B Sailboats providing their own wind

[Catsailor]

Sure, but the modern sail is a vertical wing with an adjustable wing mast on the leading edge that acts as a slat to influence the low pressure area on the top portion of the wing. If you turn the fan around and push, the sail accelerated the boat to the point where it is making wind volume over the sail by virtue of it's own velocity. There is a fan that powers boats. The outboard motor with a propeller. Water is roughly one thousand times denser than air, so there are definite limits to speed. My catamaran these days and the sailing clipper ships that did most of the cargo carriage for up to 80 years ago , went about 30 mph average. The most modern ships barely exceed that. physics.

 

[tech99]

On a reaching course, the side-force pushed on the boat and via the centerboards, are converted into forward motion.

I don't think a sideways force can push a boat forward.

My understanding is that a sail has two components of force created by the air flow: one, at right angles to the wind, called lift, and another in line with the wind, called drag. When sailing close to the wind, the lift is, by geometry, at a small angle abaft the beam, and so has a component in a direction to propel the boat.

The drag has a component slowing the boat. Both have sideways components arising from the angle of the lift and drag vectors, and these components are canceled by the centreboard. To do this, it operates at a small angle of attack to the water and generates lift to cancel the sideways forces.

The boat points at a small angle upwind of its track in order to give the centreboard the required angle of attack. Those on board see this effect as leeway.

 

[A.T.]

I don't think a sideways force can push a boat forward.

I suggest defining "sideways" and "forward" precisely, to avoid endless talking pass each other.

The vectors are explained nicely here:
http://www.onemetre.net/design/CourseTheorem/CourseTheorem.htm

 

[tech99]

The first diagram of the link does not look correct to me, because the water should be hitting the centreboard on the leeward side, to provide lift, and not as shown.

 

[A.T.]

The vectors are explained nicely here:
http://www.onemetre.net/design/CourseTheorem/CourseTheorem.htm

The first diagram of the link does not look correct to me, because the water should be hitting the centreboard on the leeward side, to provide lift, and not as shown.

The relative water flow is the same in all diagrams, and as you see the force balance works out that way. If you think that you can balance the forces with the water flow from the other side, then I would be interested to see your vector diagram.

 

[Catsailor]

I don't think a sideways force can push a boat forward.

My understanding is that a sail has two components of force created by the air flow: one, at right angles to the wind, called lift, and another in line with the wind, called drag. When sailing close to the wind, the lift is, by geometry, at a small angle abaft the beam, and so has a component in a direction to propel the boat.

The drag has a component slowing the boat. Both have sideways components arising from the angle of the lift and drag vectors, and these components are canceled by the centreboard. To do this, it operates at a small angle of attack to the water and generates lift to cancel the sideways forces.

The boat points at a small angle upwind of its track in order to give the centreboard the required angle of attack. Those on board see this effect as leeway.

So, assuming you have ice skated or roller skated. Do you put the skate behind you and push forward? Or do you push out to the side? Pushing from Behind will get you about ten miles per hour. So how come speed skaters hit 37 mph? Vectors, as explained below.

I kid you not, if my catamaran is pointed 90 degrees off the wind, and is stationary, it will take off on it's fastest point of sail. I work with an Olympic sailing coach, and he will sheet a mylar sail so tight that it is completely flat, no airfoil at all. And perfectly perpendicular to the wind. The parallelogram of the the center of effort, which is the mast, the foils and rudders creates the vectors.

We tune the vectors by raking the mast forward or aft to get it in equilibrium going forward. If the rudders are dragging one way or the other, you tune the mast. And you rake the mast for different wind speeds. Also, modern centerboards and foils are asymmetrical, in other words, underwater wings that accelerate water flowing over one side and create lift. Even boards designed in the 90s create so much lift they cavitate. In other words, the pressure is so low on one side of the board that the water boils.

It's a great sport. There are some hobie cats on your local lakes.

 

[fleeker]

The key word here being "roughly". Depending on how the sail is shaped the boat can go forward, but it's not as efficient, as just using the propeller.

