Mythbusters: Blow your own sail review

[sophiecentaur]

Oh yes, definitely. The main effect would, I'm sure, be the direction in which the air spills off the edge of the sail. For a flat, rigid sail (a board?) the faster moving air would mainly emerge at right angles to the line of the craft, with equal forces in each lateral direction - no resultant force forward or backward.

But you can't afford to be too 'intuitive' when it comes to aerodynamics.

 

[nrqed]

I cringed when I heard them say that Newton's laws predict that the boat would not move. I wish they had asked a physicist! It's obvious that if the air recoils, the boat will move. And then they concluded something to the effect that Newton's laws had been proven wrong or something to that effect.

 

[boing!]

At first, I had no explanation...the fan should be pushing backward with an equal force to the one that air is hitting the sail, but then I thought about Bernoulli. Isn't that the answer here? Put the fan back far enough from the sail and there will be more air particles hitting the sail than were originally moved by the fan. Bernoulli's principle FTW??

 

[Fewmet]

At first, I had no explanation...the fan should be pushing backward with an equal force to the one that air is hitting the sail, but then I thought about Bernoulli. Isn't that the answer here? Put the fan back far enough from the sail and there will be more air particles hitting the sail than were originally moved by the fan. Bernoulli's principle FTW??

Welcome to Physics Forums, boing!

This scenario also came up in https://www.physicsforums.com/showthread.php?t=510723". I offered an answer in terms of conservation of momentum in post #20.

 

[pallidin]

At first, I had no explanation...the fan should be pushing backward with an equal force to the one that air is hitting the sail, but then I thought about Bernoulli. Isn't that the answer here? Put the fan back far enough from the sail and there will be more air particles hitting the sail than were originally moved by the fan. Bernoulli's principle FTW??

If you "close the system" you will find that your scenerio does not work.
If you "open the system" the environmental air simply will not do as you suggest.

 

[exabrial]

I too was baffled by this, but I think I have an explanation not covered anywhere in this thread yet... I can also explain why they didn't get a result until they cranked the propeller to 11 and a had massive sail!

First, all of Newtons laws are in effect. Technically the force by the boat would be canceled if all energy is caught by there sail. Simple as that, there should be 0 net force on the system! But...

I think this is explained by the shape of their Sail. Air currents hitting the center of the sail have four escape routes: left/right, or up/down. Here's the thing, the sail is _not_ flat, it's parabolic!

The left/right escape routes are on the same plane (depth) as the entry point (the center of the sail). In other terms, the left/right escape routes are at the apex of of the parabola. Any air exiting left/right does so perpendicular to entrance of the air current, leaving 0 net force on the boat.

However, the top/bottom escape routes are _rearward_ of the apex. This means any air currents escaping top/bottom would _not_ be escaping exactly perpendicular! They would be angled slightly aft (backwards probably 15degrees), giving a net force forward!

The vertically oriented air currents are likely to be very minor, as the left/right escape routes offer much less resistance. So cranking the fan to 11 finally produces enough force to get the affect.

The oversized sail (Much LARGER than the incoming stream) allows the the vertical oriented air currents to escape around the incoming stream. A small sail would simply not allow the currents to escape around the incoming airstream, cancelling those vectors.

Two experiments would confirm this...

First, create a perfectly flat sail. Air must be able to escape equally in all directions with no pressure differences in any vector.

Second, create a massive "Angel Food Cake Pan" shaped sail and a very very VERY narrow central airstream. The shape of the "Angel Food Cake Pan" would reverse the incoming vector backwards moving the boat forward.

This was really bothering me, but finally I think I have a reasonable explanation!