[Paddlewise] Question for the engineers

Had a pleasant paddle this afternoon, down the backwater of a small dam.
Nice, easy mid-January inland paddle with a couple of friends -- a nice
thing to do in a time of year that most years sees all too much ice, snow,
and time spent on the computer rather than out in the big world.

After the paddle was over, we were sitting on the back of his boathouse,
taking in the sun and relaxing muscles too long gone unused, and one of us
mentioned a guy that he'd seen or read somewhere that had used a Smoker
canoe as the body of a Bonneville salt flats racer -- powered, I presume,
by a honking V-8. Though there are days when I'd like to have had a little
power in a canoe, this seemed a little much.

I got to wondering, however, of the aerodynamics of canoes and kayaks at
high speed, however. It's not such a dumb thing -- most of us have had our
kayaks out in relative windspeeds of 100 mph or more, say, strapped to the
rooftop rack, heading 70 mph down the highway into a 30 knot wind. Those
reading from Germany, with no speed limit autobahns, may have had
considerably higher relative windspeeds.

Now, I will admit to having spent a few bad-weather Sunday afternoons
sleeping through NASCAR races on the idiot eye. On the occasions that I
have been awake for at least part of the race I've noticed that the
announcers spend a lot of time talking about downforce, spoilers, drag, and
other things aerodynamic. I suspect that the average kayak designer rarely
spends much time thinking about spoilers on a kayak, but as I drove home I
spent a little time visualizing what happens to a kayak sitting on the roof
of a car at speed.

My question, then, is fairly simple: to throw out variables, for the sake
of disucssion, let's consider a car with a single kayak heading down a
road, with dead calm winds. Ignoring the straps and such that hold it down,
is the kayak generating upforce or downforce?

My guess is downforce, and probably not an insubstantial amount, mostly
because the area between the bottom of the boat and the roof of the car is
going to present something of a venturi that Bernoulli's principal can grab
hold of. 

But, there are complications, the biggest one being the wind being
deflected upward from the windshield and hood of the car onto the underside
of the forepart of the boat hanging out in the breeze. The boat is going to
be affected by this, of course; under some circumstances, the upforce will
raise the bow of the boat, increasing the frontal area and hence the drag,
and the situation will worsen and the boat will do a backflip before you
can say, "Did you remember the tiedown straps?"

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 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 know this is a nearly useless question, but it's something to think about
while facing the prospect that the weather is supposed to turn colder and
it may be a while before I can get out again.

-- Wes
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Wes Boyd's Kayak Place               http://www2.dmci.net/wesboyd/kayak.htm
Kayaks for Big Guys (And Gals) | Trip Reports | Places To Go | Boats & Gear
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