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Old 23-06-2010, 22:00   #1
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Challenge: Who Can Get it Right ?

Here is a question that I came across in an engineering article. This question was posed to a group of accredited and self styled engineers. I didn't make up the question but one of the respondents suggested that they pose it to some sailors. So I am taking the liberty of doing so. I do not know if this question has been aroung the block or not, but here is the question.

"You are in a sailing race on a very wide river. On land there is no wind; it is a dead calm. The race is 10 km downstream, and the river is running 5 km/hr. Your opponents decide that to make the best time, they will streamline their boats to the wind and float downstream as fast as they can. You decide on a tacking strategy, sailing back and forth across the river's width. Who wins the race? What is the winning time?"
WD
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Old 23-06-2010, 22:30   #2
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The sailor that is tacking. Because the current is moving the boat. He suddenly has 5 km/hr of wind to work with also.

Finishing time? I'm entirely to lazy to figure that out.
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Old 23-06-2010, 22:35   #3
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Okay... I'll put my sense of logic and perception (or lack thereof) on display.

My guess would be.. With no wind on land, I would guess no shore breeze since it is a river.

A tie, in two hours carried by the stream like logs...unless one got caught in an eddy like I always do when I go rafting or tubing.

or would the one that is sideways, catch more of the flow and be carried quicker than the one that is streamlined and allowing the water to rush around ?!
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Old 23-06-2010, 22:41   #4
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If there is no breeze on the land, and by inference one on the river, then the boat that goes down the middle of the river would have the greatest wind. The current would be strongest in the middle of the river also- since there is no info given on curves or obstructions, we must assume a straight ditch with no islands. Therefore the greatest current is also in the middle. One would have to assume that the boat that remains in the middle as much as possible would be the winner.
Unless I missed it, we are not given a wind velocity or direction. If there is no wind, the ones that go straight down would win in 2 hours or so.
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Old 23-06-2010, 22:49   #5
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They both drift with the current....a tie/2 hours???? That's not the right answer, is it?
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Old 23-06-2010, 23:50   #6
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They both drift with the current....a tie/2 hours???? That's not the right answer, is it?
No I'm guessing it's not...

If there's no true wind then drifting with the current will induce an apparent head wind which can be tacked into... So the tacking boat should win the theoretical race...( Of course in real life the middle of a river usually runs faster than the sides (I guess that's why they stipulate a "very wide" river) so maybe not so clear cut.)

There's no way to deduce the winning time without knowing the polars of the boat concerned, is there?
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Old 24-06-2010, 00:34   #7
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The guy who is tacking into a nice 5 knot apparent wind will go fastest.

However it doesnt say which way the current is going.

If its a contrary current those who have anchored will win.

Its probably another one of those not clearly written questions to make you laugh when you find out you're a goose....
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Old 24-06-2010, 00:47   #8
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The boat with the cold beer got it right!!
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Old 24-06-2010, 01:33   #9
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Ok, assuming equal current across the river, no wind drag on the drifting boat so it travels at 5 Km/hr. Seems safe to change the frame of reference to no current, 5 km/hr wind from finish. You have a boat motoring 5 km/hr straight down the river for 10 km, that's 2 hrs to finish. Assuming the boat sailing, tacks through 90 degrees then root 2 of 5 gives 7.07 km/hr through the water to tie. That's a 21 degree apparent wind angle. Need a fast lightweight boat to sail there faster driving.

I think that this is the DDWFTTW looking at the camel from the other end. You're trying to start a sh$t fight.


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Old 24-06-2010, 02:31   #10
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Impossible to tell without knowing the performance (polars) of the tacking boat

The boat floating down the current in a straight line will get there in 2 hours

The tacking boat uses what effectively is 5km/r (2.7kts) of wind, but has to sail an additional 4km or so. If it can move at more than 1 kt in 2.7kts of wind, it will win. If it can't, it will loose
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Old 24-06-2010, 03:04   #11
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Well look guys,

They are both immersed in the current and will both get the same downstream boost. So if the tacking boat has ANY downstream VMG generated by the apparent wind created by drifting in the still air, it must win the race. Calculating the finish times is impossible with the data given.

Cheers,

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Old 24-06-2010, 07:29   #12
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5km/r (2.7kts) of wind,

Ooops!!!
I thought it was 5 knots.

Theres a whole swag of boats that won't more in 2.7 knots let alone on the nose.
Even Sea Life the go fast racing version of the Bene 393 would begin to giggle. Who had the 'Coldest Beer' as the answer? I'm reckoning thats the solution. Lets just all get drunk and who cares who gets somehwere first


Damn, just thought of another point... where is the finish line? is it just one hundred meters in direct front where the boats started? Well then (if the cutrrent is going down river) just sail on a tacck a few hundred meters out then back and get in front of the other boats and match race them
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Old 24-06-2010, 07:31   #13
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[QUOTE]I think that this is the DDWFTTW looking at the camel from the other end. You're trying to start a sh$t fight.[QUOTE]

DDWFTTW??? Never heard of that one before, what's it stand for, please.
WD
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Old 24-06-2010, 08:02   #14
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[QUOTE=IceMan;474934][QUOTE]I think that this is the DDWFTTW looking at the camel from the other end. You're trying to start a sh$t fight.
Quote:

DDWFTTW??? Never heard of that one before, what's it stand for, please.
WD

Dead Down Wind Faster Than True Wind. It produces the longest nastiest argumentative threads.

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Old 24-06-2010, 09:05   #15
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This isn't DDW, it's DUW (dead upwind?) Neglecting wind resistance (which is negligible at 5kph for a "streamlined" boat), all boats on the river move at 5kph downstream towards the finish. The boat that tacks, if he makes any vmg at all will win. The question says the river is wide, so it should be no problem to tack within the area of max current. To solve for time, you really need the polars to be exact. But you can estimate that in a 5 kph wind, a boat that sails 3 kph at 45 degrees relative to the wind will make an additional 2kph downstream, allowing it to finish in about an hour and a half.

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