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

[ProfJohn]

Boat A with the wind is going to win.

Sail boats are better when there is a cross wind, but even the smallest boat can harness a tail wind to move it forward. And most sail boats can exceed the wind speed due to their design.

So Boat A should be able to go faster than the current.


Also, a 10mpg current does not move a boat at 10mph especially since boats are designed to cut through water with the least amount of resistance. A sail boat on a windless day tends to bob up and down and float along very very slowly.


Finally, claiming that the current creates a head wind is a stretch. Sails only work effectively if they are filled with air/wind.

 

[ProfJohn]

Winner Winner, Chicken dinner!



That's only true if the skipper points his boat straight into the wind. If he bears off the wind 40 degrees he can beat windward and make upwind progress, thereby reaching the finish line faster.

http://en.wikipedia.org/wiki/Tacking_(sailing)#Beating

Total fail! Have you ever been on a sail boat?

Sails need wind to work. Without wind they are completely worthless.

Also, without a wind a sail boat will drift. You can not even steer a sail boat without wind in its sails because the boat needs water running across its rudder to steer. Water moving from behind pushing the boat will not allow you to steer the boat properly.

Boat B will drift along about about 2 knots while Boat A will drift at 2 knots, but has 10 knots of wind filling its sail and thus allowing it to sail faster than the current.

BTW I have done this before. I have sat off shore and bobbed up and down with no wind and I have flown along at well in excess of the wind speed.

 

[DisgruntledVirus]

The one that's on a continuous pool will take off.

 

[Ruptga]

B should be a hair slower because instead of neutral wind it effectively has a 10 knot headwind. Not sure how much air drag would slow a boat though.

 

[Blackjack200]

Boat A with the wind is going to win.

Sail boats are better when there is a cross wind, but even the smallest boat can harness a tail wind to move it forward. And most sail boats can exceed the wind speed due to their design.

So Boat A should be able to go faster than the current.

But boat A does not feel any tail wind. It is being moved by the current at the same speed the wind is blowing.

And FWIW, no sailboat in the world can go faster over water than the wind on a dead rund. Boats can exceed wind speed on a beam reach.

Also, a 10mpg current does not move a boat at 10mph especially since boats are designed to cut through water with the least amount of resistance. A sail boat on a windless day tends to bob up and down and float along very very slowly.

A sail boat on a windless day will move with the current/tide, except a little slower due only to the resistance of the stationary air.

Finally, claiming that the current creates a head wind is a stretch. Sails only work effectively if they are filled with air/wind.

Not a stretch at all. In addition to the current, the movement of the boat itself will affect the apparent wind. If there is a 5 knot wind over the beam (90 degree angle to the boat), and the boat starts sailing at 5 knots forward, the apparent wind will shift to 45 degrees and will be 7 knots. The crew will then have to sheet in their sails to adjust.

 

[Blackjack200]

Total fail! Have you ever been on a sail boat?

been on a sailboat? I race them! :biggrin:

Sails need wind to work. Without wind they are completely worthless.

Also, without a wind a sail boat will drift. You can not even steer a sail boat without wind in its sails because the boat needs water running across its rudder to steer. Water moving from behind pushing the boat will not allow you to steer the boat properly.

correct.

Boat B will drift along about about 2 knots while Boat A will drift at 2 knots, but has 10 knots of wind filling its sail and thus allowing it to sail faster than the current.

2 knots? Why wouldn't boat A drift at 10 knots with the water? What is holding it back? Is it at anchor or something?

BTW I have done this before. I have sat off shore and bobbed up and down with no wind and I have flown along at well in excess of the wind speed.

then you should know that boat B, with it's 10 knot apparent wind, would win!

 

[ProfJohn]

Ok... i see where you are coming from with apparent wind etc etc.

But here is the problem:
Boat A travels down stream at 10 knots.

Can Boat B travel down stream at faster than 10 knots by using its 'head wind' ??

I think it becomes an extremely complicated problem.
i.e. Boat A is heading in a straight line towards the finish.
Boat B has to tack back and forth and thus has to travel a further distance.

 

[SlowSpyder]

Boat A. I see it as the above water parts of boat B will have to push through the still air where as the wind is moving exactly with the current for boat A.

 

[Blackjack200]

Ok... i see where you are coming from with apparent wind etc etc.

