A racing Brain teaser

[Guest]

Sorry, still don't believe there will be any apparent breeze created at only 5km/hr. If the water was raging along at 15 knots there would be enough turbulence to disrupt the boundary layer and you'd feel a breeze in your face.

[khayyam 71]

To those who are worried about hydrodynamic drag i.e. the boat going slower than the river coz of drag-this is zero, assume we are past initial boundary conditions (they have already started the race at the max speed they will achieve) the "non sailing" vessel is moving at the same speed as the water therefore no drag.

 

The sailing vessel is "taking" energy from the interface of two fluids moving at different velocities- this is actually a pretty good definition of sailing. (yes the air is a fluid - a very thin one).

 

agreed. it seems like the precise winning time is difficult to get at, though, aside from

[Guest Dry Reach]

To those who are worried about hydrodynamic drag i.e. the boat going slower than the river coz of drag-this is zero, assume we are past initial boundary conditions (they have already started the race at the max speed they will achieve) the "non sailing" vessel is moving at the same speed as the water therefore no drag.

 

The sailing vessel is "taking" energy from the interface of two fluids moving at different velocities- this is actually a pretty good definition of sailing. (yes the air is a fluid - a very thin one).[/quote

 

 

 

You cannot be accurate if you assume drag is zero! as it clearly is not.

 

Skin friction of any surface, no matter how it is presented to the "form" (water or air) it is moving through is drag. this has to be calculated into the formula to answer the "time taken " part of the question! or you are just guessing.

 

as i said in my Board example above... a "object" that presents the broadest surface area to the flow for the least mass is going to flow faster than a toothpick that weighs 5KG!

 

try pulling a 5kg bucket at 5 knots through static water versus a 5kg pipe (by its end)! it not the weight that is producing the force on the tow rope it is the Drag / friction...

 

so we can conclude that the yact tacking (bucket) will have more speed than the other boat that is pointed down the river like a pipe! If both are the same mass.

[Guest]

But surely the boat is simply floating in a moving mass of water, not moving through the water at all. So zero drag.

[Guest Dry Reach]

To those who are worried about hydrodynamic drag i.e. the boat going slower than the river coz of drag-this is zero, assume we are past initial boundary conditions (they have already started the race at the max speed they will achieve) the "non sailing" vessel is moving at the same speed as the water therefore no drag.

 

The sailing vessel is "taking" energy from the interface of two fluids moving at different velocities- this is actually a pretty good definition of sailing. (yes the air is a fluid - a very thin one).

 

 

 

 

DR sut up mate you're dribbling.

 

.

 

Sut? thanks for pointing that out "MAte", i bow to your superior knowledge on the dribbling subject!

[Guest Brucey]

[/Quote="Brucey]

Wrong air is a gas and compressable. Fluids are incompressable. It does have a moisture content but on most occasions the temperature is above the dew point and as such it is suspended in the gas.

 

DR sut up mate you're dribbling.

 

Astounded at the confusion this has caused, but now unsuprised with some of the sh*t you see on the track.

 

Apologies

 

I used the wrong term, Liquids are as I described incompresable, in fact both liquids and gas which can vary their shape easily are fluid and as such considered fluids.

 

The sailing question is surely as most have outlined ( the moving escalater is a great analogy) where to even be moving slowly, tacking into your 5km aparent (or as BOBO said, slightly less because the body of water will cause some movement of air in the same direction due to the resultant water air boundry friction) and even a mighty Varient will make some VMG into 3km/hr head wind and so b faster down river.

 

The point that a boat just going with the flow, will do slightly less than the 5km as a result of the wind drag on the rig is right, so it wont be 2hr as was pionted out but a little more.. Guess the best way there is to do a Dirty Deeds and do the Inverted thing so the sails are like a sea anchor and the hulls minimise wind resistance. But your Variant will win.

 

sh*t look now Ive caught it too, cant shut up

[Guest Dry Reach]

[/Quote="Brucey]

Wrong air is a gas and compressable. Fluids are incompressable. It does have a moisture content but on most occasions the temperature is above the dew point and as such it is suspended in the gas.

 

DR sut up mate you're dribbling.

 

Astounded at the confusion this has caused, but now unsuprised with some of the sh*t you see on the track.

 

Apologies

 

I used the wrong term, Liquids are as I described incompresable, in fact both liquids and gas which can vary their shape easily are fluid and as such considered fluids.

