Re: [Paddlewise] Stiff or Flexible?

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.

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.

> 
> I think the most valid argument for flexible boats comes from the energy
> damping effects of flexibility in waves. Ships flex and it has been shown
> that the flex reduces resistance. It was well known in sailing days that
> flexible ships made better times. Harold Saunders in "Hydrodynamics of Ship
> Design" discusses this and I suspect it has validity for kayaks.
> Unfortunately the math just boggles my mind given what one would have to do
> to determine if the elasticity of a skin kayak has enough similarity to the
> elasticity of a steel ship not to mention the enormous variations between
> boats given the method of construction. Matt mentioned the difference in
> wetness between a flexible and rigid boat of the same design. I think this
> gives some hints at what happens.

The fellow also hit on this, i.e. that the flexing and bellowing happens
more readily in chaotic seas, probably giving skin and folding kayaks a
bit of a speed advantage in such conditions for boats of equal length
and width.  
> 
> It would prove interesting to get to the bottom of this but tank testing
> costs big bucks. Unfortunately simply paddling boats and forming impressions
> doesn't cut it as science given the inconsistency of human perceptions.
> Anyway, if we actually "knew"  what would skin and hard shell people have to
> argue about?  :-)

The fellow has quite a bit of pull with the US Naval Observatory and
tried to get them involved...no dice for reasons you mentioned I guess.

best,

ralph
-- 
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Ralph Diaz . . . Folding Kayaker newsletter
PO Box 0754, New York, NY 10024
Tel: 212-724-5069; E-mail: rdiaz_at_ix.netcom.com
"Where's your sea kayak?"----"It's in the bag."
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