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OT Sail downwind faster than the wind!

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John Doe

Jan 1, 1970
0
If you enjoy reading technical argument back and forth (especially
when you suspect that one side is full of it), and have not
already done so, this one is for you.

There is much discussion about a concept called Direct Downwind
Faster Than the Wind (DDWFTTW). The idea of going directly
downwind faster than the wind using only that wind power.

Apparently they breed the silliness with the idea that you can
move directly into the wind using only the wind power. And you can
guess from that, that greater wind speed means you can move
faster.

Monty Python and a complete waste of time comes to mind.

The first place I stopped was
www.physicsforums.com
I figured there might be some good counter argument there.
 
J

John Doe

Jan 1, 1970
0
John Larkin <jjlarkin highNOTlandTHIStechnologyPART.com> wrote:

....
You can certainly make a cog railway engine, propeller-driven,
that moves into the wind. Just gear it down until it works.

I am disappointed that even John Larkin would argue something like
that.
--
 
J

John Doe

Jan 1, 1970
0
In a fantasy while lounging on an Arizonan beach.
It isn't a perpetual motion machine,

It is generating forward motion out of rearward thrust. A
perpetual motion machine generates motion out of no thrust.
The idea is worse than a perpetual motion machine.
it's a hydrodynamics problem.

You ignore the fact that there is negative pressure on the
propeller (and the craft), that is what makes the propeller spin.
Your argument is that you can turn that rearward thrust into a
greater forward thrust. It is a prima facie failure.
--
 
J

John Doe

Jan 1, 1970
0
Think about this:

You have a propeller facing into the wind, and to it you have
connected a shaft which drives the input of a gearbox.

The output of the gearbox is connected to the input of a
differential gear, and the outputs of the differential are
connected to axles with wheels on their ends, resting on the
ground.

Now imagine all this mounted to a platform with two extra
freewheeling wheels on it so it can roll along the ground.

If the propeller spins, the driven wheels _must_ turn and the
"cart" _must_ move into the wind.

Only if there were no headwind, the thing that turns the
propeller. You have headwind resistance against the propeller (and
the craft) that pushes pushes the craft backwards.
--
 
J

John Doe

Jan 1, 1970
0
John said:
You'd need a hull that's very slippery, so the wind doesn't push
much on it.

The headwind pushes on the propeller, that is enough to keep the
craft from moving forwards or to make it move backwards.
That's easy... keep most of the hull underwater. Now the
question is how much power the windmill makes for some amount of
wind load on the structure; the water propeller has to generate
more thrust than that. Marine propellers are very efficient, so
gear it down until you gave more prop thrust than wind load on
the windmill. It would be an ugly boat, but it would work.

If that were true, there would be a zillion attempts at United
States patents, with at least one successful patent. Not talking
about a silly Internet patent, talking about the United States
patent office that has deflected (millions?) of "perpetual motion
machine" patent attempts.
--
 
J

John Doe

Jan 1, 1970
0
John said:
If you accept that a wind-powered boat that uses a windmill is
not "sailing", then you win.

But sailing into the wind violates no basic laws of physics, so
other modes might be possible.

There are none to begin with.
I am reminded of an automobile torque converter that has torque
gain using fluids.

The idea that a vehicle can sail downwind faster than its tailwind
is a joke, John.
--
 
J

John Doe

Jan 1, 1970
0
Phil said:

Because you are overlooking the headwind backward pressure against
the propeller (and the craft). With the propeller attached to the
wheels, the propeller will never even rotate, it might even rotate
backwards. Would be interesting to see whether the propeller
rotates backwards depending on windspeed.

I figured it out, thanks in part to the discussion. It is a joke.
They are acting as though the propeller/sail is not part of the
vehicle. Using a propeller instead of a sail is just to add
confusion. Windspeed at the height of the sail is greater than
wind speed at the base of the vehicle, due to ground friction
against the wind. The tailwind blows against the sail and pulls
the base until the sail pulling the vehicle reaches near tailwind
speed. At that point, the base of the vehicle is moving faster
than the tailwind. And there you have the (farce) vehicle moving
faster than the tailwind.
--
 
J

John Doe

Jan 1, 1970
0
John Larkin said:
Gearboxes can have more output torque than input torque.
Any amount,
actually. If there's 1000 pounds of wind force on the windmill, adjust
the gear ratio for 2000 pounds of thrust at the wheels. Or 200,000
pounds. All you lose is speed.

You perpetually ignore the fact that there is backward pressure
against the propeller (and the craft).

You cannot get there from here, John
--
 
M

Martin Riddle

Jan 1, 1970
0
Richard Henry said:
The first notch on the bullshit meter is conservation of energy.

Wasn't it conservation of charge?
 
J

John Doe

Jan 1, 1970
0
Phil said:
No, it isn't. If you think it's obvious, post the equations.
It works for railroad cars, as I showed earlier,

Shown?

