Mythbusters predictions, place your bets.

Anonymous

Ok, so this is for people who HAVEN'T seen episodes beforehand. If you already KNOW their outcome, please don't spoil the fun.

Right, here's the idea. The Mythbusters on discovery check out commonly held beliefs, myths, thought experiments, or viral videos etc, and find out if they're true or not.
I was just checking what's on tomorrow's new episode, and thought of a silly game.
See if we can predict the outcome before the show airs.

Make sense?

Right, tomorrow's show has them testing whether a sail boat can be propelled without wind, by having a fan attatched to it, blowing at the sail.

What are your thoughts?

Personally, I'm uncertain. We know that a fan alone (without a sail) could propel the boat, just like the swamp boats in the everglades.
We also know that having a large sail effectively acting as a large parachute on a craft can act as a brake, just like a dragster.

Anyway, given those presumptions, (which may or may not be correct) I suspect the fan will basically act as a propeller, but maybe the sail will block the airflow to such an extent that it will effectively become an airbrake, cancelling out any thrust from the fan.
So, my first guess is, boat NOT moving.

What do you guys reckon?

Richie63

and the difference between a fan blowing on a sail and wind blowing on a sail is?

Anonymous

Richie63 wrote:

and the difference between a fan blowing on a sail and wind blowing on a sail is?

Fan is attatched to the boat.

So, your guess is boat will move? Fair enough.

sheepsteeth

Richie63 wrote:

and the difference between a fan blowing on a sail and wind blowing on a sail is?

w0nt the fan push back against itself as hard is it pushes the air forwards?

equal and opposites or summat

i think it wont move because of which ever one of newtons laws that is.

good thread by the way, glad i invented it.

Anonymous

I think sheeps is thinking the same as me, but in a far more eloquent way, which let's be honest, is not hard!

whyamihere

Assuming that they isolate the experiment properly (do it in a warehouse or something so no wind) then:

1. The boat will move slightly backwards, due to application of Newton's 3rd law. The fan is exerting a force toward the front of the boat, so the boat will move backwards (this is how the swamp boats in the everglades move).

2. When the air hits the sail, the impulse will counteract the force the fan exerts to move the boat backwards. It will most likely continue to move backwards slightly, due to Newton's first law, but there will be essentially no or very little acceleration.

3. When the fan is turned off, the opposite of 1 will happen, and the boat will either slightly move forwards or come to rest if it is already moving backwards.

sheepsteeth

i reckon the sail will act as an air brake htough and stop the boat from moving at all.

whyamihere

sheepsteeth wrote:

i reckon the sail will act as an air brake htough and stop the boat from moving at all.

Nope. Drag is proportional to the square of the velocity, so when the boat is only moving very slowly, the drag is essentially zero, even though it's a great big sail.

Anonymous

Great thinking, whyamihere.
After reading your point number two, it made me think that the chances of the sail balancing the fan out 100% may be slim, so yeah, the boat may well move slightly backwards. But I also think it may be dependant on the size of the sail they use. Knowing the Mythbusters, it will be frakking huge at least at some point!

But, I still go with my original guess. It will not move, at least, not in the way the myth/idea suggests
The sail is basically an unneeded additional drag. You'd be best off taking it down completely and just driving the boat with the fan. And maybe pointing the fan the other way so that the hull of the boat is going through the water efficiently!

EDIT: ah, you reposted before me.

whyamihere wrote:

sheepsteeth wrote:

i reckon the sail will act as an air brake htough and stop the boat from moving at all.

Nope. Drag is proportional to the square of the velocity, so when the boat is only moving very slowly, the drag is essentially zero, even though it's a great big sail.

Hmm. not that I'm expecting to change anyone's mind, since I really do not KNOW what's going to happen, but I think there may be more at play than just the sail being an airbrake.

If you take a helicopter, for example, and attatch it's bottom to a gigantic plate. Let's say, a huge piece of strong plywood, 50metres by 50metres. That helicopter will not take off, because it's airflow is now being deflected by the plywood, and counteracts any lift.
The downdraft needs to go down, past the aircraft, if it's just forced effectively INTO the aircraft it does nothing.


Also, I wonder what would happen if instead of a simple sail with the fan blowing on it, you stuck a curved sail, as though the boat was tacking (No idea what those sails are called). Now the fan is pulling the boat, and you have additional thrust in at least partially the same direction, from the wing shaped sail.
Would the two add up? hmm.

