A sailboat race is held on a river that runs directly North to South, with a current of 12 knots. A 25 mile race is held going downriver (south). The first heat is held early in the day when there is no wind. The second heat, also going downriver, is held later, when there is a 12 knot tailwind (from the North). In which heat are the fastest times recorded?
Going upwind, increased speed leads to increased apparent winds which
lead to even greater increased speeds and apparent winds. The
"positive feedback loop" is offset by friction with the surface.
Further, sailboats do not tend to sail faster than the wind, because of
water turbulence. But iceboats and land boats (typically with
wheels on a desert) routinely go faster than the wind.
For ice boats, the current world record is 143 miles per hour in 72
knots of wind. However, I think that the boat was not going
upwind at the time. If the boat is sailing at a 90 degree angle
to the wind, for instance, and if the sail is set to reflect the
wind straight back, then it experiences a constant acceleration that is
independent of its' speed. In practice, I think that most ice
boat records are set when the boat is sailing at about 120 degrees to
the wind
Faster than the wind ...
