Re: [Paddlewise] Perfect Boat is Plastic! Now Flex

Kirk Olsen wrote:
> 
> hmm, time to figure out how to trade hats.  My paddling choices include
> a skin/frame baidarka, and carbon fiber race surf ski.
> 
> Skin/frame boats flex very differently than plastic boats. 
> With a skin/frame boat the boat will bend as it goes over waves.  To me
> this was the most dramatic the day I paddled with a friend, he was paddling
> my VCP Pintail ('glass british heavy).  We were paddling into a headwind with
> 1 foot chop.  The PinTail was riding up over each wave and crashing off the
> top of each wave.  My baidarka was cruising straight forward with the
> waves traveling along the gunwales.  With the boat flexing as it adjusted to
> each oncoming wave.  The baidarka was much faster than the pintail headed
> into the waves.
> 
> My current take on this is a rigid boat is going to be the fastest on a
> flat water course - no flexing induced by the non-existant waves.  In
> waves I think a skin/frame boat will be faster because it adjusts to each
> wave and less forward momentum is lost to the boat coming off of one wave
> and pounding down off of the back of the wave.

I am not certain it has to do with you observed but rather with some
other phenomena.  Back about 3 years ago, one of my readers, a physicist
of some renown with some 150 patents to his name and voted to all kinds
of lists such as Industry Week's Top 50 R&D Stars To Watch, etc., took a
crack at it in an article for my newsletter.  It is quite a detailed
article that I would share with anyone who asks via back channels.  The
key point he makes is cited below in an excerpt.  The phenomenon is
phrased in terms of folding kayaks but would apply equally as well to
any skin kayak such as Kirk's.

>From Folding Kayaker Sept/Oct 1996, pp. 1-5
Scientific Look At Rough Water Drag
For Folding Kayaks Vs. Hard-Shells

"Flexible Skin In Action <subhead>
In chaotic seas, a folding kayak’s skin sections defined by its
stringers and crossribs pump in and out like a drum head and destabilize
laminar flow of water along the surface of the kayak (Handbook of Fluid
Dynamics, ibid, page 11-30).  The vibrating skin of a folding kayak is
extremely effective in pushing the critical Reynolds number of the drag
crisis down to lower Reynolds numbers.  There also may be a small
geometric effect from the less regular surface generated by the framing
effect of stringers and cross ribs on the flexible-skin analogous to
dimpling on a golf ball. 
	This “dimpling” of the surface of the flexible-skin kayak also tends to
lower the critical Reynolds number of the drag crisis.  However, because
of the large lateral size and small height of such “dimples” on a
folding kayak, the destabilization effect due to this static morphology
of the kayak skin is much less than for the dimpled golf ball.   Of the
two effects, we believe that the dynamic in-and-out motion of skin
sections of the flexible skin kayak is the dominant one that causes the
critical Reynolds number of the drag crisis to fall.  Such in-and-out
motion occurs readily in rough chaotic waters and is a common phenomenon
that many of you have often exclaimed about, i.e. the feel of the water
as it passes along the skin.
	For the drag crisis regime to cause a difference in drag between
hard-shell and flexible-skin kayaks, the Reynolds number associated with
kayak motion through water must be near the drag crisis regime.  By one
of those quirks of nature,  it is."

By now I am certain James Lofton is scratching his head saying to
himself "What! My little ole Folbot is doing all that s**t?"  Kinda my
reaction too. :-)  Anyway it is good reading.

happy paddling,

ralph diaz
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