But here is the problem:
Boat A travels down stream at 10 knots.

Can Boat B travel down stream at faster than 10 knots by using its 'head wind' ??

I think it becomes an extremely complicated problem.
i.e. Boat A is heading in a straight line towards the finish.
Boat B has to tack back and forth and thus has to travel a further distance.

Think of it this way; both boats are on a 10 knot water treadmill of sorts. Boat A is static and cannot move relative to its treadmill, Boat B has a 10 knot headwind. If he does nothing, the wind will blow him backwards on the treadmill and he will lose to boat B. But if he makes any headway against the wind (i.e. moves up on the treadmill), he will beat Boat A.

 

[Blackjack200]

Boat A. I see it as the above water parts of boat B will have to push through the still air where as the wind is moving exactly with the current for boat A.

That would be right except for the bolded.

If the skippers do not do anything, the boat in 0 wind will be slower due to the headwind and will lose. But remember that these are sailboats and we expect the skippers to use their keels, sails, and rudders to their advantage.

 

[ProfJohn]

Think of it this way; both boats are on a 10 knot water treadmill of sorts. Boat A is static and cannot move relative to its treadmill, Boat B has a 10 knot headwind. If he does nothing, the wind will blow him backwards on the treadmill and he will lose to boat B. But if he makes any headway against the wind (i.e. moves up on the treadmill), he will beat Boat A.

Wrong!

Boat A will move downstream at 10 knots. If the water and wind speed are the same the relative wind speed on the boat would be 0. So boat A won't be able to 'sail' but it would still drift downstream.

BTW the treadmill analogy doesn't work in this case. Both boats should be able to drift down stream at close to the speed of the current aka 10 knots.

 

[reksio]

Ok... i see where you are coming from with apparent wind etc etc.

But here is the problem:
Boat A travels down stream at 10 knots.

Can Boat B travel down stream at faster than 10 knots by using its 'head wind' ??

Yes

I think it becomes an extremely complicated problem.
i.e. Boat A is heading in a straight line towards the finish.
Boat B has to tack back and forth and thus has to travel a further distance.

A is not making any progress downstream relative to the water. So any progress downstream relative to the water that B makes will bring it ahead of A.

It is trivial for a boat to make some minimal progress relative to the water, in the opposite direction than the airmass moves relative to the water (tacking upwind). So B will win.

 

[Blackjack200]

Yes



A is not making any progress downstream relative to the water. So any progress downstream relative to the water that B makes will bring it ahead of A.

It is trivial for a boat to make some minimal progress relative to the water, in the opposite direction than the airmass moves relative to the water (tacking upwind). So B will win.

:thumbsup:

 

[reksio]

:thumbsup:

Okay, but what about going upstream in the same wind conditions as in your question. Can one of the boats reach a point up the river from the starting point?

 

[Blackjack200]

Okay, but what about going upstream in the same wind conditions as in your question. Can one of the boats reach a point up the river from the starting point?

If wind conditions were exactly the same as the original question, boat A would feel a 20 mph tailwind and boat B would feel a 10 mph tailwind. Boat A should win.

But I imagine you're asking if a boat could fight a 10 mph current and 10 mph apparent headwind (20 mph true wind), and still make the finish line.

I don't think my boat could do it, but this one could.

http://en.wikipedia.org/wiki/USA_17_(yacht)

 

[reksio]

If wind conditions were exactly the same as the original question, boat A would feel a 20 mph tailwind and boat B would feel a 10 mph tailwind. Boat A should win.

But I imagine you're asking if a boat could fight a 10 mph current and 10 mph apparent headwing (20 mph true wind), and still make the finish line.

I don't think my boat could do it, but this one could.

http://en.wikipedia.org/wiki/USA_17_(yacht)

To make it clear for the others:

Boat A:
Current : 10mph
True wind : 10mph down the current.

Boat B:
Current : 10mph
True wind : 0mph

Can any of the boats go against the current (up the river)?

 

[spidey07]

A will win only because it will have zero force acting against it moving forward via air.

B will have a slight force against it via air resistance.

 

[ProfJohn]

You guys are either missing or ignoring a vital point.

The current is 10 knots and headed towards the finish line.

If Boat A does nothing it will drift at 10 knots towards the finish line.

Same with Boat B.

If Boat B raises its sails and tacks into the wind it can 'sail' at a speed faster than 10 knots. However in order to tack it has to go a further distance than Boat A.

