Dynamics – Water Particle Motion, And The Forces Developed By Moving Liquids

General Discussion
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I've ridden very slowly breaking waves for long distances on a longboard riding directly to shore with no gravitational change, so I think gravity is not an absolute requirement.

Dave, I’ve always thought that when riding straight to shore, even in the soup, the wave is continually pushing up on the board and trying to raise it above sea level and gravity is continallly trying to pull the board back down again. These two balance out from our visual perspective so it doesn’t look like the board is rising and falling.

IF you go up, you gain gravitational potential energy and lose kinematic energy. If you go down, vice versa. Boards have some drag in water. If you can ride a supertanker board on a small wave without ending up lower than you started, SOMETHING is transferring energy to the board, and it is not gravity (directly).

If it were all gravity, then when you go up the wave to the same height as you started, you would lose all speed. There would be no ability to “tear the roof off” a wave.

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is it gravity and mg sin(theta)? I’ve ridden very slowly breaking waves for long distances on a longboard riding directly to shore with no gravitational change, so I think gravity is not an absolute requirement.

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No doubt by ‘no gravitational change’ you mean that the rider has neither gained or lost altitude. . . . . . and that this means that the board is not being driven via gravitational force. If so you are mistaken. Loss of gravitational potential energy relative to the earth’s surface is not a necessary result of the surfer and board being driven by gravity. You don’t have to fall relative to the earths surface (or relative to the wave) in order to be driven by gravity. An example of this effect is the way a glider can soar on an updraft. . . the glider is driven by gravity without losing height. . . and is even driven by gravity when gaining height. The myth that altitude must be lost if a surfboard is being driven by gravity has (IMO) led to another myth. . . the horizontal water jet theory

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Gravity cannot move things horizontally (at least perpendicular to the gravitational field, and on the planet Earth, that’s usually taken to be horizontal.)

Ahem, The statement above is incorrect. Gliders move horizontally, and they are moved entirely by gravity Snowboards move horizontally, and are powered by gravity Skateboards move horizontally, and are powered by gravity Surfboards move horizontally, and are powered by gravity !!

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IF you go up, you gain gravitational potential energy and lose kinematic energy. If you go down, vice versa. Boards have some drag in water. If you can ride a supertanker board on a small wave without ending up lower than you started, SOMETHING is transferring energy to the board, and it is not gravity (directly).

In tiny waves, or the soup, I think it’s more like the gravity keeps you there, in front of the wave or on the front slope of the wave. The wave energy lifts you up, gravity resists this force, and so you stay where you are just on top of the water surface. But the spot at which the wave is lifting up moves closer to shore. So you go along for the ride.

But I don’t think of it as the wave pushing the surfboard horizontally towards shore. And I certainly don’t think of it as the wave pushing horizontally away from shore like Kevin does! I think of it as the point where the wave is pushing vertically upwards moves closer to shore as it goes. We stay on that point, thanks to gravity, and so we move closer to shore as well.

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If it were all gravity, then when you go up the wave to the same height as you started, you would lose all speed. There would be no ability to "tear the roof off" a wave.

Clearlly it’s not all gravity. We can “thrust” our board for more speed back up the wave. And hey, maybe even this water jet of Kevins helps when going back up the wave (i just don’t see how it helps going towards shore…).

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There is a fow of water going up the face of the wave, thats what takes you over the falls, and its also what your fins and rail are experiencing, its not a jet of water, its parallel to the wave face. It is what gives you the ability to make it back to teh top of the wave, it also gets faster as the wave gets steeper.

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Sure. But if not for gravity, we’d never come in contact with that flow. I think that’s what TomBloke says when he tells Kevin he’s missing gravity from all his equations and diagrams.

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The jfMillbiz.home.Comcast.net reference is very interesting. It would be nice to have an author to associate with it, if you have that information I would appreciate it if you would share it. The author seems to get the fact that the something perpendicular to the bottom of the board is going on and is important

Here you go.

