster than the same hull in a less stiff material. In the stiffer hull, fewer forces dissipate and the hull will have greater efficiency. Also, as a practical design characteristic, hulls made of stiffer materials can generally be lighter. All of these are why we don’t and haven’t seen racing boats (canoes, kayaks, sailboats, shells, powerboats) with hulls that could be called flexible.

phew
i thought this was more viagra spam…

Again With The Bicycles…
In cycling a stiff frame is desired to reduce the loss of energy in side to side flex. Maybe boats and paddles flex somewhat sideways. Then you wouldn’t get back what you lost… as in springing forward.

Ditto That!!!

Well, technically speaking
Energy is never lost, it simply changes form/direction (at least according to the Laws of Thermodynamics)

Therefore, the choice between stiff or flexible really depends upon where you most desire to transfer the energy, toward yourself (in the boat) or back toward the water.

They both have advantages & disavantages.

Once again, circumstances rule…

“Unstiffness” apparently is highly desir

– Last Updated: Oct-30-06 8:53 PM EST –

Otherwise, there would not be a market for suspension bikes, and even rigid road forks would be designed differently.

A bike frame is a strange thing. It needs to be fairly stiff laterally, yet have a bit of suppleness vertically. If it doesn't, many riders will complain about getting beat up by a too-stiff frame and/or fork. (An exception might be track bikes.) Also, the wheels must be allowed to move in such a way that they will maintain tread contact with the ground. If there's too much bouncing due to a very stiff frame, some traction is lost. In some races, even a tiny bit of traction loss can make a difference in finish results.

I remember a crit in which I went from being merely one of the main pack to a 3rd place because my non-crit-specific frame was soft enough that I never lost traction on a badly bumpy corner. I was able to accelerate out of those bumps faster than the rest of the pack as they struggled to regain speed and/or control.

In a kayak, the water will provide some slippage or cushioning. And we're not SUPPOSED to bash the rocks. I like the feel of a stiffer material or structure. But obviously it's not a big deal to me, because one of my two sea kayaks is plastic.

What is better is the lesser negative
of two potentials.

With finances in mind,

It’s better to have a plastic canoe than no canoe.

If you are on a royalex budget, it’s better to have a royalex canoe than a plastic canoe.

If your budget permits, then a kevlar canoe may be better for you for performance.

A stiff and lightweight paddle may be better for performance paddling, but if you are aging and/or your body joints protest occasionally, then a flexing paddle will be better for you to keep you paddling with lesser aches.

whoa…
mickjetblue,

Are you a Politician, or are you poor?

(or perhaps a poor politician)

Who else attempts to solve technical problems via economics???

Sounded good - - -
At least to my wallet

I have a Carbon Fiber ‘taste’ and a poly ‘Pocket’

Yuck Yuck

bending
Having done some work with modal analysis, operating deflection shapes, and other fun stuff, I can say that when you put energy into a flexible structure you often get it back in unexpected ways. Putting energy into bending a flexible hull does not mean you’ll get it back in a way that’ll help propel the boat.

With a paddle, the problem can be one of control. A too-flexible paddle will tend to unload and “snap back” as it exits the water, which doesn’t contribute to propulsion at all and makes for a sloppy release.

On the other hand, many people like the feel of flexible skin-on-frame or folding kayaks, and claim they’re more comfortable in waves. Many people find that a little flex in a paddle feels more “lively” and reduces joint stress. And flexibilty is one way of increasing impact resistance.

As usual, the answers depend on what you like and what your goals are.

My stiffest boat is a wood veneer ribbed
kevlar Navarro Loon (a semi-low profile 16 footer). It’s also my fastest rower due to it’s narrow entry and smooth flank transitions. I bolted the oar socket fulcrum point through both the mahagony (sp?) inwale and outwale so that it’s plenty stiff for transferring an inside pull though the pin to an outside push through plastic spoon blades mounted on inch and a half aluminum tubes. When I row it and compare it’s motion over the water to others who are paddling (transferring power through their body to the hull), my boat seems to move much more like a water strider bug. It’s quickly jumps into motion. Guideboatguy knows what I’m talking about.

Mostly … it’s because of the efficient fulcrum point assisting the power transfer … but I think also think maybe ten percent of it’s “extra”(perceived) efficiency (compared to my other Royalex and fiberglass canoes) is due to it’s wood/kevlar composite stiffness. Hard to partition all the multi-variate aspects … because it’s also lighter than most of my other hulls … it accelerates the best of all of them and has pretty good cruising speed (4.5 mph calm water average). I can feel my strokes (just barely) “bend” the gunwale/hull structure on my other boats more than the Loon. Haven’t noticed them to “heat up” more because of the flexing so far though. Would come in handy this winter !

If I was a politician,
Hi! I’d like your vote, I’m Kevy Canoe!

I’m true and smooth to the furthest ends,

and solid throughout the inner core!

I won’t flex when the goin’ gets tough,

so pour on the paddlin’, and want no more!

Forget them poly boys and royal gals,

only I can do what you want, and always!

I power through the worst of winds and gusts,

Designed and determined, I do what I musts.

I’m Kevy Canoe, and truly the best for you.