 

The sailing question is surely as most have outlined ( the moving escalater is a great analogy) where to even be moving slowly, tacking into your 5km aparent (or as BOBO said, slightly less because the body of water will cause some movement of air in the same direction due to the resultant water air boundry friction) and even a mighty Varient will make some VMG into 3km/hr head wind and so b faster down river.

 

The point that a boat just going with the flow, will do slightly less than the 5km as a result of the wind drag on the rig is right, so it wont be 2hr as was pionted out but a little more.. Guess the best way there is to do a Dirty Deeds and do the Inverted thing so the sails are like a sea anchor and the hulls minimise wind resistance. But your Variant will win.

 

sh*t look now Ive caught it too, cant shut up

 

 

Where's that dribbling icon?... oh fart... this will do

[wheels 543]

Wow!! you guys are getting waaay to complex in your deducing. Don't bring experience into it. It is a simple maths equation. In fact I bet the person that came up with this probably has never sailed in his life. So stick to the basic mathmatic principles.

Current is 5kts. No wind, so you accept that as Zero. So boats are now moving at 5kts with current and thus wind will be 5kts opposing them. Forget friction of water, forget hydrodynamic forces, forget even what the Polars are for a specific boat. Because that is not given. You can not factor in what is not given. A you can not factor in figures that can not be arrived at if you don't have the info. So it is a simple issue of trigonometry. You know, the fandangle of the angles that dangle.

Errr, without looking back at the posts, the poster that drew it out is most likely the closest to being correct.....maybe.

[Murky 1]

Hope you've got the protest forms ready, there are going to be at least 15 factions who come away feeling hard done by.

[wheels 543]

Hehe. I was just thinking something similar. And to add that, this is a heck of a lot more fun than a simple maths question anyway.

Besides, Isn't it about time Math grew up and solved it's own problems??

[Guest Dry Reach]

But surely the boat is simply floating in a moving mass of water, not moving through the water at all. So zero drag.

 

 

ummm? well that means everthing no matter whats size shape and weight will "move" at the same speed, and we know they don't!

 

Whaka te whanau?

 

My brain hurts!

[MrWolf 0]

Wow!! you guys are getting waaay to complex in your deducing. Don't bring experience into it. It is a simple maths equation. In fact I bet the person that came up with this probably has never sailed in his life. So stick to the basic mathmatic principles.

Current is 5kts. No wind, so you accept that as Zero. So boats are now moving at 5kts with current and thus wind will be 5kts opposing them. Forget friction of water, forget hydrodynamic forces, forget even what the Polars are for a specific boat. Because that is not given. You can not factor in what is not given. A you can not factor in figures that can not be arrived at if you don't have the info. So it is a simple issue of trigonometry. You know, the fandangle of the angles that dangle.

Errr, without looking back at the posts, the poster that drew it out is most likely the closest to being correct.....maybe.

 

 

5 knots is not 5km/h.

The question is 5km/h ?

 

Should be a bottle of rum for the winner !@

[Guest Dry Reach]

But surely the boat is simply floating in a moving mass of water, not moving through the water at all. So zero drag.

 

 

AH!... you mean this scenario...

 

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.

[wheels 543]

Good spotting Mr Wolf. Darn it though, I thought we might have got right through without anyone spotting that point.

 

Dry Reach, you really must cut down on the coffee mate.

[Guest Dry Reach]

Good spotting Mr Wolf. Darn it though, I thought we might have got right through without anyone spotting that point.

 

Dry Reach, you really must cut down on the coffee mate.

 

 

20 a days ok is'nt it!

 

P.S.... Wheels...Am i helping to confuse the plebs??? ... (cr4 global spec!. )

[wheels 543]

Oops, slightly side tracking, but they say now that is you have more than fie cups a day, it may stop Alzhi........5.30PM and I think it's going to be a Frost tonight

[Guest]

20 a days ok is'nt it!

 

I bloody well hope so

[wheels 543]

And the answer is!!!!

 

you win the race, because you are sailing upwind. The winning time is: less than the time your opponents took.

 

When you all cast off you all feel a wind (a perceived wind) in your face as you float downstream. The wind is really the still air, relative to the shore, that you are moving past.

 

Relative to the river, in which you float, the wind is in your face, and all modern sailboats can sail into the wind--that is, tacking back and forth, up wind. Your path will be much longer than your opponents', but your downstream time will be faster.

 

Your opponents' maximum speed into the wind, and as measured by the land, will never be faster than the 5 KM/H current. Your speed will be faster by the speed you are able to sail upwind.

[SloopJohnB 322]

Now thats the answer I was going to use Wheels.......

[wheels 543]

Sorry I bet you to the post SJB.