28 April 2005

Apparent v. True Wind Teaser from Scuttlebutt

Peter Barrett used to ask an interesting question on sailing in current: "Two one design sailors belong to a yacht club on a river which has a current of 3 knots, but unfortunately they work opposite shifts so cannot race to settle who is faster? They agree to time how long it takes them to reach a bridge which is 2 miles downstream, the longer time buys the other a beer. The first sailor finds that on his day, the yacht club flag is limp, i.e. there is absolutely no wind. Next day the second sailor comes down and is happy to find a nice 3 knot wind in the downstream direction. Which sailor wins the bet?"

The first sailor rows his boat out to the middle of the river and starts to drift downstream at 3 knots whereupon he has a 3 knot headwind. He can beat against this headwind and so sail towards the bridge faster than a stick in the water. When the second sailor starts to drift downstream at 3 knots the 3 knot wind relative to the shore now becomes a flat calm relative to the water, so all he can do is drift at the same speed as the stick in the water. Thus contrary to common sense, (which Einstein defined as the prejudices we acquire before the age of 16) the flat calm day is better and the first sailor wins. - Peter Hinrichsen

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