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(Edited per the annotations)
It is a boat meant to operate in a flowing current e.g. a
river, and travel upstream using power extracted from
the flowing current.
This has some similarities to sailing downwind faster
than the wind, in that it extracts energy from a flowing
medium, to propel
a craft in a not-obviously-possible
direction of travel.
It
is also similar to wind-turbine-driven-propellor boats
that
can sail directly upwind, for the same reason.
In one (unlikely) incarnation, water
turbines would drive large fans (aka propellors or
airscrews) that operate in the air to
propel the boat upstream. Similar to a swamp boat,
except that the propellor is stream-powered.
I doubt that the propellor-driven version would work.
But it's easy to imagine a version that pulls itself up a
cable anchored on the banks of the river, using a
stream-driven winch.
Mythbusters - blow your own sail
http://mythbustersr.../blow-your-own-sail
[spidermother, Aug 08 2012]
Sailing directly upwind
http://www.youtube....watch?v=8Rw_qJytbG8
model boat [afinehowdoyoudo, Aug 09 2012]
DDWFTTW - Treadmill
http://www.youtube....watch?v=oTMRviy-5zY
[MechE, Aug 10 2012]
Full Sized
http://www.youtube....SCI&feature=related
[MechE, Aug 10 2012]
"A Long, Strange Trip Downwind Faster Than the Wind"
http://www.wired.co...pia/2010/08/ddwfttw
Rick Cavallero's 27 Aug 2010 Wired magazine article [CraigD, Aug 13 2012]
This boat will do it for you.
http://en.wikipedia...wiki/USA_17_(yacht)
... she could achieve a velocity made good upwind of over twice the wind speed and downwind of over 2.5 times the wind speed... [scad mientist, Aug 21 2012]
Hullaballoon
[FlyingToaster, Jul 27 2013]
Yet another thing moving faster than the airstream
https://www.youtube...watch?v=xHsXcHoJu-A
[not_morrison_rm, Jul 27 2013]
Another treadmill video
http://www.youtube....watch?v=AVDLiNbCjN8
[AusCan531, Jul 28 2013]
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Annotation:
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I don't think this would work. The boat sits in the
water, so absent any force to
propel it upstream, the boat would simply drift with
the current while the turbine remains idle.
So you'd need a large sail to catch the air, thus
resisting the current's movement
and allowing the turbine to move. This would create
pressure on the aft side of
the sail. But as the fan pushes the boat forward, the
pressure would increase on
the sail's fore side, until the pressure on both sides of
the sail was equal. At that
point, there would be no resistance to the current,
and the turbine would stop
turning. Essentially, any increase in fan speed would
result in a commensurate
loss in power at the turbine. |
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As a practical matter, you couldn't ever accelerate
fast enough to actually cause
the pressure on both sides of the sail to equalize,
because that would require
putting out as much energy as you take in. So you'd
end up with the boat simply
drifting downstream, at a speed determined by the
percentage of energy lost
between the turbine and the propeller times the
curent speed. |
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I could be totally wrong about all of that, of course. |
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I'm not going to make any hard and fast declarations, but the fans themselves act as sails. That is, as the boat starts to drift down current, they resist it. This results in some power from the screw, which provides more resistance at the fans. You could definitely produce some motion against the current, but I'm not sure if it would be enough. |
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I think this is far less likely to work than DDWFTTW, but I'm not going to declare it impossible. |
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A fan fixed on a boat will not blow the boat forwards, regardless of how the fan is powered. This one fails basic thermo. |
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Your analogy is imperfect, or at least, the title is
confusing. The equivalent of DDWFTTW would be
either *downstream* faster than the current, or
upstream *at all* (in still air); more generally, you
would have to experience forward motion relative
to both the air and the water. |
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So, as I see it, in still air and a 10 m/s current
relative to the banks, you should qualify if you
move upstream at 1 m/s relative to the banks,
which is only FTTC if you reckon the vessel's speed
relative to the water, but the current's speed
relative to some other reference, such as the
banks, which I assume is what you meant, but
which is not self-evident. Hence the confusion :-
/. |
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[RayfordSteele] A fan will blow a boat forwards - c.f.
swamp boats. The fan is't blowing on a sail, or any
such daftness (although Mythbusters showed that
that works too, albeit badly). 'Propeller' or 'airscrew'
would be better terms than 'fan'. But this idea
doesn't break the laws of thermodynamics. It's
possible to harvest energy from a velocity difference
in, and to use that energy to travel faster than both.
