Blowing your own sail

Liquidfox · 20 May 2011 at 11:35

I love watching arguments in threads i've started, makes my day that little bit easier

sanaxe1 · 20 May 2011 at 11:36

Deadbeat said:
I think it's confusing people because they believe the thrust on the sail would cause the boat to move forwards, when it's the drive from the pressure returning off the sail that's actually supplying the net force. Obviously the boat isn't propelled by the same arrangement of forces as it would be were the wind blowing.

This is true. But if some average person in the street was to ask you if they could take a fan and blow it at a sail to move it forward Yes or No the simple answer would be yes as in the real world you would go forward.

You wouldn't start going ah its this force not that force thats moving as they wouldn't give a monkeys they just want to move there boat forward with a fan and a sail it doesn't matter that its inefficient or that you could probably use another object in place of the sail as thats not the question they asked you.

alex24 · 20 May 2011 at 11:39

I can't believe how long this thread is! Just watch the second Mythbusters video and grant explains why he thinks it's apparently 'working'. Really, they should have busted it straight away, but the answer is more complex than simple Newtonian physics. The whole world is much more complex than Newtonian physics, it just works for big, simplified systems.

div0 · 20 May 2011 at 11:41

I don't need to watch a kids science video to know how it works

alex24 · 20 May 2011 at 11:42

sanaxe1 said:
This is true. But if some average person in the street was to ask you if they could take a fan and blow it at a sail to move it forward Yes or No the simple answer would be yes as in the real world you would go forward.

Only if you were being a pedant! Then you'd have to explain the whole thing to them as well.

Vonhelmet · 20 May 2011 at 13:14

Liquidfox said:
But, will it sail?

Screw that. Will it blend?

manic_man · 20 May 2011 at 13:18

Of course it would work :\

The fan itself maybe stationary, but the rotation of its blade requires energy, which in turn causes a flow of energised air to be directed towards the sail!

The fan isn't the point, the airflow it creates is.

Why you would want to use such an inefficient form of propulsion is another matter of course...

Vonhelmet · 20 May 2011 at 13:34

manic_man said:
Of course it would work :\

The fan itself maybe stationary, but the rotation of its blade requires energy, which in turn causes a flow of energised air to be directed towards the sail!

The fan isn't the point, the airflow it creates is.

Why you would want to use such an inefficient form of propulsion is another matter of course...

All well and good, but where does that airflow come from? It comes from pulling air from behind and around the fan and pushing it forwards. This lowers the pressure behind and around the fan. The pressure differential gives rise to a force pulling the fan end of things backwards, in much the same way that a pressure differential generates lift on an aircraft wing.

It's very hard to simplify this without missing things out.

PermaBanned · 20 May 2011 at 17:40

vonhelmet said:
All well and good, but where does that airflow come from? It comes from pulling air from behind and around the fan and pushing it forwards. This lowers the pressure behind and around the fan. The pressure differential gives rise to a force pulling the fan end of things backwards, in much the same way that a pressure differential generates lift on an aircraft wing.

It's very hard to simplify this without missing things out.

But by that logic hovercrafts wouldn't work?

Deadbeat · 20 May 2011 at 17:46

Superewza said:
But by that logic hovercrafts wouldn't work?

Of course they would, because the air would be pulled from the front of the boat towards the back and the airflow would push out to the rear as well. The problem with the sail is that it's a closed system (when simpified) - the force generated by the airflow on the sail (forwards) is cancelled by the force of the fan generating the airflow by pulling air in from the rear (backwards), thus they'd cancel - it's only because the airflow is deflected off the sail and returns towards the back of the boat that a net forward force is generated.

Fuzz · 20 May 2011 at 18:36

Clucking bell is this still going. I'm going to hang myself if anyone else says it'll sail.

Biohazard · 20 May 2011 at 18:50

It'll clearly sail, sail away....

Here's a nice tune for your last day on earth

Rich1988 · 20 May 2011 at 19:01

Scubascorpion said:
Clucking bell is this still going. I'm going to hang myself if anyone else says it'll sail.

I wont but they are stupid

PermaBanned · 20 May 2011 at 19:02

Deadbeat said:
Of course they would, because the air would be pulled from the front of the boat towards the back and the airflow would push out to the rear as well. The problem with the sail is that it's a closed system (when simpified) - the force generated by the airflow on the sail (forwards) is cancelled by the force of the fan generating the airflow by pulling air in from the rear (backwards), thus they'd cancel - it's only because the airflow is deflected off the sail and returns towards the back of the boat that a net forward force is generated.

So you agree that it would work? What the hell are we debating then?

Fuzz · 20 May 2011 at 20:03

Prefer Orinoco Flow personally.

Deadbeat · 20 May 2011 at 20:40

Superewza said:
So you agree that it would work? What the hell are we debating then?

I don't know, you said hovercrafts don't work!

PermaBanned · 20 May 2011 at 20:44

Deadbeat said:
I don't know, you said hovercrafts don't work!

I said they wouldn't work if the forces acting on/around a fan were balanced.

scrlk · 20 May 2011 at 21:04

This is quite a fantastic topic to read around.

Vonhelmet · 20 May 2011 at 21:27

Superewza said:
I said they wouldn't work if the forces acting on/around a fan were balanced.

The post I was responding to was suggesting that it's the airflow towards the sail that makes it move, but if it was the airflow alone, then the boat would move backwards, because of the pressure differential.

Vonhelmet · 20 May 2011 at 21:30

Scubascorpion said:
Prefer Orinoco Flow personally.

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