“Surfboard Design and Construction” by Jim Knistle

I believe Kendall indicated there’s a new version of this other great book coming out soon as well

Essential Surfing by George Orbelian 1987 (ISBN 0-9610548-2-4)

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I believe Kendall indicated there’s a new version of this other great book coming out soon as well

Essential Surfing by George Orbelian 1987 (ISBN 0-9610548-2-4)

Yes, the original author has been working on a followup for some time now, as well as another book on the history of surfboard design…

A snowboard in a big snow bowl. You start at some altitude and do a traverse. Can you end up at a higher altitude than you started?

Repeat the experiment with a skateboard in a concrete bowl. Can you hit the same altitude as the starting altitude with any velocity at all?

There is a wall of water sort of moving towards shore. The top of the wall moves faster than the bottom, and it continually shears/shoal, and this transfers momentum to the surfer. This process, in addition to gravity, is nearly the entire source of energy for the surfer.

Gravity can be enough by itself, also, people like crazy Eli surf standing waves.

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Thank you.

Kevin

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A snowboard in a big snow bowl. You start at some altitude and do a traverse. Can you end up at a higher altitude than you started?

Repeat the experiment with a skateboard in a concrete bowl. Can you hit the same altitude as the starting altitude with any velocity at all?

532×886 164 KB

You mean without pumping? Know how Danny did that, right? Starts with G.

So interesting how people can lose sight of gravity’s influence, because of its ubiquity! Same goes for the wave’s overall motion.

Keep your eyes on the balls people. Kevin the Magician is trying to distract you from the BIG OBVIOUS reality of what’s driving you.

(’ ')

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A snowboard in a big snow bowl. You start at some altitude and do a traverse. Can you end up at a higher altitude than you started?

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Not relevant. A closer example is the glider using the updraft from a slope . . . the glider is able to gain altitude, and it is moved via gravity, not ‘pushed’ by air jets. Altitude loss or gain is a red herring. . . gravity can move gliders and surfboards horizontally without any loss in altitude

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There is a wall of water sort of moving towards shore. The top of the wall moves faster than the bottom, and it continually shears/shoal, and this transfers momentum to the surfer.

No. If the surfer is moving with the wave towards the shore, he is already moving faster than the beachward movement of water particles in the top of the wave . . . . and thus cannot be ‘pushed’ by that beachward movement. . . . There was a calculator on one of Kevin’s previous threads which gave us the velocity of the water particle rotation ( expressed as vertical and horizontal velocity) . . . . one can change the size of the wave, the wave period, and the depth of water, and it spits out the water particle speed. . . it showed quite clearly that this is always much less (about a third) of the beachward speed of the wave front. .

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I’m having difficulty with this concept of “planing” since it clearly doesn’t seem necessary for a board to “plane” in order for it to move.

Hi Slim Yes it’s possible to surf a wave without planing, I often see 25 foot 12 inch wide paddle skis trimming across waves very fast, and they are ‘displacement’ hulls which are certainly not planing. . . … it sometimes happens on my 17 footer as well. .

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No. If the surfer is moving with the wave towards the shore, he is already moving faster than the beachward movement of water particles in the top of the wave . . . . and thus cannot be ‘pushed’ by that beachward movement. . . . There was a calculator on one of Kevin’s previous threads which gave us the velocity of the water particle rotation ( expressed as vertical and horizontal velocity) . . . . one can change the size of the wave, the wave period, and the depth of water, and it spits out the water particle speed. . . it showed quite clearly that this is always much less (about a third) of the beachward speed of the wave front. .

Oh yeah. Dig on that. The flow’s velocity is COMPLETELY overwhelmed, subsumed, negated, cancelled out, erased by the overall wave’s beachward momentum. Thank you Roy. That should be the end of it, but I know it’s not… Between Roy, Ken, Slim and I, your arguments are toast, Kevin. But I know you’ll just keep repeating them.