Albatrosses do it all the time. |
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spidermother, I'm aware of swamp boats, and I'm aware of albatrosses and such. I just am skeptical of the whole blowing against a sail premise. Link? |
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This isn't a fan, it's a propeller. No fixed sail. |
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As I say, the fan on a sail was done by
Mythbusters (link), and is entirely unrelated to
this
idea, but was mentioned in response to
[RayfordSteele]'s comment. When they directed a
very powerful fan (from an actual swamp boat) at
the centre of a large sail, the boat did indeed
move forwards - slowly, and with lousy control,
and in danger of capsizing - presumably due to the
large and uncontrolled airflows directed sideways,
with a puny backwards component. It might seem
paradoxical, but it does work. Similarly, a put-put
toy steam motor propels a boat forwards, even
though the water is both sucked in and blown out
via rear-facing openings. |
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This could work, except in practice. |
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(MaxwellBuchanan), I tend to agree. It works in theory, but the difference between theory and practice is that in theory, they are the same, but in practice they are not. |
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A boat cannot extract power from the current any more than I can extract power from the ambient air pressure. Without a sail or an anchor there is no relative motion, and thus no available energy. |
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[WcW] This is what got me at first with DDWFTTW, but this can extract energy. |
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The propeller acts as a sail. In still air while being moved by a current, the propeller creates some drag, which holds the boat against the current (slightly). |
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This slight holding force causes the screw to extract some energy from the water. This energy is transferred to the propeller which acts to increase the resistance to the current, and so on. |
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What DDWFTTW comes to is that the wind has more energy than a sail allows you to tap. That is it can push a sail to (near) windspeed, or it can push that same sail plus a drag of say a hundred pounds to (near) windspeed. If you take that hundred pounds of drag, and turn it into additional thrust at the sail (say, by turning the sail into a propeller, and spinning it), it allows you to travel at the sum of the thrust velocity and the wind speed. At some point that will create enough drag to equal the thrust (plus any mechanical losses), and then you can't go any faster. |
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This definitely allows a vehicle on a treadmill (equivalent to a current) in still air to move up the treadmill. This should, in theory, allow a ship to do the same. |
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I have my doubts because of the more chaotic nature of pulling energy out of water with a turbine/screw as opposed to from land with a wheel. |
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totally bonkers. the boat is never aware that there is a current. |
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[WcW]
For a moment, replace propeller with sail. If a sail boat is sitting in a stream, in still air, it will move downstream slightly slower than the current, because it sees the still air as a breeze blowing against the current. This difference is apparent as a (slight) current. |
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At t=0, that's all the propeller is, a fixed sail. |
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I think that you are getting confused. For all intents and purposes the boat that you describe could be a submarine, right? When you look at it this way it is apparent that a submarine cannot in any real way harvest energy from the current. No matter what the submarine and the current will reach the same velocity eventually and your system only proposes to make the submarine reach the velocity of the current even faster. |
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No, it definitely can't be a submarine. The key is the water/air interface. |
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Let me try again. A square rigged sailboat, in still water, with a light breeze blowing will move in the direction of the breeze. |
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This is functionally identical to that same sailboat, in a current, with still air (relative to the ground, not the sailboat). This results in the boat drifting downstream slightly slower than the current due to the retarding force of the air on the sail. |
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Please tell me you agree with this fact? |
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ETA: To clarify, please consider both cases from the reference frame of the water's surface if that helps. |
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Cultural reference to back up your point, my freedive mentor taught me to keep the boat upwind of the diver in the water. Should the motor stall, regardless of current the boat will go to the diver. |
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so, it is a sailboat? The idea is still worthless. You cannot harvest energy from the medium to which you are applying the same sort of energy and hope to gain much. Maybe in tiny areas like the turbulence at the bow a prop might harvest and re-direct a small amount of energy but there is no source of energy being wasted here. |
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The sail was to demonstrate that there is an
energy source, the differential between wind and
water. |
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Now, install a turbine in the water to harvest that
energy. Change your sail into a propeller, and
gear the two together, such that the energy
harvested from the water is applied to the air. |
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You still can't harvest more energy from the water
than the propeller applies to the air, but it is
possible to exert more force against the air than is
lost in drag at the turbine, resulting in a net gain
in position
(theoretically). And as this suggests, you are
applying them to two different media. |
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no quite the opposite, the air is a less efficient medium, thus you cannot move energy in that direction. you can try but you will only increase the drag relative to the current. |
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But there is no functional limit on the size of the
propeller relative to the turbine. What's critical
here is that it is possible for the propeller to
extract more energy from the relative wind than a
sail can. |
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Since a sail loses power as it approaches the wind
speed,
it can't extract any more energy at that point. But
the wind (relative to the surface) still has energy
available. This means that the wind can still push
a craft with a large enough sail essentially as close
to the wind speed, even if it has higher drag. |
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So lets say we have a streamlined boat, with a
large sail/propeller. In a 10m/s current, it only
drifts 1m/s because the propeller prevents any
more movement. We then toss a small turbine
out that back that adds another 100 newtons of
drag. We can arbitrarily increase the propeller
size to catch more wind and offset that additional
drag, so the boat
still only drifts 1m/s. But that 100 newtons can be
used to rotate the propeller, which increases it's
thrust by pushing back against the (relative) wind.
