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Imagine a boat, moving along in the way that a boat does. Let's assume that it creates a significant bow wave.
In the open ocean, that bow wave just travels away from the ship, eventually dissipating its energy in the form of heat. What a waste.
Now imagine the same ship travelling along a canal
with high walls on each side. The bow wave now hits the canal walls and is reflected back, hitting the aftmost sides of the ship. No big deal.
Now imagine that the ship is tapered from its wide midsection to its narrow stern. The reflected bow wave now hits the tapered rear half of the ship, and will push it forwards. In other words, some of the energy that was put into making the bow wave is now recaptured by the ship, adding to its propulsion.
Howevertheless, this will only work when the ship is travelling along a canal with walls to reflect the bow wave. It's no use in the open ocean. To quote the great American statesman: "shame".
But wait! Our ship carries outriggers on either side. Attached to these outriggers are two flat sheets of metal, lying in the vertical plane and parallel to the long axis of the ship. These sheets dip a few feet into the water. Since the sheets are quite thin, and are slicing edge-on through the water, they cause negligible additional drag. But they do reflect the bow waves, just like the walls of the canal.
So now our ship can cruise the open ocean, with its bow wave reflecting off the outriggers and striking the tapered stern, thus recapturing part of the energy that would otherwise be wasted.
This idea terminates here. Passengers wishing to make onward journeys should check the information board for details of their connecting idea. Please check that you have all your personal belongings with you before disembarking.
Busemann biplane
https://web.archive...busemann/index.html
Conceptually similar if you add an axis of symmetry [EnochLives, Aug 09 2017]
Part of the Soliton collection at Heriot-Watt University
http://www.ma.hw.ac...ESS_CANAL_BOATS.pdf
[Ling, Aug 09 2017]
Hump speed
http://www.abs-hove...ercraft-hump-speed/
About hovercraft speed issues [neutrinos_shadow, Aug 10 2017]
Wave Transmission Past Vertical Wave Barriers
https://www.google....Qy7-56Wjjd4wOV3vsdg
[scad mientist, Aug 13 2017]
Proposed shape of ship to make this work
http://www.billythe.../detail/AGWE517.jpg
So most if the ship is tapered to catch those reflected waves. [doctorremulac3, Aug 14 2017]
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That is an excellent idea because (a) it might offer additional propulsion and (b) "Slant the outriggers!" sounds really good with a pirate accent. |
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There's a strong physical similarity between surface water waves and shock/expansion waves in supersonic gas flow. A concept similar to what you describe has been proposed to reduce the wave drag on a supersonic aircraft (link). |
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Apparently, canal boats which were pulled by a
pony/horse had to climb the wave. When successful, the
effort to maintain motion reduced considerably. |
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I have no idea where I picked up that golden nugget of
information, or if it is really true. |
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Ah, here is something from Heriot-Watt University: |
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"Around 1830, when the canal passage boat services were well
established, a proprietor of
such a service in Scotland had an amazing experience. He was
William Houston, who
became the acknowledged expert in what was a “leading edge”
technology of the day.
The source of his future prosperity occurred by accident, when his
horse took fright whilst
towing an empty boat, and bolted. He decided to hang on, expecting
the resistance of the
boat to quickly tire the horse. Imagine his alarm when the boat rose up
onto its bow wave and
shot off along the canal at high speed. John Scott Russell, in his
account given to the
Edinburgh Royal Society, described how “Mr Houston had the tact to
perceive the mercantile
advantage”.
He also described Houston’s astonishment when he observed “that
the foaming stern surge,
which used to devastate the banks, had ceased”. It was this
unexpected drop in the boat’s
resistance that allowed the horse to continue so far. At 10 mph, return
day trips over the 8
miles from Paisley to Glasgow became very popular. By 1835, his
accounts showed 323,290
passenger trips in one year. The “Illustrated History of British Canals”
by Charles Hadfield
shows a tenfold increase in these five years with the number of boat
trips tripled to 12 a day
each way. Hadfield quotes the typical speed as 10 m.p.h, regularly
maintained, at fares no
higher than those for the previous 4 m.p.h. service. It was a wonder of
the times." |
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[Ling], that is awesome.
Hovercraft have a similar experience, called "getting over the hump"; it comes from the air (from the lift fans) displacing water beneath the hovercraft, but if you go faster than "hump speed", you do the same "faster, more efficient, less wakey" motion, as (effectively) the air doesn't have time to displace the water before it's moved on. |
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Nice.
