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Old 01-12-2012, 14:58   #1
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Sailing Puzzle

I had a quick search and can't find that this has been posted before.
A small puzzle to mull over:

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?

What are your thoughts?
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Old 01-12-2012, 15:23   #2
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Re: Sailing Puzzle

Second heat.
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Old 01-12-2012, 15:23   #3
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Re: Sailing Puzzle

Is this a trick question

I will try a answer anyway

The afternoon heat will be quicker although perhaps a boat that performs very well to windward could be faster in the morning (but not your average cruising boat).

In the morning, the boats will experience an apparent wind on 10 kts right on the nose. If they ignore it and stay midstream, it will be an opposing force and will slow the boat down a little. If they tack downstream, the distance travelled through water will increase although this is somewhat compensated for by the increase in boat speed however time is lost at every tack. If each board is taken right to the edge of the river (reducing the number of tacks), then the advantage of current is reduced as the current normally is weaker (slower) at the edges than at midstream. There may even be back eddies at the edges depending on the shape of the bottom.


In the afternoon the apparent wind will be 0 kts and the boat will travel the 10 miles at 10 kts.

Interesting as the morning race requires calculations for both speed and distance through water as well as speed and distance over the bottom plus the vector addition of boat speed and current speed less windage and leeway.
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Old 01-12-2012, 16:47   #4
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Re: Sailing Puzzle

Nautical miles or statute miles?
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Old 01-12-2012, 16:55   #5
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Re: Sailing Puzzle

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Nautical miles or statute miles?
LOL, lets make it nm.
Are you game to reply ?
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Old 01-12-2012, 16:59   #6
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Re: Sailing Puzzle

Might depend on type of propulsion, but I say second heat also.

Is it a laden swallow or an unladen swallow?
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Old 01-12-2012, 17:07   #7
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Re: Sailing Puzzle

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Might depend on type of propulsion, but I say second heat also.

Is it a laden swallow or an unladen swallow?
Coconuts are not compulsory
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Old 01-12-2012, 17:20   #8
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Re: Sailing Puzzle

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Originally Posted by Wotname View Post
......
If they tack downstream, the distance travelled through water will increase although this is somewhat compensated for by the increase in boat speed however time is lost at every tack. If each board is taken right to the edge of the river (reducing the number of tacks), then the advantage of current is reduced as the current normally is weaker (slower) at the edges than at midstream. There may even be back eddies at the edges depending on the shape of the bottom.
.......
To simplify things, lets say the river is one nm wide, dead straight and of uniform depth almost to the bank, no overhanging trees.

So, lets recap the conditions:
10nm
Coconuts not compulsory
1nm wide, uniform depth, straight river

Any other queries or concerns?
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Old 01-12-2012, 17:34   #9
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Re: Sailing Puzzle

1st heat. (I'm sorry to hear Wotname's bad opinion of cruising boats! despite whatever chop gets built with the apparent wind, I'm pretty sure I could make every tack)

The 2nd heat would be cancelled.No steerage, no race.
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Old 01-12-2012, 17:39   #10
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Re: Sailing Puzzle

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1st heat. (I'm sorry to hear Wotname's bad opinion of cruising boats! despite whatever chop gets built with the apparent wind, I'm pretty sure I could make every tack)

The 2nd heat would be cancelled.No steerage, no race.
Sigh. You guys are making this tough. The fifteen knot tail wind in the second heat dropped to 10 knots just as the first boat crossed the start line and the wind remained steady in speed and direction for the following hour. No cancellation.
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Old 01-12-2012, 17:52   #11
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Re: Sailing Puzzle

The morning heat with 10kts current and no wind is same as no current and 10 kts headwind, so boats might tack upwind at 2 or 3kts course made good (over water), which is 12 to 13kts over ground.

The afternoon boats will just drift at 10kts thus the morning heat has faster time.

Shoot, this is my first post here, after years of on/off lurking..
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Old 01-12-2012, 17:56   #12
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Re: Sailing Puzzle

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The morning heat with 10kts current and no wind is same as no current and 10 kts headwind, so boats might tack upwind at 2 or 3kts course made good (over water), which is 12 to 13kts over ground.

The afternoon boats will just drift at 10kts thus the morning heat has faster time.

Shoot, this is my first post here, after years of on/off lurking..
Welcome Capt Rottnest !
Glad this little challenge has brought you out of hiding
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Old 01-12-2012, 17:59   #13
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Re: Sailing Puzzle

Multis or monos?
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Old 01-12-2012, 18:04   #14
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Re: Sailing Puzzle

Second heat speed is 10 knots.

First heat let's change the frame of reference. Picture yourself in the middle of the ocean on your boat in a 10 knot current. Wind relative to the ocean floor is 0 knots. With the sails down such that you're not moving relative to the water, the boat sees 10 knots of wind. You put up sails and sail upwind. Let's say your knotmeter reads 5 knots through the water. If you tack through 90 degrees your VMG is 0.7 of that so 3.5 knots made good to the east through the water. Neither you nor the boat can detect that you're in current, all you know is that there is 10 knots of wind and you sail upwind through that parcel of water.

The entire body of water is traveling at 10 knots, your speed in this body of water is 3.5 knots, so you add the two to get 13.5 knots east relative to the bottom.

First heat wins.


The physics of sailing

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Old 01-12-2012, 18:07   #15
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Re: Sailing Puzzle

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Multis or monos?
No discriminating here, multi or mono entries permitted, vessels do not have to be blue water capable, lightweight racing anchors acceptable too. Oh, and no security issues, so weapons can be left behind (remember every ounce/gram counts when racing, so no need to disadvantage yourself unnecessarily). Politics can be discussed in the bar afterwards, but brawling is discouraged.

Ps quit stalling laddie and let's have your answer
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