The net result is a slightly faster movement
relative to the current, which produces a slightly
higher drag, which produces slightly more
propeller speed. |
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At some point you do reach the point where the
maximum efficiency of the propeller only acts to
balance out that drag and mechanical losses, but
at least on land, that is faster then the relative
wind speed, and thus faster than the current. |
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Direct down wind faster than the wind is identical
in
function to this, and is feasible. Some of the
early
tests were even closer in concept, as they were
run
on a treadmill in still air. I am posting a couple of
links to youtube videos. One is a treadmill test,
and
the other is the full size. Please note that the
full sized is an official record holder of the North
American Land Sailing Association, and they did
check for gimics/cheats. |
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still impossible, you are asking the little car to go uphill on a slope. anyone can see that that is impossible. |
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I edited my post to clarify. Please check the videos,
and explain how they work then. |
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they work well, its a real phenomenon. explain to everyone why the car does not roll uphill. |
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the car does not roll uphill for the same reason that the boat will not move faster than the current. you might be able to covert a small amount of the differential of energy, (the water moving, the air not moving) and, as in the system of the treadmill, you would allow the boat to move slightly less rapidly relative to the air (albeit a large sail will act in a relatively similar fashion) but the boat will still be moving relative to the air and thus you will still be losing ground. Think about it this way: why can't you build a boat that goes faster than the current based on the same principal, surely if it works in one direction it should work in the other. |
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//surely if it works in one direction it should work in the other.//
Shirley it would. |
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For the record the same team (full sized) also has the NALSA record for sailing direct upwind, and (I believe)that is using the reverse principal of driving the wheels from the propeller. |
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And who says the cart can't work uphill? Obviously it can't handle a hill steeper than a certain grade, and it will be slower, but it can work (on a treadmill, or if you have a wind blowing directly downhill). You did notice how the cart in that video was pulling forward against the strings, despite the only energy input being from the treadmill moving backwards. That means it is moving faster than the treadmill (current). |
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In order for the cart to be able to move downwind faster than the wind, an equivalent boat in a current (theoretically) has to be able to move upcurrent, since in a current the effective wind speed is the inverse of the current. |
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Again, there is no violation of conservation laws here, this is not over unity. It's just able to take more advantage of a slower wind speed than a sail. This isn't anything different than a sailboat's ability to sail faster than the wind on a downwind reach, because the angled sail allows more efficient energy capture than a square sail. |
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what you are doing is sailing faster than the wind, with the wind behind you. |
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Exactly. And it's been proven possible. That's exactly what happens in those videos. And it's possible because the limit on sailing directly downwind (with a sail) is windspeed, not energy. The propeller is a way to use more of the wind energy to produce an (effectively) higher wind speed by pushing the air in front of the craft backwards against the wind. |
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To my knowledge the phenomenon hasn't been demonstrated on a boat that actually has to move, displacing water as a result. |
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The increased drag of a boat relative to a land craft, the lower efficiency of a turbine relative to wheels, etc. |
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There's also the fact that it's highly unlikely that you're going to get still air, and a cross breeze could be disastrous, to say nothing of a headwind that more or less matches the current. |
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As I've said above, I have my doubts about the practicality of it in water, but not that it is THEORETICALLY possible. |
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You can get mechanical work from flowing water (a
paddle wheel or screw), but, as has been
previously noted, only if the water is flowing
relative to you. |
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So for something like the original upstream-moving
boat to work, it can’t be free-floating. It needs to
be anchored somehow to the streambed, bank, or
similar – say with bottom-reaching poles, anchors
on cables, or a gripping wheel on a fixed below or
above water cable. It could then transfer work
from the moving water via a paddle wheel,
submerged screw, or whatever, to the anchoring
poles or cables, hauling itself upstream. |
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It’s not a free-swimming boat, but a bottom/bank
crawler. |
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Physics wise, the hauling-upstream force must
exceed the pushing-downstream force on the
wheel/screw/whatever, which the hauling power
(force * distance / time) must be less than the
pushing power, so the boat’s upstream speed
(distance / time) must be less than the waters
downstream speed. Since many navigable stream
are pretty slow (1 to 3 MPH for the Mississippi, for
example), this boat wouldn’t likely be very fast,
but as it’s getting free energy, if you could build it
(or its system, if it uses prepared attachments,
like a fixed cable) to be sufficiently durable and
low-maintenance, it might be economically (and
environmentally) attractive for moving stuff that
doesn’t need to be moved fast. |
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[CraigD] Please read through all the comments. |
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The
movement against the current comes from the
propeller/ship's body acting as a sail capturing
energy from the relative motion of the current to
still air. Thus, no anchor is required. |
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Also DDWFTTW has resulted in multiples greater
than two of windspeed, which would allow
movement upstream at least as fast as the current
moving downstream. |
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I can believe a little movement against the current, but if
someone is claiming a speed against the current which is equal
to, our greater, than the speed of the current, I would not
believe it. |
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If the speed was exactly equal, for example, the sailing boat
would be in still air... |
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[Ling], the speed that is to be faster than the
current, is the relative speed between the water and
the boat. A small speed relative to the banks of the
stream, in the upstream direction, is all that I
suggested. |
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can a the mechanical energy from a more efficient medium (water) be translated via drag in to thrust in a less efficient medium producing a net positive vector of force? |
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[MechE] et al. are right. [WcW] et al. are wrong.
