h a l f b a k e r yTrying to contain nuts.
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Jim reckons that embedding a propellor into the standard fixed wing of an americas cup boat will increase windward side pressure --- but only when the wing propellor is attached to a trailing prop beneath the water line...
Ah --- thats all you need ... do some maths...
[link]
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The 'propellor' is powered by what? |
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A prop--or any large complex polygonal structure, for that
matter--affixed to Jim's fixed-wing sail will produce so
much drag as to make the sail utterly useless. You'd be
better off to get rid of the sail and just use a wind turbine
to power the screw, which I believe has already been done. |
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How are unpowered propellers going to do anything besides
add weight and drag? Maybe I just need to see a picture of
what you're talking about. |
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It's not going to increase side pressure if it's embedded now is it. Now using a texas windmill, or a whole bunch of them _as_ a sail, that might work. |
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The idea above indicates that.. |
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--- The trailing prop will spin when the vessel moves forward |
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--- The wing prop is spun by attachment to the trailing prop |
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The density of water is about 1000 times that of air and (surprisingly) the viscosity of water is only about 50 times that of air. So it appears that water is dense and slippery and air is light and viscous. |
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The amount of energy transfered from the trailing prop to the wing prop is proportional to wing and trailing prop sizes and the density and viscosity of air and water. |
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The relatively equal viscousity and massively different density means that a large wing prop can be powered by a small trailing prop. |
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Ordinarily a boat will attain maximum velocity drag equals the pressure difference across the wing by the wings area. |
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In this case as the boat increases speed the pressure difference across the wing will also increase --- up until the mechanical losses between the trailing and leading props equals the pressure differental on the wing. |
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If by 'trailing prop' you mean a propeller in the water,
please call it a screw to alleviate confusion. |
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And once again, unpowered prop = drag. Drag like the
golden age of coal mining. Drag like the drunk guy at the
party who won't go home. That's why pilots feather their
prop if an engine goes out, to reduce drag. |
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ah i see... so the water propeller is supposed to drive an air propeller ? and that would work better than pretty well anything else because ... ? |
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Okay, for every 7 points of specific gravity consumed by the yeast approximately 1 percent of alcohol by volume will be developed. Jim is probably familiar with this formula. |
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If the screw and propeller were one hundred percent efficient, this would result in no net change in velocity, since the propulsion from the propeller has to be equal to the drag from the screw. |
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Since neither is anywhere close to 100% efficient, this will reduce a significant reduction in efficiency. |
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Incorrect - conservation of energy is a rule not conservation of
velocity… |
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Consider Indiana Jones running from a ball bearing in the temple
of doom. Indy is better of on a skate board holding a roller skate
against the ball. |
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Conservation of energy says Indy can charge a battery from the
rotational momentum of the ball bearing - while traveling at the
same velocity as the ball :) |
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Indy can also transfer this energy to the skate board … |
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The maximum velocity Indy can obtain is proportional to the
total energy of the ball bearing NOT the velocity. |
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Velocity is not conserved, but energy is. If an object goes faster, it has a greater kinetic energy. This energy has to come from somewhere. |
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The ball has gravitational potential energy that Indy can pull out, otherwise it wouldn't be rolling. |
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The propeller, on the other hand, has no energy other than what comes from the screw. The screw develops its energy from drag against the water. Under theoretically perfect (non-second law) conditions, the prop can add no more energy to the craft than drag at the screw removes from it. Under real world fluid conditions, it adds significantly less, since propellers and screws are both far less than 100% efficient. |
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Physics, Madness, physics! It should be a required course
for halfbakers. A sailboat with two propellers attached is a
closed system. The wind acts as an external force. Once
the sailboat has reached its top speed it cannot accelerate
any more without a greater external force. The second
propeller (the "screw") isn't an external force because it is
turned by the flow of water over it. The water is flowing
