Re: [Paddlewise] [PaddleWise] skin on frame

Matt wrote:

> I agree with most of what Peter says and even half agree with the above,
but I would like to point out that flexing isn't the only way a kayak can
avoid being knocked around in rough conditions and that any flex at all is
absorbing some propulsive energy from the paddler. There are many aspects of
a kayak's shape that can deflect the blows and forces it may be subject to
in rough conditions. A rigid kayak doesn't have to feel like it is being
knocked around when the conditions get rough. That depends on its design
(and I think that is a very important aspect of a kayak's design).

PC: that is certainly true, and I did not mean to imply it was not.  Flex
just becomes another element that can be fine tuned and optimized that is
not available to a hardshell designer.

>Peter, what formula did you use to calculate the Reynolds number of a kayak
at about 2 billion?

the basic one from some text book Rn=(rho) x V x L/mu

where rho is density of the liquid, V is speed, L is length, mu is
viscosity.  I pluged in the values with matching units (Rn is dimensionless
so all the units should cancel out).  I did it quickly on a calculator so I
may have made an error, even the number you came up with is still way
beyound transion.

>I don't recall the Reynolds number for a speeding golf ball but for a
speeding sea kayak it is likely to be in the 7.5 million range (using the
formula Re = (speed)*(LWL/1.2791)*10^5) and your speed and LWL figures (6
ft./sec. and 16 ft.). Note: 1.2791 is the Kinematic viscosity of salt water
at 59 degrees F. (the temperature and salinity I believe towing tank results
are corrected to) and 6 ft/sec. is about 3.5 knots. Even at 33 degrees F.
the viscosity of 3.5% salt water would be 1.9323 so the Re would then be
about 5 million according to my calculations. Can you tell me where I'm
going wrong here?

your formula has built-in certain conversion factors that I would not fully
trust untill I verified it, just to make sure you put in the correct units,
for example how do you know it should not be feet per sec. or knots, or mph?
BTW, if a golf ball is 2 inches in dia, and it travels at what? 30 mph=44
f/s the Rn would be = 2.3769x44x2/12/3.7373x10^-7/32.2= 1.45 million

Transition can occur anywhere10,000 to 1,200,000 depending on the surface
roughness (the smoother the higher).  A golf ball is pretty rough compared
to a typical kayak hull.

I doub there would be anything to gain by making a hull rough.  A paddle may
be a different issue however....

Peter
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