This is indeed theoretically possible. |
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//can a the mechanical energy from a more
efficient medium (water) be translated via drag in
to thrust in a less efficient medium producing a
net positive vector of force// Drag does not enter
into it, except as a source of inefficiency. Neither
propellers nor turbines operate by drag, unless
they are really bad ones. |
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My main problem with this idea is that it is kind of
obvious; it's just one of the many permutations of
deriving power from two media with relative
motion, with the goal of moving faster than both. |
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I assert that it is impossible to build a water turbine with a higher drag/torque ratio than the torque/thrust ratio of the wind turbine that it would power, even if the inefficiency of all the other components is 0. In the other direction it may certainly be possible, but in this direction I would postulate that it is not possible to claw out more energy than is required to stay at an even pace with the current because the water turbine with inherently, immutably, produce more drag than the air turbine will be able to produce thrust. |
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//Also DDWFTTW has resulted in multiples greater than two of windspeed, which would allow movement upstream at least as fast as the current moving downstream.//
Wow! Very cool stuff, Rick Cavallaro and others recent demonstrations, on paper an in glorious 25 foot tall propeller driven land yacht form, of sailing downwind faster than the wind. I’d been aware it was possible in principle since my youthful sailing days, as many sailboats beat downwind faster than they run, but hadn’t been aware of the controversy and cool machines stoking it over the past decade. |
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The physics of it are delightful – simple, yet counter-intuitive. |
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//[CraigD] Please read through all the comments.
The movement against the current comes from the propeller/ship's body acting as a sail capturing energy from the relative motion of the current to still air. Thus, no anchor is required.//
I was focusing on the idea of getting power from a flowing water-over-waterbed current, rather than air over land or water, which I think holds promise for a practical vehicle, as well as a variety of nifty little foam and string paddle wheel toy suitable for a HS tech class project. While perhaps not as counterintuitive as Cacallaro or Jack Goodman’s big and little land yachts, it’s still pretty counterintuitively cool. |
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Also, my pronouncement that
//the hauling-upstream force must exceed the pushing-downstream force on the wheel/screw/whatever, which the hauling power (force * distance / time) must be less than the pushing power, so the boat’s upstream speed (distance / time) must be less than the waters downstream speed//
Is wrong. The physics of a directly upwind driven vehicle (exchanging water for air and pole/winch on the streambed/bank/anchor cable for wheel on the ground) are nearly the same as for downwind (all that’s different is the sign of V_{stream}) - something like: |
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P_{in} = (V_{boat} + V_{stream}) F_{paddle}
P_{out} = V_{boat} F_{cable}
F_{cable} >= F{paddle}
P{in} n >= P_{out} |
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which, assuming F_{cable} = F{paddle} gives:
V_{boat} <= V_{stream} n/((1 -n)
where n if they whole system’s power efficiency, P mean power, V velocity, and F force. |
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So if the system’s more than 50% efficient, (practically challenging for a boat, I think) the boat will get hauled upriver faster than the current. Like the DDWFTTW land yachts, as efficiency approaches 100%, the speed approaches infinity for any non-zero current/wind speed. |
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That's some pretty intensive math there. 50% efficient sounds like a "I pulled this straight out of my ass" number. lets imagine this differently, so it can mercifully have slightly more clarity; |
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the boat is sitting in still water. |
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a motorboat (the land) is going in the intended direction at 5 knots. |
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the boat is seeing a 5 knot tail wind in the intended direction. |
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Can the boat make more than 5 knots with a 5 knot tail wind? |
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(Hint: Can the hull of the boat produce a drag factor less than zero?) |
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Also, this is what I would refer to as "fringe gullibility" : if X works then Y must work too, and your doubt of X is evidence that your doubt of Y is also wrong. It's a fallacy, and also an intriguing form of crock-pottery because it would be quite easy to produce a tiny test rig to demonstrate this but you will never be able to do it because it is actually impossible. |
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The hull drag isn't a factor. An arbitrarily sized sail
can be made large enough to intercept enough air
power to keep the boat moving at, almost, wind
speed regardless of hull drag. It can then be sized
up further to provide additional power. The
propeller allows the conversion of that power into
additional speed. |
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The actual upper limit on speed should be directly
related to the power that can be extracted from
the wind, which is dependent on the relative
speed of the wind/water. |
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//That's some pretty intensive math there. 50%
efficient sounds like a "I pulled this straight out of
my ass" number.//
It’s clever math, but not mine – it lifted, with
slight rearrangements to make it prettier to my
eye, from Rick Cavallaro’s old sketch, via the
