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Old 24-06-2010, 09:10   #16
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It produces the longest nastiest argumentative threads.

John
yeah and then the jerk how started the thread saya: Ohhhh the current wasn't going that way, who said the current was going that way. And you have to sit there looking like an idiot "If you read the question corectly....."

Who gives a **** what engineers think? Only a lame brain goose sits about thinking of really wide rivers with no wind next to them.
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Old 24-06-2010, 10:23   #17
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Is this theoretical boat a monohull or catamaran. LoL
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Old 24-06-2010, 10:35   #18
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I wonder how fast BMW Oracle can go in 5 knots apparent. And how high it can point.
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Old 24-06-2010, 11:20   #19
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Anyone who's been calmed and uses their engine knows this. Just because you're on a river means nothing; you don't get something for nothing. The shortest distance wins.
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Old 24-06-2010, 15:53   #20
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Tacking at 45 to the real wind angle gives at 2 knots .7 knots vmg. That gives a finishing time of 1.75 hours.
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Old 24-06-2010, 16:07   #21
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Northern or Southern hemisphere ?
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Old 24-06-2010, 17:11   #22
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Anyone who's been calmed and uses their engine knows this. Just because you're on a river means nothing; you don't get something for nothing. The shortest distance wins.
But nobody is motoring anywhere here.

The only thing that matters is motion relative to the water. They are both in the same current so whoever gets ahead relative to the water will win.

The easiest way to think about it is to ignore the land. If the boats can't see land (and don't have GPS), then for all they know they are sitting still and the wind is 5kmh on the bow.

The boat that sits still will be pushed slightly backwards with respect to the water.

The boat which attempts to tack will make way relative to the water, if their boat is capable of doing so in that light air.

That's the "something for nothing" that sailing allows - making way into the wind.
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Old 24-06-2010, 19:03   #23
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but the tacking boat must travel a further distance.

at 45 deg tack angle, I'd be an extra 4 km ish

so it's not a matter of making 'any' headway relative to the water, it needs to make enough to cover 14km faster than the other boat does 10 km.

If it's a light sportsboat, it could do it, if it's an old clunker it won't
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Old 24-06-2010, 19:24   #24
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but the tacking boat must travel a further distance.

at 45 deg tack angle, I'd be an extra 4 km ish

so it's not a matter of making 'any' headway relative to the water, it needs to make enough to cover 14km faster than the other boat does 10 km.

If it's a light sportsboat, it could do it, if it's an old clunker it won't
No, any headway will be enough, because the current is taking both boats towards the finish line at the same VMG. Beyond that, one boat is slowed by the wind, the other is making way into it.

To put it another way, travelling across the stream does not make you any slower towards the finishing line, despite the extra distance. The current is still pulling you in the direction of the finish line at the same rate.

Think of this exactly analogous situation, which you get to just by shifting your point of thinking to "relative to the water" from "relative to the land":

Two boats are at rest in the water, with a 5kmh wind on their bows. The "finishing line" is a big piece of string, held at right angles to the wind, moving toward the boats at 5kmh. One boat stays still, pointed into the wind. One boat sails, tacking into the wind. Which boat will the string hit first?

This is exactly the same scenario.
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Old 24-06-2010, 19:51   #25
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it is impossible to answer with the information given. There has to be a current gradient... whether the flow is laminar or not, there will still be a current gradient, so that ther will have to be maximum current flow-rate mid stream and progressively less current approaching the edge, so, since all the breeze is apparent, the breeze will decrease towards the edges (because the current flow is decreased). Without more information, it is not possible to discern whether or not the benefits of using the apparent wind to tack back and forth are greater or less than the negative effect of reduced flow-rate away from mid-stream.
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Old 24-06-2010, 20:08   #26
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Agreed on the current gradient issue (mentioned earlier), but I think the thought experiment is not that sophisticated. That's why they have stipulated a "very wide" river - presumably wide enough that the current is consistent over enough of the river to tack back and forth. It's only a hypothetical, after all...
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Old 24-06-2010, 20:44   #27
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if the current is 5 kph and the true windspeed is 0, that means the apparent wind will be 2.7 knots. On the nose.

So all you need to win this race is a boat that will tack close-hauled in a 2.7 knot breeze. Or someone greedy enough for the silver to shrimp his chute.

Count me out either way. No way I'd drift a river with a 5-kph current without wind. Especially not to pick up another pickle dish.
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Old 24-06-2010, 20:45   #28
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I think paradix has expressed it very well.
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Old 24-06-2010, 20:51   #29
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Bash -
How do you figure the apparent wind will be 2.7 knots on the nose? If you are pushing straight into still air at 5 knots you are producing 5 knots of wind (or force or whatever you wish to call it).
You can produce the same effect riding a bicycle into still air. If you pedal at 5mph you get 5mph worth of breeze on your face.
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Old 24-06-2010, 20:57   #30
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Bash -
How do you figure the apparent wind will be 2.7 knots on the nose? If you are pushing straight into still air at 5 knots you are producing 5 knots of wind (or force or whatever you wish to call it).
You can produce the same effect riding a bicycle into still air. If you pedal at 5mph you get 5mph worth of breeze on your face.
reread the original post. You're going 5 kilometers per hour into the wind. That translates to 2.7 knots.

(Unless the OP was mistaken in assuming that KPH = Knots, which it does not.)
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