If there were such a device that produces forward motion from a
headwind, it would be patented already and very easy to find.
and it obviously won't work for sailplanes, because there's
nothing to push against except more air. The sailplane case *is*
a perpetual motion machine,

Sailplanes use thermals. A sixth grader knows that much.
--
 
J

John Doe

Jan 1, 1970
0
John said:
There's no perpetuasl motion, no more than a stationary windmill
has. We'd be extracting power from the wind, which people have
been doing for centuries.

Same for fighting a headwind, against the propeller and the base
vehicle.
The power drives an underwater propeller, ditto centuries.

Citations are required.
--
 
M

Martin Riddle

Jan 1, 1970
0
Jim Thompson said:
How can you sail _straight_ downwind _faster_ than the wind, and still
gain energy from the wind at your back?

You can certainly "tack" at a vector faster than the wind vector... I
think, I'm not a sailor.

But straight with the wind at your back I don't think so.

...Jim Thompson

You can't go faster than wind either against it or with it.

However, the lift generated from a sail can result in speed greater than
the wind speed when you tack into the wind.
Ice boating easily demonstrates that. At 5 miles per hour it's about 4
to 1, and at higher winds speeds its 2 to 1.

If your on a boat, your limited by the hull speed. Hull Speed = 1.34 *
(LWL)^1/2 where LWL is the length of the hull at the water line.


Cheers
 
How can you sail _straight_ downwind _faster_ than the wind, and still
gain energy from the wind at your back?

Right. At the same speed there would be no force on the propeller = no net
energy from the propeller. If you went faster the propeller would go
backwards, which runs us afoul of those who still believe in the conservation
of energy.
You can certainly "tack" at a vector faster than the wind vector... I
think, I'm not a sailor.

Certainly your speed can be faster than the wind speed, but your velocity in
the same direction (either sign) cannot be.
But straight with the wind at your back I don't think so.

I don't think so either. Energy has to come from somewhere else other than
the wind (see above).
 
J

John Doe

Jan 1, 1970
0
Jim said:
John Larkin
<jjlarkin highNOTlandTHIStechnologyPART.com> wrote:

[snip]
Gearboxes can have more output torque than input torque. Any
amount, actually. If there's 1000 pounds of wind force on the
windmill, adjust the gear ratio for 2000 pounds of thrust at
the wheels. Or 200,000 pounds. All you lose is speed. In any
given situation, there is some best gear ratio that will give
you the max speed into the wind; it's an impedance matching
problem.

John

Damn! Who would have thunk such a thing ?:)

Someone who does not recognize the fact that there is backward
pressure against a propeller and its base. Someone who does not
understand that when you perpetually move at half the distance to
your goal, you will never get there from here. I am impressed that
some engineers are bold enough to agree with such a silly idea,
that you can move into the wind using only wind power against a
propeller. Of course, when it matters, they can say "I was
just kidding!"
--
 
J

John Doe

Jan 1, 1970
0
It is easy to prove with an experiment. Between now and then, I will
go back to watching TV...
 
E

ehsjr

Jan 1, 1970
0
Jim said:
Am I missing something?

That's not the same as a sailboat trying to sail straight downwind.
How can a sailboat's speed exceed that of the wind at its back (except
by tacking).

I've never sailed, so maybe I'm missing a trick ??

...Jim Thompson

The wind velocity drops suddenly, the boat takes longer to slow down.
The speed comparison should be peak vs peak, but it isn't.

Ed
 
G

Graeme Zimmer

Jan 1, 1970
0
Wickipedia thinks that it's possible
see
http://en.wikipedia.org/wiki/Windmill_ship#Points_of_sail

and especially go see
http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/

"Skeptics think that the wind is turning the prop, and the car is turning
the wheels, and that's what makes the car go," Cavallaro said. "That's not
the case. The wheels are turning the prop. What happens is the prop thrust
pushes the vehicle." The wheels turn the prop, which turns the vehicle's
wheels, which turn the prop, which turns the vehicle's wheels. Cavallaro
knows what you're thinking."It sounds like a perpetual motion machine - but
you've got the wind as an external power source," he said.

............. Zim
 
J

Jasen Betts

Jan 1, 1970
0
I repeat: conservation of energy.

You need to arrange for the sails to travel slower then the vessel.

If you can do that, and get friction low enough, you can go faster
than the wind.

water is kind of mushy, it might work better on land.

--- news://freenews.netfront.net/ - complaints: [email protected] ---
 
N

Nobody

Jan 1, 1970
0
Which way is this magic windmill facing??
As soon as the boat moves the wind velocity at the windmill starts to
fall.
Extreme case boat spead = wind speed then mill speed and hence mill
power = ooo.ooo

Here's an idea:

Suppose that you have three or more windmills attached to horizontal arms,
all rotating about the same vertical axis. At least one of them will be
moving backward relative to the vehicle at any given time, so it would
have non-zero airspeed even if the vehicle was travelling as fast as the
wind. The forward-moving windmills would be feathered (no significant
thrust or drag).

I'm not saying that it would work, just that it's not quite as
straightforward as the above quotation suggests.
 
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