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waves shoal...without a shoaling wave the surfboard cannot go...this is an absolute requirement

Shoaling is necessary, but vague. Unless you are saying that the wave must be breaking in order to surf, which is incorrect. A better explanation is that the wave must shoal such that it is steep enough to catch the wave, with equipment characteristics and physical abilities as variables.

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I've ridden very slowly breaking waves for long distances on a longboard riding directly to shore with no gravitational change, so I think gravity is not an absolute requirement. the board can be carried along by the shearing of the wave as it shoals.

Maybe you’ve missed some of the other threads. Of course the wave force is important for the lifting part, and gravity for the falling part. Janklow has a very good analogy here:

http://www.swaylocks.com/forum/gforum.cgi?post=279952#279952

And a very useful animation is here:

http://www.crocodile-clips.com/absorb/AP4/sample/media/DJFPh063waterwave2.swf

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...the board can be carried along by the shearing of the wave as it shoals.

You have to ask yourself what is carrying the whitewater. Do you really think the momentum of the sheared part of the wave can move, in some cases, hundreds of yards. The truth is, the whitewater requires the wave and gravity combination as well, to continue to move. What happens to whitewater when it hits a deep area? Does it’s momentum carry it very far? It’s true that whitewater can push something a little bit if it’s light enough, like a surfboard without a rider on it. But you can’t actually ride the whitewater. You are riding the same wave that is moving the whitewater. You just don’t see it.

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Its also very clear to me that on a long wave face I can go up and down the wave and generate speed. I have no net change in gravity because I go up the wave and down the wave and go faster each time. Again, the up/down motion is used to transfer energy to me, but the net gain comes from the shoaling of the wave and its velocity TOWARDS SHORE.

I’m pretty sure we all agree that there is a wave and it is moving forward. This is imperative to accept in order for the gravity part to work, as well as if you buy into KCasey’s water jet theory.

Let’s take a poll and see if we agree on anything. Agree or disagree to the following:

1.) There is a wave.

2.) The wave moves forward.

3.) It is important for surfing that the wave moves forward.

4.) The wave lifts the water molecules as it passes through them.

5.) Since we and our boards float on water, we get lifted as well, and this is important to surfing.

Hopefully everyone will agree with the above. Here’s some more points that I hope aren’t too much of a stretch:

6.) When a surfer is at the crest, and the wave is steep enough, gravity helps move him down to the trough.

7.) If the surfer is at the trough of the wave, he will eventually be lifted to the crest as the wave moves forward.

8.) A surfer can use momentum from moving down from crest to trough.

9.) A surfer can use physical input (a.k.a.: pumping, weighting and unweighting, jumping) to put himself closer to the crest.

10,) The surfer must push the surfboard against the surface of the water to use physical input.

Lets see if most of us agree with at least some of these points. I have a feeling most of us do, but perhaps it difficult to put into words with the right terminology.

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Good post, KZ. (Ever get those pics of your 6’6"?) Tried to mount the animation here but no.

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Kenz, I think I agree with every point in your “poll”. Perhaps Kevin can post where he disagrees in language as clear as you used.

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That’s your intuitive description, and given it’s your description, probably works for you. It doesn’t for me.

Here is the problem. Ultimately it all has to relate back to design. You have to bring whatever it is you are attempting to describe back to bottom surface area, bottom contours, flow characteristics under the surfboard, fins orientations, template shapes, rocker, etc., that is it has to ultimately relate back to design.

Also, again, if you want to know how I’ve incorporated gravity into my treatment please see Dynamics – The Trim Equation. If you want to understand how planing is applied here please see Dynamics – The Trim Equation and Dynamics – Surfing the Force.

Up until now the major point that I have tried to make is regarding propulsion – what makes surfboards go - and my hypothesis is summarized in this and the threads referred to above. But that’s just my hypothesis; in a prior post there are a number of other references to alternative treatments (see oneula’s post with the first couple of lines

“ exactly what are you offering here that hasn’t already been presented eloquently 30 years ago in these documents

before most of the groms here were even born in this dissertation.”

You may also wish to read my replies to his question.) Not to mention any number of the repetitious posts that appear to have taken the same ‘intuitive’ approach that you’ve taken in you description.

