Sailing trivia (get in here if you like airplanes and treadmills)

[PlasmaBomb]

^ you changed the question completely.

In the original question Boat A can float to the finish line faster than Boat B can sail to the finish line.

In this question Boat A will not move at all. So Boat B wins.

I took out the trick part of the question... I though that was obvious... the intention was to make the answer obvious.

Obviously...

 

[ProfJohn]

I know absolutely nothing about sailing except this: it's spelled "tack."

Duh!!! 🙂

 

[Blackjack200]

That makes sense.

But it also assumes that using 'current' to create wind doesn't have a detrimental effect on boat speed.

Why would it? You understand that boats sail by exploiting the movement of the air over the water, right? Why would it matter what's causing that movement?

 

[PlasmaBomb]

I know absolutely nothing about sailing except this: it's spelled "tack."

lulz.

Yes it is indeed "Tack".

 

[Blackjack200]

I know absolutely nothing about sailing except this: it's spelled "tack."

Get a clew.

 

[ProfJohn]

Ok....

so Boat B travels down river at 10 knots.
It opens its sails and sails at a 45 degree angle at 10 knots.

10 knots at 45 degree angle = 7 knots down stream.

So Boat B is traveling downstream at 17 knots? Sound right?

 

[ProfJohn]

BTW this reminds me why I hate math 🙂

I am going to give this problem to my nephew the engineering student and see what he gets.

 

[Blackjack200]

Ok....

so Boat B travels down river at 10 knots.
It opens its sails and sails at a 45 degree angle at 10 knots.

10 knots at 45 degree angle = 7 knots down stream.

So Boat B is traveling downstream at 17 knots? Sound right?

It's travelling over land at 17 knots downstream.

 

[Mr.IncrediblyBored]

Is this kinda like how jumping side to side in Quake and Halflife makes you move faster forward? The folks at id and Valve must be sailors.

 

[Blackjack200]

BTW this reminds me why I hate math 🙂

I am going to give this problem to my nephew the engineering student and see what he gets.

He'll probably design a bridge so that sailboats are no longer required.

 

[ProfJohn]

He'll probably design a bridge so that sailboats are no longer required.

Nah... he'll proclaim it to be a stupid question since everyone is riding jet skis etc etc

 

[PlasmaBomb]

Ok....

so Boat B travels down river at 10 knots.
It opens its sails and sails at a 45 degree angle at 10 knots.

10 knots at 45 degree angle = 7 knots down stream.

So Boat B is traveling downstream at 17 knots? Sound right?

It's travelling at 20 knots, it's apparent speed over ground is 17 knots...

 

[reksio]

^ you changed the question completely.

No, he just changed the perspective. This is what an obsever floating in the water will see. And all observers must agree which boat wins.

 

[reksio]

Is this kinda like how jumping side to side in Quake and Halflife makes you move faster forward? The folks at id and Valve must be sailors.

They are probably just lazy programmers, and didn't nomalize the velocity vector. So when you press UP+LEFT you run 1.41 times faster than just with UP pressed. That's how it was in DOOM IIRC.

 

[reksio]

No takers on this?:

Boat A:
Current : 10mph (relative to ground)
Wind : 10mph (relative to ground, same direction as current)

Boat B:
Current : 10mph (relative to ground)
Wind : 0mph (relative to ground)

I will add a bonus case

Boat C:
Current : 20mph (relative to ground)
Wind : 10mph (relative to ground, same direction as current)

Which of those sail boats could potentially go upstream, and reach a point higher up the river than the start?

 

[Blackjack200]

No takers on this?:

Boat A:
Current : 10mph (relative to ground)
Wind : 10mph (relative to ground, same direction as current)

Boat B:
Current : 10mph (relative to ground)
Wind : 0mph (relative to ground)

I will add a bonus case

Boat C:
Current : 20mph (relative to ground)
Wind : 10mph (relative to ground, same direction as current)

Which of those sail boats could potentially go upstream, and reach a point higher up the river than the start?

