Hull Flex and Efficiency

Does anyone know the relationship between hull flex and efficiency?

I own a Seda Swift with a graphite hull. A friend has a Swift with a lightweight kevlar hull. Of the two, the graphite hull is much stiffer.

My friend claims that the greater stiffness of the graphite hull makes my boat faster in a seaway (no oilcanning etc.) While I understand his argument, neither of us can quantify it although my boat does seem to go faster in conditions.

Anyone know - is there a significant relationship between hull flex and efficiency? And if so, how would this translate to boat speed?

TIA

Switch boats
What happens when you switch boats?

Good Point
By way of background I should have mentioned that my friend and I have paddled two identical kevlar Swifts together in the past and the difference in speed in conditions was not apparent until I switched to the graphite boat. His actual comment was that he couldn’t keep up with me in the graphite boat in conditions even though he was paddling at his normal pace.

(FWIW I do not attribute the observed difference in performance to boat weight as the two boats are similar in weight.)

Sorry I omitted this.

Ferd

Is their
any difference in weight between the two kayaks that might make a difference?

Hafta do it blind
or maybe double blind. Seriously, the psychological boost from knowing you are paddling a carbon boat may be equal to or greater than any increased efficiency from the stiffer hull.

I’d be interested to know if anyone has any data on this as well. My guess is that (unless your kevlar hull is an especially flexy one) the efficiency gain with the added stiffness of carbon over kevlar would be at most in the area of 0.1 or 0.2 knots – not very noticeable unless you were both going all out in a race.

Stiff = Faster
Anytime a material deforms it is absorbing energy. Think crumple zones in cars or would you drive a nail with a rubber hammer? The Kevlar boat is taking a punch while the carbon boat is throwing a punch, not to mention what you are doing to the hydrodynamic water flow.

LOL … Grayhawk, can I use that ???

Either that, or…
… you could add another video to your site. You’re pretty handy with that hammer.

Don’t Think So
The two boats are within a couple lbs. of each other so I do not consider weight a significant factor in the observed result.

I was waiting for you two…

…to come up with some numbers or coefficient or drag data. I just chimed in as I didn’t want them paddling around blindfolded…

Really want to write a bunch on this one

– Last Updated: Oct-19-05 4:09 AM EST –

but I got a dang growling monster clawing at my good arm.
A flexy carbon (wasted material in all the wrong places) boat would be slower than a fresh (correctly layed up) kevlar one of equal weight.
Localized stiffness vs. longitudinal stiffness makes a big difference in comparing materials and boat speeds.

Does one have a rudder ? Are the seats in the exact same spot ? Are the bottoms both equally smooth?

Forgot, if the kevlar boat IS truly oilcanning, then yes it would be slower.

yes
in sprint boats which are all about speed the major push in construction in stiffness so that all energy is transfered from the paddler into forward motion. Any deformation in the boat leads to loss of energy used to propel boats. In sprint competition all boats are of standard weight so the prize of the layup is stiffness.

Friend of mine had a kevlar boat that had gotten floppy after several years and he got rid of it. I do believe that kevlar is puncture resistant but not very stiff so it will deform but not tear.

eugene

MAYBE, -BUT MAYBE NOT?
Turns out stiffness has long been sought for speed by the nuatical boat designing community.

Then a few years (decades?) back it was found that porpoises skins are flexible -no surprise there -but that the way it flexes -apparently in waves -seems to alleviate laminar flow turbulence and actually make for a faster ride.

I suppose a tank test under controlled identical conditions (speed, load/waterline depth, etc.) to measure drag would be a good first empiracal step.

Kris, any other designers familiar with this concept?

Ah yes, anything to enable us to go fast, faster, fastest, as we

-Frank in Miami

Nature
Cetaceans have dynamic rather than fixed hull forms - so can take advantage of all sorts of things a boat can’t. Same with birds wings that can change shape in every dimension including surface shape and texture.

“Rough” surfaces have also bee and continue to be studied. Sharkskin, scales, etc. There needs to be patterns and variations in the texture to optimize flow over the desired speed ranges (like our body hair). Check out some of the swimwear being developed. Simple surface coatings to simulate don’t seem to cut it though.

All could translate into high tech stuff - but probably overkill for paddlers - except that flexible skin idea which has been around for eons. Can’t see it helping straight-line flat water speed (flex would be pretty constant/stable there anyway) - but when the water’s not flat and the wind is up maybe. I’ll let you know…

Numbers…
…would run the same with any basic software available. Same shape.

Real world - some difference - but in most cases tiny. Very tiny. The 0.2 knots mentioned above is not tiny - I would call that huge. One of the layups would really have to suck to see that much difference (or be plastic and/or really scratched up - scratches are likely to make more difference - and few but racers are concerned with them).

As Pat notes below - it’s not so much the material as what is done with them. Most “Kevlar” and “Carbon” hulls are blends with glass, or all three, anyway. You just can’t say “Kevlar” or “Carbon” hull and have it mean anything as it relates to other layups, manufacturers, etc.

Stiffest might = fastest, but it comes at the price of other valuable material properties. Like with all things paddling, compromises rule.

Dang it, I knew someone was going to
bring this up … Stop making me think, I have to get to work >>> LOL