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

General Discussion
21

Actually I’m a little more interested in how nature deals with this juxtaposition of speed versus opposing force.

the works of Carpenter, Ridgeway and Kramer are more appealing to me because it’s trying to understand a real world solution to the problem of traveling quickly with minimal energy in the water mass. So it’s stuff like this and the use of applied technology to achieve some of this capability with nano particles, different construction techniques that I find more appealing than calculations. Not that I didn’t have my share of all that force vectoring stuff as an undergrad Physical Oceanography major at UW. Too bad one my favorite european professors used his knowledge of ocean and earth forces to become a professional gambler(did you know that with proper training you can predit with relative accuracy the ending position of a roulette ball on a floating gambling casino?). I guess that why I never became an physical oceanographer working for the oil companies too…

Anyway here’s some articles that tickle my fancy in the subject of biology and surfing:

http://www.spawar.navy.mil/sti/publications/reprints/marinemammals/ieee-m-b-9-93.pdf

http://www.ias.ac.in/currsci/sep252000/carpenter.PDF#search=%22carpenter%22

I can see why Morey, Dale and others really locked in to something when they went the flex and soft route.

The “Surfboard Design and Construction” article is classic and provides great insight as to why one needs to understand the physics of what’s a happening when approaching surfboard design in a proper frame of mind. Which is not unusual compared to all other professional sports too. Its one of the many “bibles” of surfboard design I have found out there…

Studying the biomechanics of surfing is another area we sorely lack testing and development in compared to our peer sports. Everyother sport looks at both the athlete and the equipment at the same time.

The problem I have with this stuff though is that this kind of info has been around many years but in the end most of the innovation we have today has come from what I see as youthful exhuberance applying techniques from other sports (like skating/snowboarding and technologies from other sporting and non-sporting industries like skiing, sailboarding, bicycles, aerospace, sailboats).

It’s this “what the fvck-I don’t care-I’m gonna try it anyway attitude” that has progressed what used to be surfing to what it is today. The technology just had to catch up to support what is forever called the “new school” decides they want to make surfing become.

I guess I don’t see the real life application of these discussions yet maybe that’s just my problem…

22

It’s regrettable that you don’t see any real world applications in these discussions. Nevertheless, the references were appreciated.

Thanks,

Kevin

23
Quote:
I am not sure what you are referring to with regards to lift and drag of displacement and 'planing forces'.

When you stand on a surfboard it supports your weight.

That means the force of gravity of your body is counteracted by an equal an opposite force.

That counteraction force has two elements - a planing element and a displacement element.

The planing element occurs because you are moving forward and the hull is angled up. The hull pushes water down. Because water moves down, you get a force up.

The displacement element occurs because the board displaces some volume of water. The board gets a lift vector equal to the weight of that volume of water.

For all surf in smaller waves, both forces are relevant. Planing causes more drag, displacement requires the board to sit lower in the water.

For more maneuverability, you use concaves to bias towards planing forces, and away from displacement. You can turn more easily, but you will bog more easily.

For more glide, use a rounded hull. This makes the board sit lower, and generate more lift from displacement. This is lower drag, simply because you are forcing less water to change its momentum.

In the extremes, you have 30+ MPH tow boards which are all planing and no appreciable displacement, and you have paddling, which is all displacement and no planing.

At all normal recreational surfing speeds, both forces are relevant, and changing from a rounded hull to a board with concaves changes glide and maneuverability.

The claims that concaves and venturis increase speed are complete and utter bs from people who do not understand the relevant physics.

24

My apologies, I’ll rephrase my question. What do you think makes surfboards go?

Thanks.

Kevin

25

Regarding the http://jfmillbiz.home.comcast.net/swaylocks/Surfboard_Design_and_Construction_1977.pdf reference.

This is a fancinating read and I highly recomend it. I have some problems with his interpretation, particularly the importance he places on buoyancy. But his reference to the interial force of the water is interesting. Not surprisingly if you combine the inertial force and the buoyant force the resulting force begins to look like the force that impacting water particles might have on the bottom of a surfboard. Also, the author seems to have understood the nature of the velocity of water particles in a wave.

