Fan on a 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.

[subductionzone]

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

[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]

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

[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.

[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]

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

[Budoshi]

One thing I noticed was that the fan was fixed while the sail was movable...

What if you tried to fix the fan on the same place as the sail... I bet you that the sail wouldn't move an inch then.... Physics,man.. you can't beat it..

[konstantinkh]

The fan and the sail are attached to the same cart. They are both fixed with respect to cart, and free with respect to rail. So I'm not sure what you are talking about.

There is a net change in momentum of the air. That means there is a net reaction force on the cart. That means the cart accelerates. That's physics.

[FlashGameDev]

that's a badass fan there.

[doremon2006]

Thanks! I also couldn't believe it would not move. Energy is taken from the battery, it cannot disappear.

[drVitall]

Is so inefficient because of the shape of the sail you choose to install. Only minor part of the air (by mass or volume) actually driving your "sailboard" forward. With more simplier yet more efficient shape you can easily get 90% "efficiency", not 10% like in your design.

PS. Getting rid of sail and rotating fan 180 degree would be most efficient way of utilizing fan power. That one I define as being 100% "efficient".

[konstantinkh]

Indeed. This is only meant as proof of concept. Many people wrongfully believe that you'd get no forward thrust from blowing air from a fan onto a sail. This simple demo proves otherwise. I'm sure you can do better with good sail design.

[Alomphega]

Just reaction, indeed, you don't think so ?