I'm not sure I understand how the shape of the sail could manage to generate more forward force than the backward force created by sucking the air behind the propeller forward, could you explain?

It can be done in theory, using a principle known as entrainment.

Even though the fan is attached to the boat, and pushes back with equal force, the air is viscous, and carries some air next to it along for the ride. You can get a little forward motion (if you angle the sail right), but its very inefficient.

It does not violate conservation of energy, since, after all, you are adding energy into the system, by whatever powers the fan. And it doesn't violate Newton;s Laws of motion because it's not a closed system.

This has been demonstrated.

When I think about this: is the affected air is carrying air next to it along for the ride, I would assume that this transfer of force to adjacent air molecules comes in response to slowing down the directly affected molecules. Basically slowed down by friction with other air molecules, and then that friction imparts momentum to its neighbors...

So I guess I don't understand how that would actually add energy...

Plus wouldn't the exact same thing happen in regard to the air flow on the opposite side of the propeller too to balance things out?

Replace the sail with a board, and let the passenger collect balls from the ground and throw them at the board, so they bounce off and leave with backwards momentum.
The boat gets the opposite forward momentum.

What is the end effect you're saying will happen here? Forward movement, zero movement or no movement?

When you fire a cannon forward from a boat, that should move the boat backward, so I imagine if you threw a ball forward there would also be backward movement. Albeit very hard to perceive since it would be so minor.

A cannonball or baseball hitting the sail, I could see moving it forward slightly, counteracting that subtle backward movement which happened half-second earlier, and creating zero net movement...

I think you may be missing the point. It is supposed to be instantly recognizable as impossible. That’s why it’s funny when it works. It’s exactly like a cartoon character picking themselves up by their bootstraps.

I'm not sure though, as this TV series is aimed at children, and I'm not sure they would actually recognize how impossible this would be in real life, sort of like they probably don't grasp the difficulty of hanging 1-handed from a rope while a pair of friends dangle from your legs, as characters not stereotyped as super-strong still manage to pull off.

It could be humor aimed at parents, but only if they're paying close attention.

Myth busters did this:

This is pretty interesting... but they only got 3mph and the one guy's theory seems to be that they got an effect like a weak fan aimed backward because the sail "bounces" a small amount of the forward-facing air backward...

I can't seem to get my head around that though. Is that a thing? What would you call that?

 

[Dale]

they got an effect like a weak fan aimed backward because the sail "bounces" a small amount of the forward-facing air backward...
Is that a thing? What would you call that?

I have no idea what you would call it, but yes, it is a thing. Jet aircraft use that idea to go in reverse. They just bounce their backward facing jet exhaust into the forward facing direction so that they can go in reverse. They use metal ducts instead of a sail, but it is the same principle.

 

[DaveC426913]

Wouldn't it simply be "directed air flow"?

 

[hutchphd]

I would call it thrust! The gases in a rocket engine do not care how many times they bounce around the combustion chamber...it is only the speed and direction of their final exit that matters.
Just a (more or less efficient) ducted fan.

 

[Ibix]

They use metal ducts instead of a sail, but it is the same principle.

...but this design is considerably more efficient than a fan and sail!

 

[Baluncore]

Wouldn't it simply be "directed air flow"?

“Vector thrust”.
“Reverse Thrust” brakes on jet aircraft, also VTOL.
https://en.wikipedia.org/wiki/Thrust_vectoring#Vectoring_nozzles
Also used on jet boats (water) for steering and reverse.

 

[DaveC426913]

“Vector thrust”.
“Reverse Thrust” brakes on jet aircraft, also VTOL.
Also used on jet boats (water) for steering and reverse.

Agree. But I was looking for a general term.

 

[A.T.]

I'm not sure I understand how the shape of the sail could manage to generate more forward force than the backward force created by sucking the air behind the propeller forward, could you explain?

Did you watch die video starting at 4:00? There is a diagram at 4:34.

The diagram is basically the same as for thrust reversal on jet planes:

Here in action:

I imagine if you threw a ball forward there would also be backward movement. Albeit very hard to perceive since it would be so minor.

