Re: [Paddlewise] Stiff or Flexible?

Dave wrote;

(SNIP)

>
> I have been intrigued by the many thoughtful posts on the "stiff vs
flexible"
> issue, and share Rick's interest in whether there is any *hard data* to
show
> that one is really superior to the other.
>
> I suspect such data would be very difficult to acquire, though the
efficiency
> of dolphins, porpoises, and the like may suggest the issue is worth
exploring.
>
> I bet the US Navy has data, though I also bet a "flexible" submarine is an
> engineering impossibility, sans genetically "expanding" a whale and
equipping
> it with a conning tower and living quarters midships (midwhale?).  <vbg>
>
> Who got data?  We all have beliefs on this issue.

Max Kramer developed a rubbery material called "Laminflo" that was supposed
to reduce skin friction by up to 50% but I believe experiments to replicate
his results have not always proven successful. This dates back to the early
sixties ( I think) and I could not find the tech papers on it in my files.
Not sure I ever even had them. In any case, I think enough research was done
on dolphins etc. to establish that the skin contributed to their efficiency
but they also have very efficient propulsion systems and I am not sure if
that was factored into the original research.

In any case, the idea hasn't caught on so maybe it doesn't really work as
well as thought. I saw a sample of it and I doubt if a skin kayak has
similar characteristics as this stuff sort of felt like skin over fat - soft
but resilient. I believe George Dyson suggests that a seal skin kayak would
have that feel. I have not felt one so can't say.

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.

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.

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?  :-)

Cheers,

John Winters
Redwing Designs
Web site address, http://home.ican.net/~735769



***************************************************************************
PaddleWise Paddling Mailing List - All postings copyright the author and not
to be reproduced outside PaddleWise without author's permission
Submissions:     paddlewise_at_lists.intelenet.net
Subscriptions:   paddlewise-request_at_lists.intelenet.net
Website:         http://www.paddlewise.net/
***************************************************************************