Re: [Paddlewise] Question for the engineers

Wes wrote:

[Re:  aerodynamics of a sea kayak in transit on a roof rack:]

> Let's remove some complications and reduce it to simplicity. Mount that
> boat on a rack on the back of a semi-trailer for the test, just to get rid
> of the upward force [from a car's windshield]
> and try to get the airflow coming at it from directly
> in front. Make a special rack for the test, say, a pair of rods that would
> fit through fore and aft grab loops on the end of the boat, so the boat can
> move up or down as necessary without moving backward, and put some scales
> under the boat. Upforce or downforce? Little or lots? Why do you think so?

I'll take a SWAG at this, from the perspective of a sometime aeronautical
engineer wannabee and longtime model airplane flier.  (Impressive credentials,
eh?)

I'd say it's close to a wash, mainly because while the main airfoil (the hull)
is shaped to provide a downward force (aka "downforce") the effect is weak
compared to what a NASCAR racer develops because they exploit a variation of
the ground effect, using a path that is very close to the ground.  In addition,
the deck of the kayak is airfoiled, also, to produce some upward force ("lift"
in airplane terms), albeit not to the degree the hull is airfoiled.  Finally,
the beam of a kayak ("wingspan" if it were a true wing) is so small much of the
effect of the moving air would be to form non-lifting vortices spinning off the
sides of the yak. (This is one of the reasons long wings are more efficient
than short wings.)

I don't want to do the experiment of driving down the road at highway speeds
with my yak untethered in its cradles to see if it would take off, but a friend
did, at 35-40 mph.  Result:  it had not moved a shake when he pulled over to
the side of the road.  He was shaking, however, at the prospect of what might
have happened.

The bottom line in this may be that variations in sidewind and buffetting from
passing semi-trailers have more upset potential than the "aerodynamic" effects
of yak airfoil.  Strap (and bow- and stern-tie) that baby down.  Nobody needs a
17-foot fiberglass projectile impaled in the windshield!

--
Dave Kruger
Astoria, OR
not an engineer, either

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