Once again, it would be greatly appreciated if you would be kind enough to supply the name of the author and the title of the monograph.

Thanks,

Kevin

26

Please consider this as a continuation of the prior post…

Regarding the fin dynamics reference. I’ve reread the post a few times now, but the relevance remains unclear.

My guess is that you are referring to my suggestion that the standard cant and toe-in found on modern shortboards may aid in propulsion (under certain circumstances.) In particular perhaps it was in regards to the curious direction of the post lateral fin wake in the picture of the (poor) fellow in the tube that I posted in my last thread.

If so you are correct. The point was that this propulsion makes sense if you accept the nature of the propulsive flow that I have been describing. There is no need to invoke turbulent flow, the propulsion is not a consequence of the motion of the surfboard. Perhaps you believe if you were to place canted and toed fin in a nose-to-tail stream you would see such a wake pattern. If so how would such a pattern provide propulsion? For that matter what exactly is making the surfboard go in the picture? From the lift from the fins because of the flow, which arises from the lift of the fins?

I am not disputing that the standard tri-fin setup simple works. I’ve just always had a problem with when and why it works, and for that matter when and why it doesn’t? I’ve never believed that it was because of the foil. I saw the use of foil as structural for the most part, though it does provide for better flow characteristics. But if you accept that propulsion is a consequence of the flow impacting the bottom of the board, or any other surface exposed to that flow, then the tri-fin makes a lot of sense (under certain circumstances.) I kept writing ‘under certain circumstances’ because there is a down-side to cant and toe-in, it can start to produce more drag then propulsion. Nevertheless, as commonly applied, the natural evolution, or the ‘natural selection’ process of the marketplace has more or less provided builders with an optimal setup which seems to work well a broad range of conditions.

Once again, thanks for the references, they were interesting reads.

Kevin

27
Quote:
My apologies, I'll rephrase my question. What do you [blakestah] think makes surfboards go?

Oh, I can’t wait for this. And it’s my dream that Dave’s answer contains the word gravity in it at least once (unlike Kevin’s).

28

“…for me the forces Savitsky describes are what makes the surfboard go, please see this and prior posts. Once again, that flow that makes a surfboard go, is different from the relative motion (flow) which the surfer experiences. The flow that causes the planing is actually close to perpendicular to the motion the surfer experiences.” – KCasey

I just had to clip that in here for some recognition.

I love that: “The flow that makes a surfboard go.” Seussian influence I believe. Makes sense.

GRAVITY v. DEPRAVITY

The flow that makes a surfboard go

isn’t a wave that we ride, oh no.

It’s a flow that makes us go–

I say that it’s so, so it’s so!

“Gravity!” you shout? You can’t credibly doubt

the Truth that these diagrams show…

But you say that you do!

(Well I’ll say nothing new,

admit proof I’m without –

these louts are about to tire out.)

29

You still haven’t realised that a horizontal water movement is only able to apply a horizontal force to an object which offers an opposing force. In the horizontal plane the only significant opposing force is inertia, and once this is overcome any horizontal water movement which occurs is unable to exert a force on the surfboard.

A surfboard which is riding a wave in trim offers no significant opposing force to any horizontal water movement in the wave because it is already travelling beachward with the wave (and is thus exceeding the horizontal water particle movement in the wave)

The opposing force which you see occurring is almost entirely due to gravity opposing the upwards movement of the wave. … .this is a force which is entirely in the vertical plane, it is not a horizontal force. . … . you have hijacked this vertical force, pretended that it isn’t due to gravity, and then ‘explained’ it as horizontal water jet force which does not exist.

When confronted with this and other objections you ignore them.

You are thus, in my opinion, not engaging in rational discussion… . . . . and quoting supposedly academic sources willy nilly and out of context doesn’t help at all.

Now front up and answer the objection above please. … . concisely and logically .

