Fan on a Sailboat

Demonstrating reverse thrust generated by the sail acting on the focused air current from the fan installed on the 'sailboat'. In this case, a cart is used for simplicity, and a simple 'sail' is fashioned from a paper towel.

The two common misconceptions are that this is either a good propulsion method or that it provides no propulsion whatsoever. Video aims to demonstrate that there is some propulsion, but it is far less efficient than that of the fan alone.

Note that contradicting results can be achieved by using a different geometry of the sail or air flow from the fan. This video concentrates on the configuration similar to ones used for running downwind. A real boat running with spinnaker should achieve similar results.

Category:

Education

Tags:

• Fan
• Sailboat

[Zeitherius]

O.O but... my physics book says this shouldn't work

[konstantinkh]

@Zeitherius Short version, books tend to oversimplify the problem. There are setups where what your books says is certainly true. This setup shows that it isn't always so. The conservation of momentum and Newton's 3rd Law, from which argument is drawn, holds either way, of course. Any introduction level book in any subject will have such flaws. One must be aware of that when reading them.

[silverrahul]

@konstantinkh. dis is makin me go mad. coz wen i thought bout it i thought fan wud try 2 pull boat backwads by jet effect & sail wud try 2 push forward. but they wud cancel each other, hence no movement. But d stupid real world gets in the way of elegant physics & as shown in ur vid, boat moves forward. But still, i cant figure out y . i know u explained b4 , but still i dont understand . Can u just tell me in simple terms, wich force works in wich direction & y the resultant force is forwards?

[konstantinkh]

@silverrahul It's the Newton's 3rd law. The fan takes static air and makes it move. The reaction force pushes on the fan backwards. Sail takes that moving air and stops it. Reaction force pushes it forward. If that was all, the forces would be in balance, like you say. But this particular sail doesn't just stop the air, it reflects part of the flow back. So it must bring it to stop, then accelerate it again. The additional reaction force from accelerating air back is the net force forward.

[Lexorinox]

Damn this got me :D !

There are full discussions on yahoo answers about thisand it says that the boat would still go backwords because all the air isnt captured by the sail => this sounds realistic so the video made me lol?

Could u plz explain forces working here xD

[konstantinkh]

@Lexorinox The forces here are not trivial. There is lift at the fan and pressure at the sail, but simple explanation for why one is stronger than another does not exist.

What you should be looking at instead is the net flow of air after it interacted with the fan and the sail. Here, the flow is reflected back, so the cart is pushed forward. Situation where not all air is caught is also possible if the fan produces a wider spread stream of air.

[subductionzone]

Hey KSquared, did you watch Mythbusters tonight? I am watching it now, no mention of your video yet.

[Lexorinox]

@konstantinkh Interesting, thank you very much for the explanation :)

[konstantinkh]

@Lexorinox The forces here are not trivial. There is lift at the fan and pressure at the sail, but simple explanation for why one is stronger than another does not exist.

What you should be looking at instead, is the net flow of air after it interacted with the fan and the sail. Here, the flow is reflected back, so the cart is pushed forward. Situation where not all air is caught is also possible if the fan produces a wider spread stream of air.

[Lexorinox]

also forgot; is this maybe something with bernoullis laws and higher pressure/lower pressure,