“posted on the internet” link in the linked wired
article. |
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The 50% efficiency is not of anal origin. It’s a “I
did some simple algebra on the immediately
preceding equation, assuming any interested
reader would follow it” number, that is:
given: V_{boat} <= V_{stream} n/((1 -n);
V_{boat} = V_{stream} = 1
then: 1 <= n/((1 -n);
1 – n <= n;
1 <= 2n;
1/2 <= n;
So “n >= 1/2" reads “more than 50% efficient”. |
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//lets imagine this differently, so it can mercifully
have slightly more clarity//
I think the confusion is that, though the
underlying physics is only slightly different, MechE
and I are describing very different machines. |
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His has a sail/propeller in the air, connected to
some sort of paddle/screw in the water. It’s
power comes from the speed of the air relative to
the water. |
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Mine is essentially Cavallaro’s land yacht rolling
along the bottom of the river (or equivalently,
floating connected to a cable connected to the
riverbed or banks). Mine ignores the air (it could
be completely underwater). It’s power comes
from the speed of the water relative to the land. |
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Both find, with only slightly different math, the
same “50% efficient move upstream at the same
speed as the water moved downstream” (assume
still air for MechE’s machine) and “100% efficient
moves upstream at an infinite speed”. |
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You gotta love the tasty crunchy nuggets of
infinity in classical mechanics. |
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So long as you roll/pull a cable you don't need 50% efficiency, any level of efficiency will produce forward motion and that equation is completely without application. |
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Ok, one damn big umbrella shaped thingy made of something with neutral buoyancy, one long cable, a pulley system. |
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The cable runs the length of the river, one end of the cable goes to the boat, the other to the umbrella thingy, which goes into the river, and is dragged downstream by the current. |
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Via the leverage in the pulley system, it pulls the boat upstream faster than the current. |
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Looking at the treadmill video, I wondered if the air
dragged along by the belt is really a different case. It
would be slower than the belt speed, decreasing as the
height increases. This seems opposite of the river case. |
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No matter what any air drag is realized as a decrease in the relative speed between the air and the surface, reducing available power, increasing the impressiveness of the result. |
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[WcW] is attacking a straw man. An efficiency of 50%
is required to move upstream in still air at the same
speed as the current (or, equivalently, to move
downwind in still water at twice the speed of the
wind). This follows in a straightforward manner from
the fact that, for uniform motion, thrust equals
drag. Yes, an efficiency greater than zero is all that
is required to move at all, but nobody here has said
otherwise. |
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BTW, the simplest model I can think of to
demonstrate the basic principle is a pair of wheels
on a fixed axle, which acts as a spool for a length
of cord; think cable reel with the cable partly
payed out. If the cord runs out from beneath the
axle, then pulling the cord will cause the device
to move faster than the speed with which the
cord is pulled, which corresponds to a mechanical
advantage = (wheel diameter - axle diameter) /
wheel diameter) less than one. The fluid version
substitutes (lossy) inclined planes (aerofoils) for
wheels. |
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I'm not assailing a straw man, the idea has been edited to reflect the dubious nature of the thesis which is still embodied by the title. The title cannot be defended as an idea, but the boat winch concept is fine. It really must be pointed out that the boat winch concept also would work fine on a submarine vessel and thus the "sailing" aspect of the idea is entirely dispensed with. |
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The idea of the winch was edited in to illustrate a
similar concept that is less counter-intuitive than
the airscrew-driven craft, not to negate the original
idea. |
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I checked my old text books and found laws of
(classical) physics relating to Conservation of
Momentum, and Conservation of Energy. Didn't find
anything making absolute statements about limits
on drag/torque ratio and torque/thrust ratios. |
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As I understand it, upstream propulsion using
stream power violates neither conservation of
momentum, nor conservation of energy. |
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Ok, demonstrate it. I'm not incredulous. massive sail? massive fan? I want to see this baby crawling upstream in your endless pool. It wont work for exactly the same reason that the upwind systems do work. As your speed approaches the speed of the current you loose your "anchor" with the wind. You have nothing to push against, and when you go even faster you are "dragging" your anchor, clearly that is impossible. Please tell me that you can understand this. |
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[WcW] We've shown you a version on a treadmill, which is functionally identical to this. What are you not understanding? |
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it is in no way functionally identical to this. A slinky that can walk down stairs is very different from a slinky that can walk up stairs. A video of a slinky going down stairs is not proof that it can go back up stairs. |
