Re: brain teaser question

Gordon (
Sat, 22 May 1999 11:11:25 -0700

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West Wight Potter Website at URL
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>If the apparent wind is head on as implied in the question, the "lift force"
>generated would be at a right angle (or close to) to the wind and the
>direction of the boat. The drag force created over the sail and the boat
>would oppose the forward motion of the boat. I think the drag force of the
>sail overcomes the very small forward force of the lift.
>Jim Nolan

That's a good point. The resistance (drag) offered by the relative wind
WILL theoretically oppose the downstream drift, but that effect is probably
negligible at 2 kn. The force of the much denser water on the hull and keel
should overpower the slight air drag on the abovewater parts of the boat, I
would think.

If Sailor #2 can tack at 45-50 degrees to the apparent wind the boat should
travel downstream at something greater than just the 2 kn drift. The fact
that we can tack upwind in a zero current condition demonstrates that the
sail's lift is more than sufficient to overcome the combined air and water
resistance (drag). (The boat will accelerate until the lift and drag are
balanced, then continue at a steady speed.)

In navigation, it is assumed that the boat will move with the current at
the speed of the current, modified by the boat's direction and speed
through the water. I don't think that air drag is ever considered.

Here is a way you could test the effect of air drag on downstream drift:
Sometime when you are drifting in a current, sails down, on a windless day,
throw a chip in the water and watch it. If the air drag of your boat in the
apparent wind is significant, the chip on the surface of the water might
eventually leave you behind as you suggest because it has less air drag.
Then if you raise and set your sails to use the relative wind you can
probably outrun the chip, even though you have increased your drag by
raising the sails. (The problem with this test is in knowing that there is
indeed zero true wind, since you will be feeling the apparent wind induced
by the current.)

Harry Gordon
P14 #234, Manatee
Mountain View, CA