Re: [Paddlewise] [PaddleWise] Skin on Frame Boats...

Matt wrote:

>> Dimples on a golf ball work by delaying separation of
the airflow around the back of the ball so that it doesn't have as big of a
"wake" retarding it. They are only effective in a certain range of
Reynolds's numbers (a relationship between the speed the length effected and
the density of the fluid). ...Kayaks operate in a different range than
golf balls.

This is exactly correct.  The reduction in drag that can be gained by
increasing the surface roughness, as in a golf ball, occurs over a very
narrow range of Reynolds number.  As I recall it is in the 600,000 to
1,000,000 range (about where a golf ball is at).  It occurs in the
"transition" range, where the flow transitions from laminar to turbulent.
Kayak hulls are way beyond transition;  2 billion Rn (this is for a 16ft
LWL, 6 ft/sec speed, in cold water).  There would be no benefit to the
dimpling of a kayak hull.

>>Peter used the word "dead" to describe the rigor mortis of "stiff" kayaks
compared to the squishy movement of a SOF in waves. wouldn't that make a
partially inflated rubber ducky the ultimate in undeadness?

C'mon Matt, you know better than this, I would not consider a "squishy" SOF
a good design.  Compare it to the stiffness of skis,  too soft and they are
"dead" and too stiff and they are "dead", but the right amount of flex for
the type of terrain and speed you like and the ski is described as "lively".
Same with the suspension on a car, too soft or too stiff for the conditions
yields a poor ride.

I always qualified this statement with "the right amount of flex" or "in a
well designed SOF", you gave examples of the many poor ways to design a
kayak.  There are many ways to "kill" the performance of a kayak, too soft,
too stiff, too heavy, too wide, poorly shaped, etc., etc. as you enumerated.
Having the ability to flex gives the designer/builder one more feature to
adjust and optimize.  That option is not available in a hard shell design.
Too much flex takes energy from the paddler and makes it hard to control,
too stiff tends to knock the paddler around in rough conditions (also making
it hard to control).  On smooth water there would be little difference I
suspect.  Also, what is the "optimum" amount of flex would be subject to
individual tastes, most commercial folders are too flexible for my tastes
for example.   It is even possible to install a truss adjustment and have
the flex of the hull adjustable for different conditions/paddlers that could
be adjusted from the cockpit.  One option not possible with a hard shell.

I have built a hard shell, and have used many others.  And I have built and
used a number of SOF kayaks.  My personal observation is that I prefer the
ease and economy of building a SOF compared to other construction methods
(the last one I built for my daughter this year, a Greenland type, cost $43
and weights 20 lb., 15' 6" x 19").  There are many things to like about
other construction methods, overall I just like SOF the best.

Peter
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