Half a croissant, on a plate, with a sign in front of it saying '50c'
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Bow-wave capture for boats

  [vote for,

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.

MaxwellBuchanan, Aug 09 2017

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 09 2017]

Wave Transmission Past Vertical Wave Barriers https://www.google....Qy7-56Wjjd4wOV3vsdg
[scad mientist, Aug 12 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]


       Slant the outriggers too?
LimpNotes, Aug 09 2017

       That is an excellent idea because (a) it might offer additional propulsion and (b) "Slant the outriggers!" sounds really good with a pirate accent.
MaxwellBuchanan, Aug 09 2017

       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).   

EnochLives, Aug 09 2017

       Apparently, canal boats which were pulled by a pony/horse had to climb the wave. When successful, the effort to maintain motion reduced considerably.   

       I have no idea where I picked up that golden nugget of information, or if it is really true.
Ling, Aug 09 2017

       Ah, here is something from Heriot-Watt University:   

       "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."
Ling, Aug 09 2017

       [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.
neutrinos_shadow, Aug 09 2017

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. (+)

       Somehow this reminds me of the ekranoplan: by operating close to the ground, it can use smaller wings to achieve the required lift.   

       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.   

       However, I'm going to vote [+] anyway, just because it's about time somebody invented the sideways ekranoplan-boat.
Wrongfellow, Aug 12 2017

       Have we accounted for the energy loss that occurs when the waves hit the side of the planes?   

       Have we accounted for the energy it takes to channel the water between the planes and the boat hull?
RayfordSteele, Aug 12 2017

       This is f-in' brilliant.   

       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.   

       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.   

       If it works. [+] regardless.
doctorremulac3, Aug 12 2017

       No, this doesn't pass the physics sniff test.   

       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.
RayfordSteele, Aug 12 2017

       Could be right. The air in the jet models is compressible, water is not.   

       Still, even if it doesn't work, really clever.   

       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.
doctorremulac3, Aug 12 2017

       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.
RayfordSteele, Aug 12 2017

       tap tap tap ...this thing isn't on, is it?   

       Interesting idea. But does a vertical sheet that extends "a few feet into the water" actually reflect a wave?   

       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.
scad mientist, Aug 12 2017

       Wait: what's that tapping noise?
pertinax, Aug 12 2017

       //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.   

       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.
MaxwellBuchanan, Aug 13 2017

       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.   

       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.   

       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.   

       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.   

       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.   

       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.   

       Still damned clever, whether it works or not.
doctorremulac3, Aug 13 2017

       Enjoy your extra half bun, sir.
21 Quest, Aug 13 2017

       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.   

       I think we need a java-enabled animation.
RayfordSteele, Aug 13 2017

       ... to display ripples in coffee.
pertinax, Aug 14 2017

       Hmm. I have a half-feeling that this may not work.   

       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.   

       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.   

       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.   

       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.
MaxwellBuchanan, Aug 14 2017

       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.   

       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.   

       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.   

       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.
8th of 7, Aug 14 2017

       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.   

       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.   

       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.   

       So like a teardrop with a sharp front instead of rounded. (See link)
doctorremulac3, Aug 14 2017

       Attempting to reclaim turbulent water energy would be not worth the induced drag.
RayfordSteele, Aug 14 2017

       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.   

       If the verdict is "guilty, death by firing squad" I'll say "Eh, oh well. Just another day at the office."   

       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."   

       I rest my case.
doctorremulac3, Aug 14 2017

       What we need, is the ship to tow its own canal along with it.
Ling, Aug 14 2017

       If Escher were alive we could get him to do the design drawings.
doctorremulac3, Aug 14 2017

       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 …
8th of 7, Aug 14 2017

       So, it's not a method of securing a special hair style for those who are about to go on a cruise?
xenzag, Aug 14 2017

       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...).
neutrinos_shadow, Aug 14 2017

       The defense proposes that rendering of the final verdict be deferred untill this new evidence is entered into the record.   

       And we all should be working neutrino but we're not so join the club. Work can kiss my ass.
doctorremulac3, Aug 14 2017

       Neutrino - are you not describing a hydrofoil?
xenzag, Aug 14 2017

       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.
8th of 7, Aug 14 2017

       //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.//   

       Does my stern look big in this ?
bigsleep, Aug 14 2017

       Can this boat capture other hairstyles, or is it limited to the blow-wave?
Ian Tindale, Aug 14 2017


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