:frowning:

30
Quote:

What I haven’t seen much of is discussion of the relative contribution to surfboard lift and drag of displacement and “planing forces”…

Planing forces, such as the reactive jet forces in the analogy, re-direct water downward. They

Blakestah, Just to keep Kevin honest I have top remind you that his ‘jet forces’ analogy supposedly only occurs horizontally. You are correctly talking about planing ‘water jet’ forces which are in the vertical plane. . . . but that’s excluded in Kevin’s theory. . . . he has stated repeatedly that the force which makes a surfboard gois horizontal flow. . . but there is no significant horizontal opposing force other than inertia. . . . .so what happens is that we start discussing what really does happen (and which includes gravity is the main force opposing wave movement) . . . . and it’s then kind of assumed that all kevin’s prior horizontal jet stuff is correct. . . this allows him to escape into design discussions supposedly based on his theory without fronting up to the fatal faults in that theory. … . . it reminds me of tactics used by bent politicians. Cheers roy

31

Greg Griffin has a pretty good handle on how the fin affects the way a board will work.

I have one of his 5 fin fish boards (a 6’ x 20") and it has lift in the tail and it accelerates like a race car. The board has a flat bottom from nose to tail. I wasn’t quite sure what he was talking about when he told us the board would lift up in the tail, but I have definitely felt the board lift like there was something pushing it up. When you push down to turn or do little pumps from rail to rail it makes the board move forward a little faster than other boards.

People look at the board with a lot of doubt, but once they ride it they know that there’s something special going on. I even felt the same way when I first saw these multiple fin boards, but I’m sold. The combination of size (short), shape (wide) and number of fins make it loose, yet it can make extremely hard turns. You can go from a longboard to this thing and turn as hard as you would a longboard and it feels solid.

Greg has a deep understanding of how the board and the fins have to match precisely to get the best end product. I think that combination is very specific to each type of board, but maybe not so much to the type of wave. Just need enough of a wall to get it on the rail and it will go.

32
Quote:
Quote:

You can’t change the laws of physics JIM ! . . . . . Even in the name of art . … . . . . Physics is physics mate. . . and Kevin’s theory is incorrect… . . . . . . … . . it’s not an art issue. :frowning: Good Lord ! I just saw the “jet impacting bottom” diagram again, it’s BUNKUM!!! .

wrong

the evolution of man and developement of culture and societies

has been through creative thought and radical freethinking individuals lack of acceptence in

the staus quo

whether it be science, language or art

alchemists thought they could change lead into gold

roy stewart said they cant because its goes against the laws of physics

so they all say "thanks roy and slim,

you were right.

well stop wasting our time with trying to turn lead into gold"

modern metalurgy ceases to exist .

ideas and creative thought process, lead to more ideas and more creative thinking.

which in turn can lead to fundemental changes in humans understanding of the the universe

all of a sudden the imposible becomes possible

It is fascist thinking that attempts to discredit creative thought processes by using

widely accepted concepts on what is “fact”

oxymorons prevail in the modern world

and fact and fiction is blurred

the universes is filled with paradoxes

and things exist beyond human comprehension.

this is a witch hunt

Silly,

This is a physics thread. … . you have told us that you don’t undertand physics and that you don’t have much interest in physics. . … . . . perhaps that’s why you keep trying to solve the argument on a social, and emotional, and personal level.

The trouble with solving it on the vibe level is that the vibe is completely irrelevant. … it’s just dry physics.

A truth hunt, not a witch hunt.

Get what I mean?

regards,

Roy

PS You mention fascism. . … . presenting logical objections to Kevin’s theory is NOT fascism. . … . . . … fascism is more akin to the " get off this thread if you don’t agree with Kevin" stuff which some of us have been subjected to from yourself and Kevin. It’s a public forum. … . no special no touchee ‘Indian reservations’ for Kevin please !

.

33
Quote:
So, The Wave Is A Jet?
Quote:
Wow, The Wave Is A Jet

Bwahahahahahahahahhaahahhhahhahahahahahhahah

hahahahahahahahhaahahhahahahahahahaha

hahahahahahahahahahahahahahahahaha

hahahahahahahahahahahahahahahahah

hahahahahahahahaha…

…my side hurts…I’m going to get a drink of water…

…those that believe this nonsense should put down the physics books and go surfing…you don’t even have to go surfing, just go out into the waves and observe.

