B Sailboats providing their own wind

[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.