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Pirate Escape

Science brain teasers require understanding of the physical or biological world and the laws that govern it.

 

Puzzle ID:#16701
Fun:*** (2.42)
Difficulty:*** (2.28)
Category:Science
Submitted By:JakeAadminus*mod!!!
Corrected By:boodler

 

 

 



A pirate is floating down a river in a modern sailboat after having just pillaged a small village upstream. The current is 3 knots with respect to the land. The wind is zero knots, with respect to the land. The pirate wants to proceed downriver as quickly as possible to avoid capture.

Should he raise the sails, or not?




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Comments

Rowsdower**
Feb 24, 2004

Okay, all I know about tacking is that a ship can sail directly into the wind by sailing at angles to the wind, or with a cross wind by angling the sails. How does one calculate relative speeds and such?
(user deleted)
Feb 24, 2004

Um, I didn't quite follow this one...Could you explain it?
Palsha**
Feb 24, 2004

Rowsdowner...you have to calculate the velocity relative to the land to the boat, then you have to calculate the velocity of boat, without any other incidents. It's like a physics problem which I love! Let's see some more jake
curtiss82**
Feb 25, 2004

I disagree. I think that the zig zagging would slow you down because it will greatly increase the distance you will have to travel. Even though you might be traveling a little over 3 knots, you might double or triple (maybe more) the distance traveled from the zig zagging. As far as I know, tacking is for when the wind is actualling blowing and from watching sale boats on TV, it takes a really long time to travel a short distance because of all the zig zagging. I bring up the idea that the wind has to be blowing because when you zig zag, you will pretty much be traveling perpendicular to the current and you will lose the force pushing you down the river.
bluetwo*
Feb 25, 2004

he should not raise the sails. he should turn on the motor (as most "modern sailboat[s]" have one).
curtiss82**
Feb 25, 2004

There is another thing that I want to mention but forgot about in my last comment about why there needs to be an actual wind instead of apparent one. When a person sails against the wind, they turn sideways and the wind that they were traveling head on into now becomes a cross wind. They then can gain speed going sideways by tacking, turn back into the wind to travel in the direction they desire, and then back the other way before they lose their speed. In the situation described in the problem, the pirate will always be traveling head on into the apparent wind making it impossible for him to tack and gain speed. With this, the additional distance traveled, and the loss of the current pushing the pirate (as described in my last comment) I would have to say that the answer is wrong and that it would be better to leave the sails down and travel straight down the river.
(user deleted)
Feb 26, 2004

Jake, these teasers are old, stale chestnuts that have already been answered many times. curtiss82, the sails on the boat can't tell the difference between "an actual wind" and "an apparent one". Are you trying to tell us is that a sailboat cannot make headway into a wind? This has been proven wrong by wind-surfers and beachcombers all the world over. It is natural to assume that the pirate must not raise his sails, but this is only if his sail can't be moved. If they can, I'll race you any day.
curtiss82**
Feb 26, 2004

Anonymous, go to http://www.physlink.com/Education/AskExperts/ae438.cfm , it explains how tacking works. In order to tack, you have to position the boat at an angle near 90 degrees because as the triangular sail inflates with a wind it creates an airfoil shape. As subsequent wind passes around the sail (airfoil), negative pressure is induced out front of and on the leeward side of the sail. This in turn causes surrounding air to rush into the sail and propel the boat further. This sail/airfoil action is compounded, as the boat travels faster, the wind around the sail creates more negative pressure, causing the boat to travel faster causing more negative pressure and so forth. In other words, the wind has to be at a 90 degree angle in order to travel around the sail and induce the negative pressure. In this teaser, there is no actual wind blowing, there is an aparent wind from the boat traveling through still air. So no matter what position the boat is in, the wind will always be head on and never at a 90 degree angle.
curtiss82**
Feb 26, 2004

One more thing, even if the tacking worked, common sense tells me that the increase speed is going to be too small to compensate for the increase distance that the pirate would travel from zig zagging because the wind speed is so small.
(user deleted)
Feb 26, 2004

curtiss82, when I read your post I was a little skeptical about Bernoulli being the major factor in tacking. Then I visited the site that you mentioned and found out that it is indeed only a minor factor. Tacking is mainly achieved by the shapes of the hull and keel of a boat and the angle of the boat and sail relative to the wind - not the shape of the sail.
(user deleted)
Feb 26, 2004