You might be able to get a bit more oomph out of the flat side panels by causing them to undulate alongside the boat. There would still be minimal resistance but the shape would create a series of moving 2d parabolic reflectors to channel the wake to optimal pressure points on the hull. (+) |
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Somehow this reminds me of the ekranoplan: by operating close to the
ground, it can use smaller wings to achieve the required lift. |
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I'm not quite sure why it reminds me of this, because the mechanism
you described doesn't seem to have much in common with Wikipedia's
"Ground effect vehicle" page. |
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However, I'm going to vote [+] anyway, just because it's about time
somebody invented the sideways ekranoplan-boat. |
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Have we accounted for the energy loss that occurs when
the waves hit the side of the planes? |
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Have we accounted for the energy it takes to channel the
water between the planes and the boat hull? |
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Seems like you'd be reclaiming that kinetic energy
that's just tossed aside. Even if there were issues
with practical application
of this, and I'm not saying there are, some ideas
are
brilliant whether they work or not. |
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But it seems like this really could. Throw a couple
of thin steel plates on the side of a ship and save
10 percent on fuel? That's a big deal. |
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If it works. [+] regardless. |
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No, this doesn't pass the physics sniff test. |
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The boat must continually displace the water in front of it.
Hence the water is pushed to the sides, creating a bow
wake and wave. As water is incompressible it needs
someplace to go. You're going to be raising the water's
elevation in the pocket a small amount and perhaps forcing
it to travel
faster through the channel between the plane and hull,
creating more drag. I foresee a large net loss overall. |
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Could be right. The air in the jet models is
compressible, water is
not. |
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Still, even if it doesn't work, really clever. |
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What if you allowed the outriggers to swim like a
fish in
a serpentine fashion and tapped the energy that
way? Grab the outward moving energy such that
you're not adding any friction to the forward
movement. |
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Maybe if the outriggers caught a spray-velocity water at a
glancing blow and directed it rearward without actually
dipping into the waterline themselves, but if your hull is
doing that, you'd be better off eliminating the spray by
redesigning the hull. |
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tap tap tap ...this thing isn't on, is it? |
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Interesting idea. But does a vertical sheet that extends "a
few feet into the water" actually reflect a wave? |
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One Google search later, it looks to me like the answer is
basically no. I found a paper [link] that is comparing 3 ways
of predicting the reflection. All three theories say that
you'd need the wall to reach a significant depth to make a
difference. For example, to get about half of the wave
reflected, the wall must extend ~20% of the way to the
ocean floor. |
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Wait: what's that tapping noise? |
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//to get about half of the wave reflected, the wall must extend ~20% of the way to the ocean floor.// Say what? That sounds wrong. Unless the ocean is very shallow, I can't believe that waves have any significant component in the depths. |
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I can believe that the depth of the reflector has to be related to the wavelength or amplitude, but not that the reflector has to extend so deep into the ocean. |
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Most of that kinetic energy is traveling on the
same plane as the depth of the ship's hull so no,
just as deep as the ship goes would do it. That
shockwave is going where there's the least
resistance which is up. Lots of pressure down
below while the closer to the surface, the less
pressure there is so that's where your displaced water is
going to go. |
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But to visualize if this would work, I'm thinking of a
single ball hitting the bow, bouncing off, hitting
the outrigger plate and then bouncing back at the
angled ship stern imparting some measure of
forward energy to the ship. With that model it
works. |
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On the "that model doesn't work" argument, that
ball doesn't push back on the outrigger reflector
plate much. That wall of water you're churning up
against it might. |
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The displaced water that the ship pushes out of
the way now has kinetic energy and it's going
someplace. You're redirecting that mass to that re-
directing wall so you can bounce it back, but it's
going to have to fit in a space that's already filled
with water meaning it'll have to pile up against
that wall. |
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Does that cause enough friction to negate the push
you're getting when it bounces back and hits the
ship in the rear? I dont' know. |
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I also think you might have to consider the wavelength of
the
reflected water bump so since you can't adjust the length
of the ship and you'd want to vary the speed, I
think you'd have to move the plates in and out
depending on how fast you were going to bounce
that reflected wave into the right spot. |
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Still damned clever, whether it works or not. |
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Enjoy your extra half bun, sir. |
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I still say no. Water waves operating in different
directions will interact and partially cancel, raising
the overall depth a smidge where they do. Raising
that level represents a net energy loss. |
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I think we need a java-enabled animation. |
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... to display ripples in coffee. |
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Hmm. I have a half-feeling that this may not work. |
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My initial assumption was that a wave represents water moving horizontally. After all, the wave moves horzontally, so it seems as though water must be moving horizontally. |