Are you familiar with momentum conservation? If the balls (initially at rest) end up with negative momentum after bouncing from the board, the boat will get the opposite forward momentum.

 

[Nik_2213]

Slightly off-topic:
1) My compliments to any Hobie Cat sailor.
Long, long ago, when such 'roof-toppers' were considered barely more than toys, I saw one ordered off the water to allow racing dinghies to compete. The 'Commodore' had been equally curt to my brother and I, who had fun GRP slalom/kayak canoes. Anyhow, four or five big groups of dinghies were marshalled, started on their way to turn at distant buoy.

Then the Hobie guy pours himself a long drink, adds ice from a thermos, sets it down. He slides the Hobie back into the water. Doesn't bother with jib, just hauls the main, glides off. Goes really, really wide of the out-bound dinghies and marker buoy. Slides effortlessly past the lead dinghies, all now with spinnaker & trapeze deployed. Is across the line, back up the beach in time for his long drink to still have ice.

Drinking slowly, he'd just enough to politely toast the first crews from the lead group as they flogged their finely-tuned craft across the line...

Even from a distance, we could see those racing dinghies' upstaged 'Commodore' had gone red as a stop-light...

2) D'you remember a book called 'The Forty Knot Sailboat' ? Author envisaged a foil-riding, wing-sail 'proa' configuration, was widely scorned. If anything, he understated the potential...

 

[A.T.]

The "apparent wind" explanation is also mis-leading.

Yes, the "go faster and create more wind" part omits that at higher speed the apparent wind also comes more from the front.

But as you note, this not at all what the OP means by "own wind", and the mods already had to delete this derail from this thread once.

 

[rcgldr]

Yes, the "go faster and create more wind" part omits that at higher speed the apparent wind also comes more from the front.

But as you note, this not at all what the OP means by "own wind", and the mods already had to delete this derail from this thread once.

I created a new thread for this and deleted my posts related to apparent wind.

 

[rcgldr]

video - "some planes can back up on their own"

Most commercial airliners can do this, but this can only be done at airports where the terminals are built / designed to handle the reverse thrust used to backup.

 

[Herman Trivilino]

Robert Beck Clark wrote an article on this in either TPT or AJP, back in, maybe, the 1980's.

He tells the story of how he put this sailboat question on a freshman physics test. The correct answer was thought to be that it would be impossible to get a fan blowing towards a sail, all mounted on a boat, and use it to propel the boat forward. Newton's Third Law was thought to be the correct explanation of why.

Well, one student who took the test argued successfully that it could be done in such a way as to propel the boat, and gave the correct reason why using the principles he had been taught. When a grad student was grading this student's test he was stumped by the answer and brought it to Professor Clark. They eventually got it all straightened out, realizing Professor Clark was wrong and the student was right!

At the time one could purchase from one of the scientific supply houses this so-called Newton's Third Law demonstrator that had a fan mounted on a cart, blowing towards the (rigid) sail that is also mounted on the cart. The lecturer could turn on the fan and demonstrate that it indeed didn't propel the cart.

I can just imagine the designers of that device, playing around with different fan sizes and speeds, and different sail sizes and shapes, all in an attempt to get things arranged so the cart wasn't propelled. They had to go to great efforts, I imagine, to make the thing work right so that it would obey the "laws" of physics and not propel the cart!

 

[A.T.]

Robert Beck Clark wrote an article on this in either TPT or AJP, back in, maybe, the 1980's.

Ironically airliners were using thrust reversal even back then. Yet apparently the myth about its impossibility persists even today.

 

[kith]

At the time one could purchase from one of the scientific supply houses this so-called Newton's Third Law demonstrator that had a fan mounted on a cart, blowing towards the (rigid) sail that is also mounted on the cart. The lecturer could turn on the fan and demonstrate that it indeed didn't propel the cart.

I remember seeing such a device at a science fair a few years ago. The fair itself was outstanding and had many ingenious demonstration experiments for the vistors to play with (tacking with small land sailing vehicles, water containers which could produce bubbles on demand which then would sink a ship, etc.). Given this, I was really surprised to find a device which demonstrated wrong physics among them.