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?

The second heat is simple: both water and wind move the boat downstream at 12 knots--there's nothing that can change that.

The first heat has the water carrying the boat into what amounts to a headwind of 12 knots. Ignore the dry land, which is moving backward at 12 knots relative to the water.  It's the relative wind, the headwind of 12 knots that is relevant. There is a sailing practice called tacking that enables a sailboat to travel zig-zag, but net into the wind, using that headwind to progress in that direction.  By progressing into the headwind relative to the water, the boat is traveling more than 12 knots relative to the land.  This means the faster time will be recorded in that first heat.

Posted by Charlie on 2007-02-13 09:17:02