[Paddlewise] Stiffness

735769 wrote:

> I think Ralph Diaz posted an article by a fellow with good credentials
> suggesting that the skin boat would have a resistance reduction due to a
> similar effect to that of a golf ball (ridges replicating dimples -
correct
> me if I got this wrong Ralph) . I doubt if this occurs in any significant
> way since golf ball dimples create turbulent flow at the surface and
reduce
> the wake size. On a blunt object like a golf ball reducing wake size has a
> huge effect. On a streamlined body like a kayak there is very little wake
to
> reduce so the added resistance due to turbulent flow would probably give
an
> net negative effect. This idea (dimpled surface) has had many kicks at the
> cat and always shows up poorly in the tank.  Do not confuse dimples with
> microgrooves which do have a positive effect under special circumstances.

Ralph answered:
Actually the fellow raised the golf ball dimple effect but largely
discounted it for different reasons than you have...because of the large
surface of a kayak and relatively shallow height of "dimples" in a
flexible skin.  What he maintained lowered the critical Reynolds number
of the drag crisis was the bellowing of a flexible skin boat (foldable
or Northern indigeneous).  The dynamic in and out motion of the skin
sections between frame members is what has the effect on the Reynolds
number.  The rest of the explanation is complex.  I have shared the
article from Folding Kayaker with you, Matt and anyone else who asked
for it.  If some one has asked for the article and didn't get it, I
apologize.  I sent off a batch initially, then got waylaid and wound up
sending another batch much later and may have lost a name or two.>>>

My take on this article was much like John Winters'.
I think the author was, like George Dyson, a very creative thinker and I
admire that. I also think he was wrong in his conclusions and like George
seemed to be reaching (very creatively I might ad) to explain a foregone
conclusion. John Winters got the catch, which is even if you can get a kayak
into the Reynolds number regime that makes a golf ball fly so much farther
with dimples than without, it is a regime of more turbulence that is
delaying separation (eddymaking) from the golf ball until it has traveled
further around the blunt back of it resulting in a narrower (less drag) wake
than with a smooth ball. That narrower wake more than makes up for the added
drag due to the extra turbulence. No matter how this turbulence is created
(with dimples or a vibrating skin) it is not going to help delay separation
on an already streamlined stern because there is no separation occurring on
the streamlined shape until very near the back end where the boat and
resulting wake are already very narrow. If a kayak had a very blunt rounded
stern this just might work but most kayaks are fine enough in the stern that
creating more turbulence (and therefore more skin friction) will just hurt
performance rather than help.

For my friend George Dyson an his believers:
Seal skins are not like a dolphins skin and don't act like it (when dead and
sewn together the seams would ruin the laminar flow even if it did work like
dolphin skin (or whatever the dolphin does with it) is suspected of doing
(promoting laminar flow).
Whale oil is not like WSR301 (a four million atom long molecule) and if it
were it would have to be injected into the water in front of the boat anyway
to help maintain laminar flow not just smeared on the skin.
A bifid bow does not act like a bulbous ships bow and at speed is out of the
water anyway so isn't doing much at all.
 (These were all theories of George's that were published by him in years
ago in Sea Kayaker and later The New York Times.)

Creative types need analytical types around to shoot down their wilder
theories. Without creative types analytical types would have a whole lot
less to shoot at (and a lot of discoveries wouldn't have been made at all).

Paddle stiffness:
A little flex in the shaft of the paddle can be good if you load up the
paddle on entry to the water and the rebound takes place when the blade is
at a more efficient (more vertical) angle than when you flexed it. This
takes a very resilient and relatively stiff shaft as if it weren't quick to
respond or too soft the loss in flexing would be wasted (such as if it
didn't unload until after you withdrew the blade from the water--I have used
paddles like this--but not for very long). Graphite and stiff glass and wood
shafts seem to work the best. Stiffer aluminum shafts don't flex as much and
don't cushion the shock of a hard stroke as well so one must hold back a
little on the power. I have trouble seeing any advantage in more flexible
blades, to me they just lose bite and also controllability. More thoughts on
what makes for a good paddle can be found under "Paddles" on our website
below.
Matt Broze
http://www.marinerkayaks.com

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