over it because the wind is pushing the boat. So now the
same amount of wind is pushing the boat and pushing the
propeller through the water. The boat slows down. The
only way for the boat to achieve a faster speed given the
same velocity of wind is with a larger or more efficient sail. |
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Ok so you missed the anno about indiana jones... |
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A sail boat travels faster than the wind all the time, (surfers and) land yachts do the same. The fastest is apprently an ice yacht that regularly goes twice as fast as the wind velocity. |
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The anno about Indy above should have been enough --- but simply the wind weighs in more than the boat so there is loads of energy to tap. Jim just found a way.... |
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Sail craft travel faster than the wind when they are sailing
across the wind, because the sail acts as a wedge. That has
nothing to do with this idea (and I did respond to the Indy
comment). You are adding effective sail power by
increasing drag, and the two have to balance in this
instance. |
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Please ignore most of my former comments, as I have been reading up on Direct Downwind Faster than the Wind (DDWFTTW), which makes this conceptually baked. |
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In order for it to work on a sail boat (as opposed to a land sailer) however, it needs to be the case that the screw can't "slip" in the water, which isn't the case. So it almost definitely still won't work in a water sailing environment. |
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In addition, it would produce no net benefit in any wind other than a true tail wind as these already take advantage of the sail/keel effect to produce a net speed faster than the wind, and do so more efficiently. In fact, even based on the DDWFTTW results, it appears that tacking downwind is actually faster than this idea can produce. Meaning it is faster to run at an angle to the wind, back and forth across the wind, because the sail ends up being more efficient than the propeller. |
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So the net result is that the idea only works theoretically in one direction, and a sailing craft can already do better than that. |
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// Ok so you missed the anno about indiana jones...
// |
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He didn't miss it, he just ignored it because it was total
rubbish. |
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Look, I have no more formal education in physics than
what you get in high school--so none, really--but I have a
helluva lot of real-world applied physics knowledge. So let
me break it down for you, one layman to another: |
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If you put an unpowered screw on a sailboat, it will
produce drag. If you put an unpowered propeller on a
motorboat, it will create drag. Because of inherent
physichal properties of an object that has mass, including
but not limited to weight, inertia, and anterior surface
friction, neither of those devices are able to produce as
much
or more thrust than the drag they create. That is why
motorboats use powered screws, sailboats use sails, and
every prop-driven vehicle in common usage uses a powered
propeller. |
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I'm sorry, but no matter which way you slice it, it just
won't work. Yes, you may be going faster than the wind,
but you'll be going slower than you would with out the
unpowered prop/screw/whatever. |
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And Indy was running away from the giant stone ball in
Raiders of the Lost Ark, not Temple of Doom. |
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Well at least this idea was useful for bringing DDWFTTW to my attention. |
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Be sure to check out the autogiro boat link from [NothionToby]. That seems like the most likely way that this idea coud actually be useful. However I have my doubts that the autogiro boat could approach the efficiency of a sail in normal conditions, so adding on a screw in the water probably wouldn't make up the difference. I'm basing this on my vague recollection that giroplanes are not great in the efficiency department, but are used because of other good characteristics. |
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The article suggests that the autogiro boat worked very well, but there is no data saying how it compared to a standard sail other than possibly being able to sail closer to the wind. I'm no sailing expert, but does that generally indicate a better sail overall, or is that just one aspect that can be traded off for other benefits? |
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Of course as you can see by the quality of the above argument, I could be completely wrong. Maybe autogro sails can be better than standard ones, and adding the prop could enhance this even further. |
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"Conservation of Energy is NOT the same as Conservation of Momentum". |
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"Fluid drag; Worse Than You Ever Thought It Was Going To be". |
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"Entropy: It Doesn't Hate You, It Just Is." |
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"How To Nail Jelly To The Ceiling". |
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Sigh --- this is my reduction. |
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It doesnt matter how you figure it when it comes to conservation of energy you have got to have big balls... |
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Sure... I conserve energy all the time. |
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