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Please post a video of a vehicle paddling itself upstream in still air and I will recant my objections. Such a demonstration would be so easy to build. |
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I would even accept a video of a boat going faster than wind speed with a tail wind. So simple to demonstrate if it were possible. |
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Either would be fine. Please, either one. |
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You are simply ignoring my very clearly worded objections. |
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//I would even accept a video of a boat going faster than wind speed with a tail wind// |
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[WCW] Read the "Wired" link posted by [CraigD]. Very interesting. |
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A vehicle on a treadmill, still air; is conceptually identical
to a vehicle in current, still air. |
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[CraigD] mentioned it, but I wanted to see for myself so I found a boat that can beat downwind faster than the wind (see link). This can achieve VMG (velocity made good) downwind 2.5 times the wind speed (zigzagging at almost 4x the wind speed). |
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If a boat were to use some kind of screw in the water instead of a keel and a propeller instead of a sail, I don't see any reason why it would not be possible to create a SUFFTW boat. It is probably easier (or at least better understood) how to make an efficient water craft with a keel and sail rather than screw and propeller, so I wouldn't expect to ever achieve even close to 1.5x the speed of the current going upstream in still air (equivalent to 2.5x the wind speed in still water). |
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[WcW] //Such a demonstration would be so easy to build. I would even accept a video of a boat going faster than wind speed with a tail wind.// No one is going to throw together a demo since it is difficult of efficiently harness the water and wind. I assume (but could be wrong) that there are few sailboats that can achieve a VMG downwind equal to or greater than the wind speed because any boat that can do that has little use for a spinnaker (which most have). |
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I somehow doubt that it will ever be built because I doubt such a craft could win sailboat races probably wouldn't be as much fun to use as a leisure craft and otherwise has no practical purpose other than to prove a point. |
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Still I see no evidence that such a device can work. As such a
boat beating downwind would need to produce more thrust in
the air than it drags in the water which does appear impossible
to do. Can you do it by zig zagging? Maybe? Anyway nobody
seems to have found a way to do it yet even under more
favorable conditions (air/ice air/earth) |
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Found a way to do what? Sail direct downwind faster
than the wind? Yes they have. That's exactly what
the full size link shows. Sail downwind faster than
the wind by tacking? Yes, that's what Scad's link
describes. Move up treadmill in still air? Yes, that's
what my first link shows. What, exactly, haven't we
shown? |
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What matters is that the work done on the
turbine by the water exceeds the work done
by the propellor on the air, by a margin that
accounts for the total system efficiency. That is
not a problem, because the speed of the water is
greater than the speed of the air, while the forces
are approximately equal. Drag is inevitably
present, but for good aerofoils is relatively small.
It is quite plausible for the thrust of the propellor
to exceed the drag of the turbine. |
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I think the practicality or otherwise of such
devices is mostly beside the point, and not a
reason not to build it. After all, racing boats - and
a lot of other things humans do - are mostly
useless. But the same machine could not
efficiently travel DDWFTTW and DDCFTTC, as good
turbines make bad propellors, and vice versa. |
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A simple version might consist of a rigid rod with a
propellor at one end, and a turbine at the other,
the pitch of the turbine exceeding that of the
propellor, supported on bearings by a hull (or
hydrofoils) at a slant. |
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(I'm concentrating on the original winch-less
version, because it is more challenging. I still
think you need to drop the 'faster than the
current' requirement, because it is an arbitrary
benchmark, much as Mach 2 is arbitrary in a way
that Mach 1 is not. If it is simply included to
match the 'faster than the wind' requirement,
then you may have not properly understood the
symmetry.) |
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// But the same machine could not efficiently travel DDWFTTW and DDCFTTC, as good turbines make bad propellors, and vice versa. // |
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Is that actually true for applications like these? Clearly the screw on your typical outboard motor would be practically useless as a turbine, but it seems to me that it is designed small to avoid hitting bottom in shallow water, and although the low aspect ratio blades make it only effective at high speed, that's not a concern since it can be turned at high speed no matter the speed of the boat, and low speed efficiency doesn't matter much. On the other hand, a water turbine must work at low speed or it is useless. Similarly with airplane propellers: if you want to go fast enough to fly and don't want to have a really tall landing gear, you get a typical propeller, but ultra efficient low speeds aircraft (human powered for example) have long narrow propellers more similar to wind turbines. I suspect that for relatively low speeds as in both of these applications, high aspect ration blades would be most efficient for both propeller and turbine. |