34

holey cow

what makes a surfboard go?

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

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. the board can be carried along by the shearing of the wave as it shoals. But I cannot ride a wave in the open ocean where the water is deep enough no shoaling occurs.

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. Certainly within those trips up and down the wave face I gain and lose speed with gravitational potential…but in the end I go MUCH faster than I would go simply riding the wave once from wave to trough. Just as certainly I start close to the peak and end in front, so I do have a net gravitational potential loss (and velocity gain)…

SO I have to conclude that whereas shoaling is an absolute requirement, some speed, and in some cases most speed, can come from consideration of gravity alone.

And thrusters work depends not on three fins, but only two. The inside rail fin and the rear fin. You can set up rotating fins so that the outside rail fin turns in toe-out and offers no resistance to the flow, AND YOU GET SUBSTANTIALLY MORE THRUST. The counteraction of the fins when you go straight offers only drag. The critical aspect of the thruster in a turn is that the rear and rail fin both stabilize against centripetal acceleration in a turn, but their angular difference allows the front inside rail fin to generate speed in the direction of the board centerline in addition to accelerationally holding the turn…

This has been known for decades by anyone who works with fins a lot (the why a thruster works part, not the rotating fin part - that was only a year or two ago). People like Greg Griffen even broke it up further by using multiple rear fins as stabilizers, either as a quad or five fin, and making more thrusty boards.

35

Thanks.

This – what makes surfboards go - is the topic of this thread, see initial post and of prior threads, in particular “Dynamics – The Trim Equation”. My reading of your post suggests to me that we disagree as to the fundamental mechanism, please see the above initial post and prior post ‘Dynamics – The Trim Equation’.

By the way, in analyzing things mechanically, it’s a standard strategy to consider the steady state case, if one exists. That’s what is dealt with in ‘Dynamics – The Trim Equation’ and subsequent threads (including this one.) Generally, if you understand the steady state case you can then look at other scenarios and decide what else has come into play (a matter of accounting.)

On Fins…

That many of the various kinds of fin setups work has never been an issue with me, they obviously do (with respect to the conditions for which they were designed.) What has been an issue with me has been when and why; and I believe, understanding propulsion is the first step in answering these questions.

Fins are important, but regrettably they’ve yet to be discussed in these threads in any great detail. Neither have (in any detail) those design features that impact the motion of the surfboard post propulsion, see prior thread ‘Dynamics – The Trim Equation’

Thanks again,

Kevin

36

Dave. WTF?

When you were riding those ripples to shore on your LB, what force kept you sitting in front of the wave?

When you were carving up and down on a bigger wave, what force were you exploiting? “No gravitational changes?!?”

I guess even intelligent people lose sight of the forest when someone is pointing at a tree. Perhaps especially in Kevin’s case, because the tree ain’t there. Not your fault, Dave. You were just looking elsewhere while this pre-Galilean misdirected you.

Kevin, you have disagreement(s) about what other people identify because you are physically, provably WRONG. You don’t understand the first thing about it all, so you have no coherent understanding or comments to make, flowing from your fundamental misunderstanding.

The wave is not a jet. See my steel wave example in previous thread Dynamics blah blah blah and my citation of Occam’s Razor then add the conclusions together for some clarity. Or comment. Where do you find error with those two posts exactly?

You may ignore this post as you do all the others of mine that pointed out how badly you’re fooling yourself, but you prove my points by doing so.

I would also refer you to Ken’s post above for some cogent quantitive assessment of your overall shtick.

37
Quote:

SO I have to conclude that whereas shoaling is an absolute requirement, some speed, and in some cases most speed, can come from consideration of gravity alone.

Beautiful!

Quote:
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. But since the point at which the wave is lifting up is continually moving closer to shore, the board moves towards shore.

The same thing holds in the case of a trimming longboard - there is no visual change in elevation, but we are always falling and always being lifted by the wave. When these two are in balance, we can just stand there and do nothing (if the wave was flawless). Since the location in which the wave is lifting us is moving down the line closer to shore, the surfboard moves down the line closer to shore.