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But we know that's not the case. A water wave is more like a ripple in a sheet of fabric - the water moves up as the wave passes a point, and then down as the wave passes. If we had a series of markers suspended in the water just below the surface, we'd see each marker move up and then down as the wave passes, but the markers wouldn't move sideways. |
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This means that my initial assumption - that a wave hitting the side of a boat will give it a push in the direction of wave travel - was wrong. |
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Put it another way: suppose the boat were standing still, and a series of waves hit it from behind. The boat wouldn't move forwards, it would only bob up and down. Hence no propulsion. |
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What happens is that the wave would lift the stern of the boat, pivoting around its centre of mass, and converting kinetic energy into potential energy. |
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The boat then attempts to move back to the position with the lowest potential energy. Depending on the relative sizes of the wave and the boat, the boat either slides "down" the following wave (wave > boat), like a surfer, or displaces water backwards as it descends (wave <= boat); either action will impart some forward motion to the boat, albeit very small in some cases. |
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The answer is the solution already established by the super catamarans. Two streamlined, torpeo-shaped hulls are fully submerged to provide buoyancy, and support the main structure by pylons having a narrow aspect ratio. No part of the main hull is submerged. |
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The pylons generate very little wake, which obviously removes the need for outboard deflector plates to attempt to recover energy. The widely-spread twin hulls provide excellent stability. |
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I was assuming it's not about a wave hitting it from
behind but two matching waves hitting a wedge
shaped stern from the sides. Like squeezing a
watermelon seed to shoot it from
your fingers. Those two waves come together on
either side of
that wedge they would raise the stern a bit but
also push it forward. |
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Maybe the way to make this work is to have your
propellor at the FRONT of the bow. Then you'd
combine your bow wave with the engine
wake.Reclaiming all THAT waisted energy might be
worth
it. |
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I'm also thinking that your ship would need to be
teardrop shaped so most of the boat is tapered at
the end so that no matter where the wave hit it
you'd get some forward movement. |
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So like a teardrop with a sharp front instead of
rounded. (See link) |
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Attempting to reclaim turbulent water energy would be not
worth the induced drag. |
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I've basically appointed myself the lawyer for this
idea. Just like a lawyer might think his client is
guilty
as hell but advocate for him anyway, I want to see
this idea get its fair trial. |
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If the verdict is "guilty, death by firing squad" I'll
say
"Eh, oh well. Just
another day at the office." |
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Now who among you would look into the sweet,
innocent eyes of this idea and not think that it
would work? Haven't you yourself had days where
you didn't feel like working? Remember what Jesus
said "Let those who cast the first stone be like an
oxen unto a privit hedge, black in thy sight oh
lord." |
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What we need, is the ship to tow its own canal along with
it. |
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If Escher were alive we could get him to do the
design drawings. |
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Giving Mr. Escher access to a 3d CAD system would probably
cause the Universe to implode, or at the very leastt collapse into
two dimensions … |
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So, it's not a method of securing a special hair style for
those who are about to go on a cruise? |
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I have searched, unsuccessfully, for a boat I saw in a science magazine many years ago (mid '90s?).
The hull was shaped such that the bow-wave, rising as it moved back along the boat (it was a jet- or other fast-boat) was deflected back over and down, effectively providing some extra buoyancy, lifting the stern slightly. I'll keep digging when I have a chance (I should be working...). |
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The defense proposes that rendering of the final
verdict be
deferred
untill this new evidence is entered into the record. |
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And we all should be working neutrino but we're not
so join the club. Work can kiss my ass. |
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Neutrino - are you not describing a hydrofoil? |
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No, that's a "twin-vortex hull". It's - like this idea - a monohull with outriggers, but not a trimaran; the tube-like "wings" catch the bow-wave and deflect it downwards, lifting the main hull without the need for a submerged hydrofoil and thus allowing a much shallower draught with high speed. |
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I think it might be able to trap the odd mullet. |
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@neutrinos_shadow: Sea knife. |
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I've have always wondered why the horses aren't out front using the plane to top off the roosters. |
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This idea sounds like it might work - there must be
some fatal flaw with it I haven’t seen... |
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Did somebody figure out if this would work or not?
This has to be explored. |
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Yesterday I was arguing about Nepalese jogging
pants while this idea lies dormant. Time to have a
little review of time management. |
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Somebody has to explore this, really. The bow
wave capture, not the Nepalese jogging pants. |
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What happens if you add a horizontal element to
this in the form of slats that keep the
wave from arching up on the outriggers and losing
that energy? It appears that all the energy is lost
due to the waves moving up, that's been discussed,
but would horizontal slats capture that energy
before it's expended by moving vertically? |
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