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One difference: DDWFTTW will probably tend to have higher speeds than DDCFTTC, so the props/turbines might need to be optimized for different speeds, but that's not an inherent difference between props and turbines. It seems to me that a craft designed to work well with a 10kt. current could work well in a 10kt. wind. One problem is that you’d need to use a symmetrical profile on the blades. (or else have a variable pitch, variable twist, and reversible direction). An asymmetrical profile is more efficient when close to stall speed, but I think I saw (but can’t find it now) that at normal speeds a symmetrical profile has similar efficiency. I’m not sure if that would only apply to higher air speeds (not applicable to this craft), or if that is relative to stall speed. If it is relative to stall speed, then a symmetrical profile should work fine since the prop/turbine should be designed to operate far from stall speed anyway. Of course you’d need either variable pitch or variable gearing to switch modes. |
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[SpiderMother] Am I missing something significant here? |
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Can a shaft with a turbine on one end and a prop on the other propel itself upstream? I believe that the different properties of the materials involved make this impossible. It would be so easy to demonstrate this if it were possible, and the applications are countless. |
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Except you said in your previous anno //Anyway nobody seems to have found a way to do it yet even under more favorable conditions (air/ice air/earth)// which is conclusively disproven, since multiple people have done air/earth. |
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And it's not simple to test unless you have an endless pool, or a river with flat water and perfectly still air, neither of which I happen to have. Conceptually, if you can do it with wheels, there is no reason you can't with a turbine. |
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Direct Down Current (DDC) is equivalent to Direct Up Wind (DUW). Direct Down Wind (DDW) is equivalent to (DUC). (Except that typical wind/water speed differentials may be different) |
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For DDC and DUW, creating a prototype boat is pretty easy. For the most part, the boat will go the same speed as the water because of drag on the hull, and it's pretty easy to pull enough energy from the wind to move slowly in the desired direction. |
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For DDW and DUC, it's hard to build a boat since the boat must have very low drag in the water, but it might be relatively easy to create blimp that drags a turbine in the water to power its propellers. It will in general be moving at the same speed as the wind and will just need to extract enough energy from the water turbine to move slowly forward. |
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Of course this highlights the difference between optimizing for these two scenarios, in contradiction to my previous post. Although I still think building a single boat that can do both might be possible, since water drag can be very small in a well designed multi-hull. Building a DUW blimp doesn't seem likely... Although, the principle behind the hullaballoon might actually result in very low air resistance in a lighter than air craft, since the weight bearing gas envelope doesn’t have to move through the air. |
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You don't really need a blimp to try this, just moving water, still air and some sort of suspension. I'm not incredulous; I have given it a lot of thought. |
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//One problem is that you’d need to use a
symmetrical profile on the blades. (or else have a
variable pitch, variable twist, and reversible
direction)// That is precisely what I meant.
Symmetrical aerofoils are less efficient, and
efficiency is paramount here. Likewise, I agree
that the turbine and the propeller would be low
aspect ratio. No, I don't think you're missing
anything. |
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//For DDW and DUC, it's hard to build a boat since
the boat must have very low drag in the water//
Hence my suggesting hydrofoils. |
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//the different properties of the materials
involved make this impossible// Not necessarily.
The water-turbine will simply be much smaller
than the air-propeller, reflecting the different
densities, just as the keels of yachts are smaller
than the sails. |
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//simple to test// Start with a model in still water
in a wind tunnel, or still air over a flow tank.
Surely someone has tried this! |
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I was in the process of editing that wp entry. |
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It was written like a conspiracy theory, rather than
as a scientific entry. |
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I hope to get back to it soon. Look at the page
history. |
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//drive large fans...that operate in the air to propel the boat upstream. |
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Easily done. Get a load of (for example, Justin Bieber) fans and then propel them off the boat by railgun, should provide sufficient thrust to go upstream and possibly (with salvo fire) up a moving waterfall as well. |
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If you had a boat that could travel upstream, if you
put it in still water it would spontaneously start to
move forward. Which suggests there is an error
somewhere. |
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Thank god you showed up MB. I was just starting to loose confidence in the unworkability of this idea. |
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No need to thank him; it was actually I who brought
our meeting to a close. |
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And, with a wind over the still water, you can.