I would say B and C should be able to make progress upstream (as viewed by an observer on land).

 

[PlasmaBomb]

No takers on this?:

Boat A:
Current : 10mph (relative to ground)
Wind : 10mph (relative to ground, same direction as current)

Boat B:
Current : 10mph (relative to ground)
Wind : 0mph (relative to ground)

I will add a bonus case

Boat C:
Current : 20mph (relative to ground)
Wind : 10mph (relative to ground, same direction as current)

Which of those sail boats could potentially go upstream, and reach a point higher up the river than the start?

Like this?

A: Which ever one was sensible enough to bring an outboard...

 

[reksio]

Like this?

Yes, thanks for the picture.

A: Which ever one was sensible enough to bring an outboard...

No motors. Just sailing upstream. Which one(s) can do it?

 

[reksio]

I would say B and C should be able to make progress upstream (as viewed by an observer on land).

:thumbsup: Correct, efficient sail boats could do it. It is symmetrical to the downstream race:

Downstream race : Relative to the airmass B is going downstream, faster than the stream.

Upstream race : Relative to the water B (and C) are going downwind, faster than the wind.

 

[reksio]

Next question:

Can a glider (plane without engine) stay airborne permanently in an area with only still air and some downdrafts(sinking air), but no updrafts(rising air)?

 

[PlasmaBomb]

:thumbsup: Correct, efficient sail boats could do it. It is symmetrical to the downstream race:

Downstream race : Relative to the airmass B is going downstream, faster than the stream.

Upstream race : Relative to the water B (and C) are going downwind, faster than the wind.

I would just like to point out most boats couldn't make progress upstream.

e.g. B -
Downstream = - 10kt
Apparent wind = 10kt
Full run = 5kt (the boats "forward" movement cancels out some of the apparent wind)

Speed over ground = - 10 + 5 = -5kt

Even the 18ft open class boat only manages 12kt at ~35 degrees off the wind so it's apparent over ground speed = - 10 + (cos35)*12 = -0.2kt over ground.

Your best bet is to drop anchor and wait for the current to slow (if in a tidal region). Been there done that at the narrows in Strangford lough (7.5kt tidal).

 

[PlasmaBomb]

Next question:

Can a glider (plane without engine) stay airborne permanently in an area with only still air and some downdrafts(sinking air), but no updrafts(rising air)?

Nasa says No.

 

[DominionSeraph]

Next question:

Can a glider (plane without engine) stay airborne permanently in an area with only still air and some downdrafts(sinking air), but no updrafts(rising air)?

No. Matter has a finite lifespan. Even if the glider was resistant to normal structural decay, the atoms that make up the air and glider are not eternal.
Big Rip or Big Crunch could come before that, though.

A glider could stay "airborne" in the gasses collected at a Lagrangian point for a very long time.

But no.

 

[reksio]

I would just like to point out most boats couldn't make progress upstream.

e.g. B -
Downstream = - 10kt
Apparent wind = 10kt
Full run = 5kt (the boats "forward" movement cancels out some of the apparent wind)

Speed over ground = - 10 + 5 = -5kt

Even the 18ft open class boat only manages 12kt at ~35 degrees off the wind so it's apparent over ground speed = - 10 + (cos35)*12 = -0.2kt over ground.

Your best bet is to drop anchor and wait for the current to slow (if in a tidal region). Been there done that at the narrows in Strangford lough (7.5kt tidal).

Correct. Most sail boats wouldn't make it. But the USA-17 that BlackJack mentioned could do the following trick:

Start at the downwind mark and simultaneously release a balloon at the upwind mark. While the balloon floats:

upwind mark -> downwind mark

the boat goes:

downwind mark -> upwind mark -> downwind mark

and still beats the ballon.

 

[reksio]

Nasa says No.

Where exactly do they say this?