Re: [Paddlewise] Did Hobie Cat sucker punch top racers?

Peter, I think the "stiction" you refer to is commonly referred to as "static
friction," which is greater than the "sliding friction" between two
contacting solid surfaces.  Stiction is greater than sliding friction
because:   when two solids sit in close contact, intermolecular forces
between the surfaces cause the surfaces to interpenetrate slightly.  This
increases the drag force, relative to when the surfaces are already sliding
past one another -- because the interpenetration can not occur to as great a
degree when the surfaces move.

This effect can only occur when the molecules on opposing surfaces are fixed
in place on their own surface, as they are in solids.  If one of the surfaces
is a liquid (e.g., water contacting a kayak hull), the rapid motion of the
liquid's molecules means the interpenetration does not change appreciably
from static to moving conditions.  Hence, no "stiction" when one surface is
liquid (or a fluid, actually -- same situation if one surface is a gas).

As for whether the boundary layer (the thin layer of liquid presumed to move
in concert with the solid surface) interacts differently with the adjacent
liquid layers:  I doubt it.  There is no basis in intermolecular forces which
would support that.  As for any "transient increase in turbulence" at
startup:  my understanding is that turbulence is driven by increased relative
velocity of the two surfaces.  That would suggest no increase in drag at
startup, inasmuch as the two surfaces are not moving yet.  Turbulence is the
result of relative motion.  Until the surfaces move, there is no turbulence.

If you are interested in pursuing this further, I think vortex theory as it
relates to turbulence may deal with any initial conditions, inasmuch as
vortex initiation must occur at startup ... and there may be some effect
inherent in accelerating the first set of molecules into the first set of
vortices which might give the effect you are looking for ... but I doubt it.

I must confess I think any effect such as this would be extremely small, and
very difficult to measure or document.  That is a gut feeling, and not backed
by any solid theory or specific expertise in fluid mechanics ... which
someone else might have.

--
Dave Kruger
Astoria, OR

----- Original Message ----- 
From: "PeterO" <rebyl_kayak_at_iprimus.com.au>

> I'm trying to understand if there is a transient increase in drag when a
> hull or a paddle goes from stationary to forward movement or from reverse
to
> forward movement. I'm told back channel that boundary layer effects on a
> hull would not cause a transient increase in drag under these
circumstances,
> (thanks Matt). Is it known whether something analogous to stiction in
solids
> occurs when a hull or paddle starts to move through water or reverse its
> movement in water? Perhaps a transient increase in turbulence at the bow or
> stern or at the edges of the paddle?
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