So the question is whether the improvement in Boat B's speed will over come the extra distance it has to travel. I believe the answer is almost certainly no.

Here is why:
Assume that the race is 10 nautical miles in length in a straight line down river.
Boat A will take one hour to finish that distance.

Boat B has to tact and most boats can only tack at a 40-50 degree angle to the wind. So to make the math simple let's assume that the boats in this race can not tack at an angle greater than 45 degrees.

Simple math tells us that for a boat to travel 10 miles downstream at a 45 degree angle will require the boat to travel a total distance of 14 miles.

So the question becomes: can Boat B tack into the wind at greater than 14 knots?

Right now the world record for boat speed vs wind speed is 1.7 aka 1 knot of wind gets you 1.7 knots of speed. And that is certainly based on a boat going with the wind. I have not found a record for a boat going into the wind, but it is certainly slower than 1.4.

 

[ProfJohn]

Here is a diagram that shows boat speed vs wind speed at various angles.

The fastest boat on the chart can barely reach wind speed when sailing into the wind.

So in effect Boat B could sail into the wind, but would only be able to reach 10 knots and would actually get to the finish line faster by coasting on the current alone.
http://sites.google.com/site/yoavraz2/sailingboatspeedvs.windspeed

 

[ProfJohn]

BTW racing upstream into the current would make the race more like the treadmill effect you describe since you would be fighting the river current in order to make progress.

 

[PlasmaBomb]

Trick question is trick...

Redrawn for simplicity -

Boats A and B are identical and sailed by equally skilled crew.

Both are same distance from finish line.

What will be the result ? A B Tie

Why ?

 

[PlasmaBomb]

Here is a diagram that shows boat speed vs wind speed at various angles.

The fastest boat on the chart can barely reach wind speed when sailing into the wind.

So in effect Boat B could sail into the wind, but would only be able to reach 10 knots and would actually get to the finish line faster by coasting on the current alone.
http://sites.google.com/site/yoavraz2/sailingboatspeedvs.windspeed

You are forgetting to add 10 to the result...

So for the slowest boat (half-toner) at at 30 degree offset the distance will be 1/0.86 = 115% of the straight line distance. However the speed will be 10 + chart speed (2) = 12 knots. Thus it's speed is 120% of the drifting speed and it only has to cover 115% of the distance. Boat B wins.

The more "racy" the boat the faster it will be on a beat. For example the Tornado will manage over 6 knots at a 45 degree beat, while only having to go an extra ~30% distance.

 

[ProfJohn]

^ 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.

 

[ProfJohn]

You are forgetting to add 10 to the result...

So for the slowest boat (half-toner) at at 30 degree offset the distance will be 1/0.86 = 115% of the straight line distance. However the speed will be 10 + chart speed (2) = 12 knots. Thus it's speed is 120% of the drifting speed and it only has to cover 115% of the distance. Boat B wins.

The more "racy" the boat the faster it will be on a beat. For example the Tornado will manage over 6 knots at a 45 degree beat, while only having to go an extra ~30% distance.

That makes sense.

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

 

[mugs]

You guys are either missing or ignoring a vital point.

The current is 10 knots and headed towards the finish line.

If Boat A does nothing it will drift at 10 knots towards the finish line.

Same with Boat B.

If Boat B raises its sails and tacts into the wind it can 'sail' at a speed faster than 10 knots. However in order to tact it has to go a further distance than Boat A.

So the question is whether the improvement in Boat B's speed will over come the extra distance it has to travel. I believe the answer is almost certainly no.

Here is why:
Assume that the race is 10 nautical miles in length in a straight line down river.
Boat A will take one hour to finish that distance.

Boat B has to tact and most boats can only tact at a 40-50 degree angle to the wind. So to make the math simple let's assume that the boats in this race can not tact at an angle greater than 45 degrees.

Simple math tells us that for a boat to travel 10 miles downstream at a 45 degree angle will require the boat to travel a total distance of 14 miles.

So the question becomes: can Boat B tact into the wind at greater than 14 knots?

Right now the world record for boat speed vs wind speed is 1.7 aka 1 knot of wind gets you 1.7 knots of speed. And that is certainly based on a boat going with the wind. I have not found a record for a boat going into the wind, but it is certainly slower than 1.4.

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