Should be easy:
A river runs straight from West to East at 10 knots. A 10 mile race is held: the boats sail downstream, from West to East. The first heat is held in the morning, when there is no wind. The second heat is held in the afternoon, when there is a 10 knot wind from the West. In which heat are the faster times recorded?
Ok, I'll be the first goose to have a crack....
Both heats record the same time?
Reason: In 2nd heat the wind is blowing at same speed as the current making the apparent wind 0 and therefore identical to the first heat.
The morning. All motion is relative. In the afternoon you're sitting in the water with no wind and the finish line comes to meet you in 1 hr. In the morning the finish line comes towards your position in the water at the same speed, but you have the option of uphauling and beating into a 10 knot breeze to meet it.
easy:
second heat faster - 0 apparent wind = 0 drag.
first heat a touch slower as the apparent wind is 10 knots (river running at 10 knots on a calm morning) the wrong way - causing some drag
or is it?
the 10 knots apparent wind (the wrong way) may be allowing the boats to sail INTO the wind (especially if the boats were starboard serenitys..) thus gaining some extra ground? hmm - more to this than meets the eye....
Easy, you were actually in the bar all afternoon cause there wasn't enough wind to have a good sail and you ended up too drunk to care.
OK, so the river runs straight but you didn't say the race was straight so what if the course was a slalom style course down the river. So in the first race the boats may have sailed the course with apparent wind and scored a time. The second race was all DNFs as no-one was able to make it across the river to round the buoys with the zero apparent wind.
Wouldn't bet my shirt but I'll guess the first race.
Dunno, I went to school when they taught metric, not bushells, poles, cables, chains and furlongs.
Its easier to work tens and zeros.
Answer to puzzle.
The faster heat is the one with no wind. When the wind and the water both move W to E at 10 kt, the boats drift down the river at 10 kt, with their sails hanging limp. In the heat with no wind (as measured on the land), a drifting boat has a headwind of 10 kt. You can tack into that.
Of course, you don't get something for nothing. In the heat with wind, the river does very little work on the boat. In the heat without wind, it exerts much greater force on the boat, in particular on the keel or centreboard. Much of that work goes into disturbing the air downwind of the boat's sails.