It has been my (written) belief that the first step in making sense of design is to understand propulsion. If that can be understood, and it’s clear what are the critical properties and parameters that govern it, then good design will naturally ‘fall out’ as a consequence.

If after reading my threads you disagree, fine. If you feel the best way to determine what is happening is by consensus, that’s fine too. In either case, if it is your intend to take the approach you insist on taking, please consider starting a thread which takes that approach. It’s very apparent that many others seem to understand what you’ve written and are happy with your approach in general, and may even be able to help you develop it further.

Kevin

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Occam’s Razor.

We will likely show up to debunk pseudo-science as long as you keep posting it, because you are obnoxious in your repetition, and you’re wrong.

In terms of the understanding we have of the realities of wave surfing and the fact that everything necessary is very well accounted for in observable quantities, your term “intuitive” is simply insulting to us, physics, and science in general. It’s not “intuitive.” Gravity is not “intuitive.” The wave’s motion beachward and the grade on its face are not “intuitive.” The fact that you make this remark about our understanding is chokingly rich with irony. You have an intuition. It’s wrong or it is of the facts which we are describing and which are already describing themselves in perfectly visible phenomena.

Don’t be such a microthinking abstractionist. Go surfing (you do surf?). See which forces you can actually discern and which ones dominate your ride. HINT: Your 12 kt motion over water doesn’t mean there is a 12 kt flow going the other way .

(You will admit that your arrows of force all are in sync with the direction that the surface of the face is moving toward at any one moment?)

Your hypothesis is superfluous. Observe a wave. Its beachward motion. Its grade. Remember gravity. (BTW, Kevvie, you can’t only “deal with” gravity in one thread-- LOL!!! How old are you?)

Occam’s Razor.

(You never did say exactly what you thought the flow was doing exactly or which direction it’s doing it in…you say it’s a circle?! LOL {*} You’re so confused–the particles [molecules?] you keep mentioning are the wave itself. Yeah there’s a flow–of a waveform through water! Call it a flow, a process, a progression, a radiation, whatever–it’s a wave)

BTW, in terms of design, which is supposedly your underlying concern, I don’t think today’s hot surfers or pros find any particular shortcomings. (I do, since the boards only work that well in one direction, but that’s food for other trains of thought–please do not apply) Your theoretical expertise is absolutely unneeded.

Your repetition of your hypothesis and ideas flowing from it are obnoxious without any evidence–and once again, that would be proof that shows the influence of gravity and the wave’s motion combined are overwhelmed by your “flow.” LOL!!

Occam’s Razor.

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The real life answer is that not every wave even waves within a set will break exactly the same way. Not every break will have a wave that breaks exactly the same way every time. And not every surfer will surf exactly the same way on every wave. So in a sense design is always the art of compromise.

Traditionally design is about trying to build something that best fits the rider’s style and conditions of use and then slowly tailoring the design based off the rider’s feedback off of each new generation of the design produced for that specific rider in question.

For a designer to take a bunch of theorems and laws of physics and hydrodynamics and use that solely as the base for design without any rider feedback or understanding of the break’s other conditions while a good intention it isn’t going to work out for the end user… There’s alot of good intentions sitting in the design junkyard because of this… FP anyone?

These are good fundementals to start with but in the end it’s always the interaction of the end user and the builder that leads to the best real life end product. That is of course if the builder and end user can communicate effectively with each other. Builders will understand that last statement.

I still don’t understand what your saying…

Seems like its saying we don’t know how a surfboard works in the first place.

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Kevin,

If you were really interested in the physics of surfing you would be replying to the questions asked here in physical terms.

Instead you ask those who don’t agree with you to go and start another thread.

Try answering this:

How can a beachward water particle motion of 4mph ‘propel’ a board which is travelling beachward at 10 mph ? (Helpful hint: it can’t. . . )

Here’s another question:

Please explain how density implies pressure (helpful hint: It doesn’t. . .)

:slight_smile: