sailing puzzle

Richard O'Brien · Post by **Richard O'Brien** » Tue Nov 15, 2005 5:17 pm

Two people own identical sailboats. The have gotten tired of
racing them the normal way, and decide to hold a different kind of
race: a race against time over a preset course. The race may be
held at any time by either boat, and the person who picks the best
weather conditions will win.

The course is 5 miles long, and runs downstream on a straight
river whose current is 5 miles per hour. (You may neglect such
considerations as eddies near the shore--assume laminar flow at
5 mph).

One person sails the course on a day when a 5 mph wind is
blowing straight down the river. The other person sails the course
in a flat calm. Who wins, and why?

courtesy of Dr. Kenhelm Philips, Univ of Alaska

answer to be published manana'

Gerald Gordon · Post by **Gerald Gordon** » Tue Nov 15, 2005 5:50 pm

Duh, now let's see...sailed the course in a flat calm??? Kind of makes you wonder how that works.

Frank C · Post by **Frank C** » Tue Nov 15, 2005 6:25 pm

Same time, one hour elapsed for both boats. Both boats travel with the current, and the 5 mph wind provides no added progress, as his apparent wind is zero.

(Assumes that the guy in flat calm leaves sails furled ... if he chose to pole out a sail, he would retard the downstream progress he's making with the current ... but that wouldn't be smart, would it?)

baldbaby2000 · Post by **baldbaby2000** » Tue Nov 15, 2005 6:40 pm

I would think it would be the guy who had the wind behind him. With the current pushing him at 5 mph and the air moving at 5 mph there would be no air resistance since he's moving the same speed as the air (the air's frame of reference and the water's frame of reference are the same). The guy on the calm day would have an apparent headwind which would slow him down even if his sails weren't up.

Bb

DLT · Post by **DLT** » Tue Nov 15, 2005 6:59 pm

I vote with BB for the exact same reason...

Sloop John B · Post by **Sloop John B** » Tue Nov 15, 2005 7:13 pm

The one person wins and the other person loses. Check back to Richard's scenerio to see who is who.

I think Richard screwed up the puzzle. If he posed it correctly, it would have been a tie.

Richard O'Brien · Post by **Richard O'Brien** » Tue Nov 15, 2005 7:35 pm

Sloop John B wrote:
I think Richard screwed up the puzzle. If he posed it correctly, it would have been a tie.

Puzzle is posed correctly!

yuri · Post by **yuri** » Tue Nov 15, 2005 7:52 pm

In principle BB is right, unless the guy sailing in calm will do tacking. He will have apparent head wind which he will be using for his advantage being close hauled. He may win the race. Am I correct?

Frank C · Post by **Frank C** » Tue Nov 15, 2005 8:15 pm

yuri wrote:In principle BB is right, unless the guy sailing in calm will do tacking. He will have apparent head wind which he will be using for his advantage being close hauled. He may win the race. Am I correct?

Hmmm - excellent point, Watson (er, yuri - welcome aboard)!

Sloop John B · Post by **Sloop John B** » Tue Nov 15, 2005 8:36 pm

Yeah, okay.

'One person' wins because he has wind and current. The 'other guy' just has current. This is as simple as football about who is going to win.

Inject something we didn't know about for your solution

Oh, now wait. A five mile current with a five mile wind would put 'one person' in a flutter.

A five mile current would put 'other guy' in a five mile current.

Yeah, I suppose they cross the finish line together, one fluttering and the other being drug with the current

Hey man, this is too complicated for us liberal arts types.

yuri · Post by **yuri** » Tue Nov 15, 2005 9:56 pm

Actually, the guy sailing in calm is the only one who can do something. Try to think relative to the water (imagine no river banks) - the one sailing downwind (5 kt) with 5 kt current will be actually in a complete calm conditions - so he has no ability to control his progress. The one who is sailing on the calm day will simply have to sail against the 5 kt wind. He will tack and he will win the race. Period.

richandlori · Post by **richandlori** » Tue Nov 15, 2005 10:23 pm

Easy, neither will win, they both lose for not just firing up their Honda 50 and quietly motoring away,,,,

adm · Post by **adm** » Wed Nov 16, 2005 1:48 am

Despite forming consensus it is actually other way around.
It is comparison of velocity make good over ground (VMG) in two situations:

1. One guy with no wind and 5 kn current (VMG = 5 kn)

2. Other guy with no current but 5 kn head wind.

Who win this...???? To use description from puzzle: the person who sails the course on a day when wind is blowing straigh down the river. It may seems like paradox but do not forget that by sailing he/she will create drag which will slow him/her down. To move boat through water more avaiable energy will be wasted for friction and other drag the in case where boat will be standing still in the water (but still moving over ground).

mike uk · Post by **mike uk** » Wed Nov 16, 2005 2:18 am

Hmm... maybe this isn't a "sailing puzzle" at all. Maybe it's one of those logic puzzles with a twist.

Dr. Kenhelm Philips, Univ of Alaska is apparently the author so a quick look at his work might give us a clue.

Google.. Doh. Nothing found. Does he exist? Does the river exist?

Oh well. I'll look forward to manana.

craiglaforce · Post by **craiglaforce** » Wed Nov 16, 2005 3:25 am

The guy sailing with the wind behind him would win. And the reason is "Duh".

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