There is essentially no difference between a pirate boat floating down a river at 3 knots and a sail boat sitting in a harbour where there is a 3 knot wind blowing. If the sail boat can tack into the wind, the pirate boat can do the same. I wish I could explain it better.
unklemykeAus*
Feb 29, 2004

If our pirate is not extremely familiar with the river in question, he could get himself in a peck of trouble trying to tack across it - sandbars, shallows, snags, - you name it. There is a reason that even the most seasoned captains are required to use the services of a pilot on the Mississippi - and that's a really BIG river. I think he'd be farther ahead sticking t the main channel and hoping his pursuers don't have something that's motorized.
bbngk
Mar 03, 2004

A pirate in a modern sailboat ?
jimbo*au*
Mar 04, 2004

I am afraid I agree with Curtis. Take the exaggerated case of tacking at right angles to the current. The windforce produces lateral movement of the boat but no progress downstream. I have done this in a canoe. A thrust with the paddle will propel the canoe across the river with very little dorwnriver component. Also, the extra zig-zag distance travelled through tacking would be a major factor in slowing the downstream progress.
KatelinAus*
Mar 08, 2004

Debates are very interesting...
(user deleted)
Mar 11, 2004

jimbo, taking an exaggerated case proves nothing. Here's why: let's take an even more exaggerated case where the pirate is "sailing with the wind", it's obvious in this case that the pirate should not raise his sails if he is going to "sail with the wind". If I can find some reference to this, I will post it here.
jaycr*us*
Mar 16, 2004

I figured that his sail would the Jolly Rodger on it and he may want to keep it hidden.
Palsha**
Mar 22, 2004

curtiss, you have too much time on your hands. Anylaze how you can use your time more wisely than trying to act like your smart because you sound dumb due to your wrong answer.
curtiss82**
Mar 22, 2004

Palsha, what credentials do you have to make that kind of comment. I guarantee you that I am smarter than you and I have the credentials to back it up. It is my opinions that tacking will not help in this situations and other people seem to agree.
Palsha**
Mar 29, 2004

My creds. is the following- I live under the bridge under I-40 and I dropped out of middle school. If you are smarter than me, then I must crap bigger turds than you. Now what are your creds.?
curtiss82**
Mar 29, 2004

A B.S. in Chemical Engineering from the Colorado School of Mines, and I'll be attending graduate school at MIT this fall to get my Ph.D.
Palsha**
Mar 29, 2004

curt....don't take me seriously in my above post. I'm just giving you a hard time. I'm getting my B.S. in Biology & Chemistry from Duke at the end of this semester, then off to medical school at ECU or Duke. It all depends on my finances. MIT is a very very very nice school! Great education! I think they've had the most Nobel Prize winners to come from their. What are you studying in for your Ph.D?
KatelinAus*
Mar 29, 2004

...definately interesting...
JakeAus*
Apr 05, 2004

I forgot to mention that the pirate has a cannon and a really cool parrot. So, he aims the cannon backwards and fires it to gain speed, and the parrot grabs onto the bow and pulls the boat forwards as fast as its little wings can carry it.

Also, the pirate sits behind the sails and blows as hard as his lungs can. The wind leaving your mouth travels at 20MPH, so he would be adding this speed to his boat. Its just a matter of physics.
KatelinAus*
Apr 16, 2004

GO JAKE!!!!!
mad-adeAhu*
Apr 18, 2004

well, now you put it like that
hersheykiss8908*
Jun 27, 2004

i agree with all of curtiss82's comments. i think teasers should be challenging, not impossible and confusing like this one.
drussel3*us*
Jun 02, 2005

I am confused. Wouldn't raising a sail actually cause drag on the boat, causing it to go slower???
markmonninAus*
Sep 21, 2005

YAR... a pirate's life for me.
markmonninAus*
Sep 21, 2005

YAR... a pirate's life for me.
stil*us*
Dec 20, 2006

At 3 knots, I could limp a seventeen-minute mile. I can't picture that as being enough to make my kilt flap. Is it enough to fill a sail?
Even though it could add as much as 41% to the length of the course taken, turning the boat to one side or the other should make the effect of the river's current greater.



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