MythBusters -Blow Your Own Sale

[RockMonkey]

How the crap did that work? I can't really wrap my brain around any way they can make the boat go forward. The best case scenario is you can catch 100% of the backwards thrust, causing the boat to go nowhere. Realistically, you shouldn't be able to catch all the thrust, so the boat should move backwards. Why the crap did it move forward with excess thrust?

The only thing I can think of is the wind blows the sale into an airfoil shape, bowed forward. With the thrust high enough, more wind escapes around the top and bottom of the sail, and flows over it's front surface like a wing, causing a low pressure zone at the front of the sail, and pulling the boat forward.

Any better explanations? That was one of the better myths they've tested, with one of the more surprising results!

 

[Badger]

Missed the episode so I guess I will have to find it.

 

[muleskinner]

Missed the episode so I guess I will have to find it.

http://eztv.it/ep/27394/mythbusters-s09e06-blow-your-own-sail-hdtv-xvid-fqm/

I haven't seen it yet either. Maybe tonight.

 

[Cherokeester]

I will have to watch it, I have to admit though I thought you were talking about sales in the store sense. Sails are on sailboats.

 

[Badger]

Ok watched it. They explained it for the most part. They believe the sail is acting as a reflector. They could have tested it a bit further by using a wind meter behind the boat to confirm that indeed the wind was being redirected by the sail and sent backward which int turn creates forward motion.

 

[RockMonkey]

Basically, they took an air boat, mounted a big square sail to the front of it, and turned the fan around so it blows air into the sail. The damn thing went forward 3 mph. It's like sitting in your rolling office chair, lifting your feet off the ground, and pushing it down the hall by pushing against the back rest. Unpossible!

 

[RockMonkey]

Ok watched it. They explained it for the most part. They believe the sail is acting as a reflector. They could have tested it a bit further by using a wind meter behind the boat to confirm that indeed the wind was being redirected by the sail and sent backward which int turn creates forward motion.

Basically, isn't that how a square sail works? That sounds like BS to me. Like some random words they threw together because they didn't understand why the boat was moving forward.

 

[RockMonkey]

I will have to watch it, I have to admit though I thought you were talking about sales in the store sense. Sails are on sailboats.

D'oh! I suck at the spelling... Strangely I spelled it both ways in this thread.

 

[JoeT]

Good episode, I did like the rocket powered skateboard portion. I didn't see anything for sale but I did see a sail.

 

[Spork]

I saw the previews for it but missed the show. From what I saw and wild guesses I think basically all the sail is doing is directing the air from the fan backwards. The forward motion was because they are redirecting enough of the air to move forward. A smaller sail or larger distance between the fan and sail would probably wouldn't have redirected enough and they would have moved backwards.

 

[RockMonkey]

I saw the previews for it but missed the show. From what I saw and wild guesses I think basically all the sail is doing is directing the air from the fan backwards. The forward motion was because they are redirecting enough of the air to move forward. A smaller sail or larger distance between the fan and sail would probably wouldn't have redirected enough and they would have moved backwards.

So the air hits the center of the sail, blows along the curved surface to the edges, where it is redirected backwards. The sail is a giant, inefficient thrust vectoring device? Okay, I could buy that as a possibility, and it's simpler than my explanation.

That's also vaguely similar to the "bounce" theory that they halfheartedly proposed on the show.

 

[Badger]

Hell that's what I said for the most part. My guess is that more of the air /thrust is acting upon the water surface more then actually creating thrust against the air. Air coming off the bottom edge of the sail is pressing against the water pushing the boat forward?

If you notice he had to go side to side with it to keep it straight. Also they never looked into different types of sails. How many single mast boats do you see with an old style square sheet. Wondering what could be done witha fan put in front of a proper sail. Then it might work more like it is supposed to and more like how Monkey first thought.

 

[I Lean]

Sounds vaguely like a thrust reverser on a commercial jet. I wonder if they could actually back up using them?

Then again, if it moved the boat at 3mph, there's not a lot of excess energy there to get a heavy anything moving on land.

 

[iceaxe]

Hell that's what I said for the most part. My guess is that more of the air /thrust is acting upon the water surface more then actually creating thrust against the air. Air coming off the bottom edge of the sail is pressing against the water pushing the boat forward?

Thinking of it as a crude thrust reverser makes much more sense. Providing enough thrust to over-ride the cancelling effects with the re-direction of enough of the air to the rear to enable an opposite reaction of foreward motion.
Any more traditional sail shape and more of the air would easily escape past it such the fan would simply be thrusting you backward would be my take on using a square sail. If you really need to move foreward and have that much thrust avialable and don't know to simply point it backward.....

Sounds vaguely like a thrust reverser on a commercial jet. I wonder if they could actually back up using them?

Jets back up with thrust reverse? Go to the next airshow featuring a C-17 demo. (edit, understanding that turbofans are a little different than the turbojets with the reversing clamshells that close behind the engine, but to the same effect)

The airboat was interesting, I was hoping they would do a jet engine on a small boat with another heat resistant sail.

 

[Badger]

You missed what I said about the traditional style wedge shaped sail and the fan. I said placing the fan in front of the sail or should I say off center and to the side of the sail. You would basically be creating you own wind and redirecting the thrust more like a true sail.

That's one thing the Mythbusters failed to realize. A sail will also work with the wind coming from the front/side of the boat. In order to sail down wind you usually use a spinnaker which is a huge ass sail.

 

[Unimog]

Like Badger said, a sailboat doesn't move by the wind just blowing it unless it is just going down-wind (the slowest direction). There are a lot of complex things going on, but the main force is the 'lift' on the sail, just like the lift produced by an airplane wing. The other factor is the shape and existence of a keel/daggerboard/centerboard to keep it from just blowing sideways and turning the lateral force into forward movement.

 

[iceaxe]

You missed what I said about the traditional style wedge shaped sail and the fan. I said placing the fan in front of the sail or should I say off center and to the side of the sail. You would basically be creating you own wind and redirecting the thrust more like a true sail.

That's one thing the Mythbusters failed to realize. A sail will also work with the wind coming from the front/side of the boat. In order to sail down wind you usually use a spinnaker which is a huge ass sail.

Yes, I see what you were getting at now, got thrown off track somehow in your post.

Now then, short of calculating those complexities, if the position was right could you have thrust from the source and from the air volume it generates acted on by the traditional sail?, so maybe you get a boat tacking upwind agianst it's own generated air on top of the thrust from that generation, kind of a supercharged sailboat?
Find a stable way to mount the fan in an effective spot - outriggers on both sides and something for counterbalance like a bunch of mythbusters hanging off the opposite side from the fan about to go in the drink, might be entertaining.