We've already covered that. In fact, that's exactly
what the second link shows. |
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The current in the water is always going to exert more force on the boat than the air. the best case scenario is a boat with no drag in the water at all, in that case, the sum of the forces is zero. In all other cases the sum is going to be negative meaning that the boat moves with the current at a rate less than a boat with no interaction with the air at all. |
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In that case it's impossible to sail upstream at all. |
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Again, the key thing is that it's possible to pull more
power out of moving air then is needed to propel a
boat at the speed of the wind. |
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And [WcW], given your contention, please explain
that second link. |
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I can: its not the same thing. |
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I'm obviously missing something (apart from hair, charisma or dress sense) here ..surely boats can sail upstream? Otherwise how would fellucas have done all that trade up and down the Nile over all those millennia? |
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I include tacking as "sailing upstream". |
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A submersible tow car Would work, by using the
current with a turbine, and wheels to propel it
forward. |
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[nmrm] That's rather the point. [WcW]'s statement
requires that a boat not be able to sail upstream. My
statement is to counter the absurdity of that. Heck,
ships can sail upstream and upcurrent combined as
long as the body of water is wide enough to tack. |
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Since the propeller can be treated as tacking even
when the boat is sailing straight, this is clearly
(theoretically) feasible. |
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you can sail directly upstream. You cannot in still air paddle up current. |
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//You cannot in still air paddle up current.// Ah,
well, actually you can. All that is required is that
you can paddle faster than the current. |
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You can also sail upstream, given suitable sailage,
adequate wind, and preferably a boat. |
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What you cannot do, howevertheless, is arrange
any combination of turbines, propellors and
suchlike, using _only_ the flow of the river and
with no part of it remaining fixed to the river bed,
to travel upstream. As was noted above, such a
boat would, when placed in still water,
spontaneously move forward at an ever-increasing
rate. |
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another best case scenario is a boat that sits atop the water and moves downstream at a very low rate because it presents no drag in the water at all and presents a massive drag on the air. |
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The Dutch already do that with Holland. |
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A lot of people assume that Holland goes all the way
down, but in fact it's built on an enormous floating
platform of reeds. |
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It's a little-known fact that the windmills are actually
giant fans which can be rotated as necessary to help
keep the country in position. |
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huh ? of course you can. As long as you have access to at least two media moving with different velocities you should be able to move in whatever direction you want to by tacking. |
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If you've a headwind of 20kph and a head-current of 2kph you can still tack in your watercraft, right ? |
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Likewise a 20kph head-current and 2kph headwind would be childsplay for an airship-sailor to tack into. |
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//of course you can. As long as you have access to at
least two media moving with different velocities you
should be able to move in whatever direction you
want to by tacking.// |
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Ahem. //What you cannot do, howevertheless, is
arrange any combination of turbines, propellors and
suchlike, using _only_ the flow of the river and with
no part of it remaining fixed to the river bed, to
travel upstream.// |
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toss a propeller into a stream and some of the stream's energy will turn the propeller and some of the stream's energy will push the propeller (linearly). |
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Arrghhh!!!! [marked-for-tagline] |
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//using _only_ the flow of the river// |
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Which is completely irrelevant to the idea, since it
is based on using the air/water interface, not only
the flow of the river. |
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oohhhhhh, is //flow of the river// supposed to mean output as well as input ? That's okay; I realize English isn't your first language. |
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A large, efficient, slow propeller is a close approximation to your an air tyre, or to a stationary winch, or to a mule named Sal. Which is why this idea is indeed theoretically possible. |
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But as I said, it's just one of the 72 permutations on the theme. (Coming soon: sailing down-plasma faster than the plasma). |
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Remembering Waterhouse's Navy maths test misunderstanding in the Cryptonomicon...surely the flow rate of the river is highest at the centre? |
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So if the boat lurks near the banks has something remarkably like a very long wooden spoon, with a pivot point that spikes into the river bed, then the difference in flow rate would tend to push the boat upstream..re-site spiky thing, repeat... |
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//using _only_ the flow of the river and with no part
of it remaining fixed to the river bed, to travel
upstream.// |
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^ mathematically you should be able to do it taking energy from the water and using it to move through air. |
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On the other hand, if you're specifying that system energy input and output both occur in the media of the flowing water current then I think we can all agree: "yes, duh" though if the river had a different speed at the centre than near the banks... |
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////using _only_ the flow of the river and with no part of it remaining fixed to the river bed, to travel upstream.//// |
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"remaining" it doesn't remain fixed, it is relocated. |
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//if the river had a different speed at the centre than near the banks... |
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Ahem, if you were to look two comments upwards. |
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<returns to whittling entire Sequoia to make enormous wooden spoon. ...cuts finger> |
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//Ahem, if you were to look two comments upwards// I was - that would be the neatest iteration: a watercraft that proceeds upstream, powered by water moving downstream, using only the river as energy media. |
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Surely what is called for here is not sailing or
paddling upstream, so much as wading upstream. |
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The craft has legs which it can anchor on the
riverbed. The legs hold turbines which capture
energy from the flow of the stream past the
stationary legs. When a useful amount of energy
has been accumulated, the craft takes a step
forward. It probably doesn't try to keep the
turbines working during the step. |
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The parts of the legs visible above the waterline
must be decorated with gondolier costumes. |
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marks for the gondolier costumes, I'm thinking if you grouped them in threes, you'd have a fairly nifty tripod, just ply 'em, with garlic and antibiotics at regular intervals. |
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With my version, there's always the off-chance of being able to outsource the whole thing to flocks of spoonbills and charge twitchers for birdwatching rights. |
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