Re: [Paddlewise] Graphite for Bottoms

Douglas wrote;

>This reminds me of an engineering course I took a while ago.  If I
remember
>correctly, fluid friction against a solid surface can be reduced if the
>solid surface has some roughness to it.  The principle, as I remember, is
>that a smooth surface allows laminar flow to occur near the wall, while a
>rough surface creates turbulence near the wall.  The smooth wall (boat)
has
>to drag along a thick boundary layer of water, while the rough surface
will
>carry only a thin layer.  Similar to why dimples help a golf ball fly far.

Laminar flow is the exception rather than the rule on kayaks (and other
surface vessels as well). Typically the flow becomes turbulent only a short
distance from the bow as the Reynolds number rises rather rapidly with
length. In theory laminar flow can be retained for the full length of a
kayak but in practice this does not occur due to yaw, particulate in the
water, surface condition (mostly rough bows), wave turbulence, etc.  To
date the only "rough" surface that reduces friction (for boats) comes in
the form of microgrooves commonly called structuring. These grooves work
only when oriented properly to the flow and if they have the right depth
for the speed. This requirement for precfision makes them impractical for
kayaks.  There was an article in Scientific American about microgrooves but
I don't know if it is still on their web site. It explianed how they work
and had some nifty pictures of the flow.

Try this URL although some of the stuff ddi not come up for me
http://www.contrib.andrew.cmu.edu/~zu22/html/supercomp/0197moin.html

The golf ball business pops up a lot but does not apply. The dimples on the
golf ball reduce the wake size not the surface friction of the ball. In the
case of streamlined bodies like kayaks, the wake size has already been
reduced to the near minimum and separation drag rarely amounts to much.

>
>So maybe roughing up the surface with sanding, or by creating a bumpy
>coating with various fillers, could reduce drag.  Since I'm not practicing
>hydrodynamic engineering right now, I have no idea whether this would work
>or what size "bumps" are required.   Any ideas?

There have been a number of research projects into this and some
interesting configurations have surfaced but the problem of flow alignment
with grooves or bumps seems to foul up the works.


Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/

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