Sailboats providing their own wind

[A.T.]

The mis-applications of "intuition" to aerodynamics and propulsion systems are a little more common than just mine.

You can consider throwing balls to see that the concept doesn't violate momentum conservation. Aerodynamics then just makes it less efficient, but not impossible.

 

[A.T.]

As I recall I did a calculation at the time which showed you could go upwind at half the windspeed (ignoring hull friction and assuming a "perfect " propeller).

There is no such speed limit for going directly upwind or downwind in terms of true wind multiple. It only depends on the efficiency, which imposes practical limits:

https://orbit.dtu.dk/fedora/objects/orbit:55484/datastreams/file_3748519/content

With less resistance on land more than twice the true wind was achieved in both directions:
https://en.wikipedia.org/wiki/Blackbird_(land_yacht)#Achievements

 

[hutchphd]

There is no such speed limit for going directly upwind or downwind in terms of true wind multiple. It only depends on the efficiency, which imposes practical limits:

https://orbit.dtu.dk/fedora/objects/orbit:55484/datastreams/file_3748519/content

With less resistance on land more than twice the true wind was achieved in both directions:
https://en.wikipedia.org/wiki/Blackbird_(land_yacht)#Achievements

Very interesting references. I guess I assumed the Betz limit for my calculation...(it was a long while ago and not really my field! I remember wondering at the time why there was no such limit on a sail. ) Thanks much for the clarification.

 

[A.T.]

Very interesting references. I guess I assumed the Betz limit for my calculation.

There is an old paper by Blackford, that derives a speed limit of 2 x windspeed, by assuming that the maximal upwind speed will be achieved at the Betz limit:

http://202.38.64.11/~cxyu/AJP_pushmepullyouboat.pdf
That assumption is never justified, and doesn't make much sense: The Betz limit only maximizes the energy extracted in the reference frame of the windmill. To maximize upwind speed you have to balance energy extraction with the drag of the windmill. Since drag doesn't play any role in the Betz limit, there is little reason to assume that it presents the optimum for this application.

 

[jbriggs444]

Just for laughs, I'll recommend Niven's Flying Sorcerers. The mis-applications of "intuition" to aerodynamics and propulsion systems are a little more common than just mine.

Yep. Orbur and Wilville tried it both ways. The etymology of "Purple" was cute in that one too.

 

[tech99]

There is a video of a model windmill boat going directly into the wind at this URL:-

 

[Baluncore]

Vectors are more fun, and a one dimensional analysis is unrealistic.
Sail boats operate at the interface between two fluids with different velocities.
I have a keel boat, so set my sail at 45°, then mount the fan blowing across the boat onto the sail, so as to propel the boat forwards.

 

[sysprog]

However, if you have a windmill on a boat, driving a water propeller, it can push the boat against the wind.

Jacques Cousteau implemented a not-especially-dissimilar concept on his supplementally-wind-powered (primarily diesel powered) vessel Alcyone.

Regular wind-powered vessels collect more wind power; however, the especially inventive person Mssr. Jacques Cousteau dreamed up a way to make a wind turbine that could drive a propeller so that a vessel could go directly into a headwind as an alternative to sailors' tacking -- he was also the inventor of the demand regulator, without which we would not have SCUBA -- what an exceptional man of science and engineering he was.

 

[A.T.]

Jacques Cousteau dreamed up a way to make a wind turbine that could drive a propeller so that a vessel could go directly into a headwind

He probably wasn't the first. The optician Bernhard Schmidt actually build such boats in the 1920s:

https://en.wikipedia.org/wiki/Bernhard_Schmidt#Bergedorf

http://www.friedensblitz.de/sterne/schmidt/Schmidt3.html

I also remember reading about some French models, which might have been earlier.

 

[Catsailor]

Apparently, none of you are multi-hull sailors. Multi-hulls do create their own wind. And regularly exceed windspeed by two or three times. On a reaching course, the side-force pushed on the boat and via the centerboards, are converted into forward motion. As the air accelerates over the wing shaped sails, it creates apparent wind. As the apparent wind increases, the sails go faster and create more wind. The only limit is drag, which is why iceboats can exceed 120 MPH. Maritime architect Nat Herreshoff knew this more than a century ago when he designed the catamaran Amarylis in 1874. At around 20' it blazed past 120' yachts, so much so it was banned from racing. Even the lowly 40 year old Hobie 16 can easily exceed 2x wind-speed, and the big foiling Trimarans now racing across the Atlantic go 3x or more, on hydrofoils. https://sailinganarchy.com/2019/11/05/fantastique/

 

[A.T.]

Apparently, none of you are multi-hull sailors.

Apparently, you missed the point.

On a reaching course,...

Sure, but not directly upwind or downwind.

 

[Catsailor]

Apparently, you missed the point.Sure, but not directly upwind or downwind.

We tack upwind , and even downwind. It's faster than the wind on either point of sail.. My point is, postulating a motor facing the wrong way on boats that have been perfected over the centuries for their speed, economy and versatility is beyond silly I have gone 30 MPH on the ocean on my 18'. Physics puzzles are fine. But boats were Man's first vehicles. The engineering, banking, insurance and investing industries all relate back to historic water transport. The boat I currently sail used Formula One computer aided design with millions of data points. Then there are cartoon boats with fans on them. Then there is the difference between beating a dead horse, or in some cases, riding one.

 

[A.T.]

Physics puzzles are fine.

And physics forums are for physics puzzles.

 

[Catsailor]

And physics forums are for physics puzzles.

As pertains to the title of the thread, yes sailboats can provide their own wind. Apparent wind. Three times Utah Catamaran State Champion here.

 

[berkeman]

Three times Utah Catamaran State Champion here.

Wait, isn't Utah a desert?

 

[Baluncore]

On a reaching course, the side-force pushed on the boat and via the centerboards, are converted into forward motion. As the air accelerates over the wing shaped sails, it creates apparent wind.

That "apparent wind" is the vector sum of the wind and the boat velocity. There is no new or faster wind generated on the boat. It is still impossible to continue sailing a conventional sail cat directly into the wind, or when there is no wind.

 

[DaveC426913]

As pertains to the title of the thread, yes sailboats can provide their own wind. Apparent wind. Three times Utah Catamaran State Champion here.

Just a plain ol sailor here with no prizes.

This isn't about efficiency; it's about a physics thought experiment, as specified.

(After all, a racecar driver could always come by and pooh pooh your paltry 30MPH: "If speed is what you want, trying to use sails is beyond silly. You should get a Formula One.")

 

[A.T.]

As pertains to the title of the thread, yes sailboats can provide their own wind. Apparent wind.

Try reading the actual question, not just the title. This is not what the OP means by "provide their own wind".

 

[Catsailor]

Wait, isn't Utah a desert?

Uh, 40 large high altitude reservoirs, One Pleistocene lake at 6000'that is 104 square miles, 400,000 years old. Three high volume whitewater rivers, Colorado, San Juan, and the Green, Lake Powell, 162,000 acres, 1900 miles of shoreline. We do ok.

 

[PAllen]

Myth busters did this:

Hey, but suppose you had a large bent tube on the intake side of the fan arranged so the intake of the fan was drawn from in front of the sail? That is, the intake tube would bend around the sail. I think you could go a lot faster that way.

 

[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.