The Northern Monkey

is the fan at the side or the front?

whyamihere

The Northern Monkey wrote:

is the fan at the side or the front?

I read it as having the fan at the back, pointing forwards at the sail, which is in the middle.

The Northern Monkey

I don't know why, but i'm thinking that if its at the back it wont work, but at the side it might :?

Depends on the type of boat too I reckon... does it have a keel?

Anonymous

whyamihere wrote:

The Northern Monkey wrote:

is the fan at the side or the front?

I read it as having the fan at the back, pointing forwards at the sail, which is in the middle.

So did I, I think that's the general idea behind the story as commonly repeated, or seen in cartoons.
But I think the monkey might be onto something.

Richie63

I have now built the experiment using a small indoor helicopter attached to a plastic food container in the bath photos and results to follow once its charged up

cat_with_no_tail

So would it make and difference if you were to mount the fan closer to / further away from the sail or do the 2 opposing forces cancel each other out (more-or-less) each time?

I'm rubbish with stuff like this.

The Northern Monkey

Richie63 wrote:

I have now built the experiment using a small indoor helicopter attached to a plastic food container in the bath photos and results to follow once its charged up

Anonymous

Richie63 wrote:

I have now built the experiment using a small indoor helicopter attached to a plastic food container in the bath photos and results to follow once its charged up

Would you be willing to keep it till after the show?
(if everyone else is in agrement)

Richie63

so far it spins on itself --need to isolate tail rotor

Richie63

Ok missed that post from Yeehaa and will post a photo only

Anonymous

Richie63 wrote:

Ok missed that post from Yeehaa and will post a photo only

I don't intend to be mean, I just think it would be fun to get as many people's thoughst in as possible before finding out what happens, if that makes sense to everyone?

sheepsteeth

it is the best idea i think

Richie63

But what happened?

Anonymous

Considering how big that "fan" is when compared to the sail, I think I've got a pretty solid idea of what heppened
But hey, I migh tbe surprised, that's where the fun is.

fletch8928

My take on a subject I can only guess at is this.

Doesnt natural wind also go around the sail and causes a drop in pressure at the front? Which has got to help with moving forwards! The fan made wind will probably only blow directly at the sail, causing it to bellow out but not realy move, unless the initial burst of wind puffs the sail out quickly, which punches a hole through the air infront of the sail. On the flip side of that, is that the sail may push forwards into thick air and the boat could move backwards.

I actually have no idea, but a couple of bob says it wont do anything that resembles useful movement.

Great topic.

Anonymous

haha, I just realised the toy helicopter is called "Super Overlord"
Why don't real aircraft have names like that instead of Huey, or Apache?

sheepsteeth

yeehaamcgee wrote:

"Super Overlord"

thats what i call my knob,

ddraver

I think It will make a difference if they set the fan up on the back of the boat pushing the boat forward (to use the technical term, if the boat is running) or at the side (is the boat reaching/beating)

If the former then it's a matter of how much backward force will be created by the fan sucking the air through it vs how much wind the fan will create and how efficiently the sail converts that force into forward motion

Even with this there are so many variations in sail design/fan design/ hull design that I don't think the myth can be proved either way. If it's the latter case then the are hundreds more...

Anonymous

They're usually fairly well thought out experiments, DDraver, so I'd imagine they'll find a way to eliminate all variables apart from what they're actually testing.

ddraver

I Know (and a s a geologist I am professionally obliged to be as vague as possible) but sailing is phenomenally complex so i don't think it is possible

Put simply you ll get different results with different fans and different sails...

When is it on btw?

bartimaeus

Surely they should just stick the fan at the back, point backwards - then the boat will go.

Otherwise Newton's 3rd law says the fan pushes the boat backwards, while the wind on the sails make it go forwards. My guess is that the sail is less efficient than the fan (dead downwind sailing is rubbish as the airflow over the sail stalls - which is why a reach is fastest) so I'm guessing that if anything the boat would try to go backwards... but it would need a big fan to make it move.

Cferg

Depends on the size of the sail surely.

If the fan was pointed away from the sail, and the force produced by the fan was large enough to overcome all other forces acting against it ( friction, mass, gravity, air resistance etc.) then the boat would move.

However when the fan is turned round, the drag produced by the sail will depend on the size(area) of the sail. Drag produced is dependant on the surface area. So my guess is that it will move, however in what direction I'm unsure though it could probably be worked out if all the variables were known through a few simple calculations if like Yeeha says they keep variables to a minimum.

Never seen mythbusters before but I may now watch it !