Gravity is the force that keeps us in the wave, so to speak, so it is indeed a requirement, is it not? If there were no gravity, when the wave first hits us from below, we’d fly up and out into space, never to return…

Quote:

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.

Clear as day to most of us. Surely we can generate speed with our muscles acting on the board rail to rail and whatnot. (just as I can ride a skateboard uphill without taking my feet off of it - pump side to side…) But Kevin talks like he believes gravity has nothing to do with surfing and that the surfboard travels towards shore only because of water that is flowing away from shore.

38

It’s been said to me in a private discussion with another member that what Kevin really means to say is simply that the water flow up the wave is necessary to get the surfboard planing, at which point gravity takes over. I think he’s saying a lot more than that, but I disagree even if that’s all he were saying.

I believe two things:

  1. A surfboard planes (to the extent that it ever does) when it reaches a high enough speed. Falling down the wave face thanks to gravity provides this initial speed, and we keep it up by using our muscle power and board shape to propel ourselves back up the wave and down again and all that jazz, or just stand there and let the balance of gravity and upward lifting of the wave propel us shoreward.

possible objection: well, then how does the surfboard move with the wave intially as you are taking off, when it’s not falling down the wave yet? doesn’t there need to be a jet of water moving up the wave in order to raise your board up and make it plane?

  1. No. A surfboard does not need to plane in order to move. Obvious since I can move my shortboard easily with little hand paddles even when it’s completely underwater. So…as the wave arrives, the wave energy moves you without planing for a while until you are high enough in the air (so to speak) that you can begin to fall down. The wave angle gets steeper so much of the board is sticking out into the air in front of the wave and therefore not in the water so less board is in the water and therefore less drag - as one angles the nose down more and gravity gives one speed as he falls, allowing his feet to land on the board, he stands up and somewhere between the top and bottom of this fall, the board reaches enough speed that it “planes”.

Kevin doesn’t see this gain in speed from falling with gravity so he requires a force of water flowing under the board fast enough to get the board to plane.

39

What I’ve really meant to say is what I have written.

If you interested in the role of gravity, please see the thread ‘Dynamics – The Trim Equation.’

I would suggest, though it’s completely up to you, that in your analyses you keep the following in mind.

A force is required to change the motion of an object.

Liquids interact with solid objects via a force, usually expressed as force per unit area –i.e. pressure.

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

You may also wish to read my initial post of this thread.

I would also recommend reading my comments to Blakestah in the above post regarding steady state analyses.

Regarding what planing is or how it’s being applied here please read Dynamics – The Trim Equation, or Dynamics – Surfing the Force.

You seem quite convinced in your own explanation as to ‘what makes a surfboard go’. I actually don’t understand your description well enough to disagree or agree. It’s also apparent that a number of others seem to understand your approach. I suggest you consider starting a thread that outlines your approach so that others might help you develop it further, but of course that’s completely up to you too.

Thanks,

Kevin

40
Quote:

You seem quite convinced in your own explanation as to ‘what makes a surfboard go’. I actually don’t understand your description well enough to disagree or agree.

Simple. The wave lifts the board above sea level. Gravity causes the board to want to fall back down. Since the point in space at which the wave is lifting upwards moves closer to shore as time progresses, the surfboard also moves closer to shore with the wave.

Picture some sort of mechanical road rolling in to “shore” in which the ground rises up in series on its way, like a wave does, and me on a skateboard. I would ‘surf’ my way towards shore even though there is no motion towards me like the water flow you are talking about.

Are you saying the difference between the skateboard (which has wheels) in my case and the surfboard is that upward water flow is necessary to make the surfboard board “plane”? 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. Heck, I can move it forward quite easily with only my arm muscles when it’s complete under water. I also believe that the board planes once it reaches a certain speed and dropping into the wave provides this speed.

Imagine a surfer “air dropping” into a wave. It happens, and guys make the waves. They gained enough speed, from the gravity of falling, for their boards to be planing when they hit the surface - they did this without any water interacting with their boards. it was all gravity making them fall.