Forces in Surfing

General Discussion
81

…water is moving up the face of a wave.

Face water moves up vertically relative to earth (earth is an easier reference). Face water ‘moves’ in the opposing direction relative to the beachward motion of the wave energy.

Thus, the net velocity vector of the face water, relative to the forward motion of the wave energy and surfer is up the face.

And even tho much of the waves energy has already changed due to breaking action, water DOES ‘pile up’ on the beach, momentarily. But usually returns back to the water’s edge due to the beach slope and gravity.

By the way, how to you ‘pile up’ water? Magic?

82

Craftee,

YES, water is piled up by the forces of magic. Magic does exist in surfing. Evidenced by: When a board reaches 37 mph, it becomes frictionless, and proceeds to INCREASE speed exponentialy, regardless of wave size, and ONLY the force of gravity keeps the board and rider in contact with the earth. (using your earth reference) However, I’m not sure if the phenomenon occurs in the northern hemisphere.

83

right I’m lost and this thread seems to be dying

craftee - i think you’re lost too

Bill - magic? I’m a non believer

and finally water does not pile up on the beach (on a large scale), I am trying to illustrate the point that the surface of the water is a medium for the wave in the same way as the hose or the carpet.

And with all these waves there is no net mass transport, i.e. the waves just don’t keep going up the beach and swamp the land.

I feel i have to be pedantic now and say, there are nonlinear effects which cause mass transport on a small scale, i.e. rips

Al out

84

Gee, Bill! … ahhhh.

Right.

Phew! Man, I didn’t know what the hell was goin’ on for a while there.

Oh boy!

Shoot.

You’re a madman.

85
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What I don't understand is how you think we are using the wave energy? It takes energy to pick us up to the crest (giving us potential energy) but once we drop I'm not sure we affect the wave that much (i.e. absorb some of its energy), we're just riding along it.

That is totally dependent on where you are on the wave, up high, mid way or at the bottom. Read my previous post about dynamic changes in energy transfer as it relates to surfing. Top/bottom/repeat type surfing involves fast changes in energy transfer OF THE SURFER and has almost no affect on energy changes of the wave. The energy changes of the wave happen whether the surfer is there or not. Two seperate things requiring two seperate analysis.

HOWEVER, if you know how to smack a powerfull lip and come down with it, you know the wave’s energy can affect the riders energy. A pitching wave’s lip is a high energy packed mass of moving water, moving faster than the wave itself. Good surfers have been harnessing this high energy zone for decades. Surfers who know how to smack a powerfull lip, particularly with a small lightweight board, realize this better than those who cannot.

Yes, when you are at the trough of the wave the water flow is moving away from the beach, that is why bottom turns fling you back to the top of the wave.

And when you are at the crest of the wave the forward motion energy of the wave is transfered to down the line, or off the bottom projection.

86
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You can delete the fin(s) and still move down the line though, although that surface area is useful lifting area. I would argue the finless dynamics in perfect trim and snowboarding down and around a perfectly conical mountain are exactly the same.

Your rail catches some of the wave energy to lift you up against gravity. The fins and the rail have similar functions.

87

And with all these waves there is no net mass transport, i.e. the waves just don’t keep going up the beach and swamp the land.

  1. waves break, losing much/most of their energy before reaching the shore

  2. at that shoreline point, the mass of water going UP the beach (most beaches have slopes), combined with gravitational force, makes the available wave energy to ‘swamp the land’, very weak or non-existant in comparison to the competing weight (weight=water mass x gravity) of the water being moved. Therefore, weight force wins by a long shot and lands dont get swamped. Thank God for small waves and big land masses with slopes.

  3. very rarely, there is enough wave energy to overcome competing gravitational affects and lands do get swamped. tsunamis come to mind.

  4. beach erosion is net mass transport. (is there no beach erosion where you live? there’s heaps over here and its a major ‘problem’…particularly to wealthy types living right on the beach)

and this thread seems to be dying

There are numerous threads in the archives on the exact same subject. Not once, has there been a final definitive answer that everyone can agree upon. All threads die, but thank God our minds dont - starting new threads is part of this process.

Youre seven posts into Sways now, so here’s a wise little tip from Voltaire:

“All things must come to an end, meanwhile we must amuze ourselves”

Welcome to Sways?

88

Yes, when you are at the trough of the wave the water flow is moving away from the beach, that is why bottom turns fling you back to the top of the wave.

And when you are at the crest of the wave the forward motion energy of the wave is transfered to down the line, or off the bottom projection.

Hmm, not exactly what I had in mind. In progressive top to bottom surfing, you have maximum potential energy at the top, with some kinetic. After a fast drop to the absolute bottom, you have zero potential and theoretically, maximum kinetic. It is this kinetic which allows good bottom turns.

With enough kinetic at the bottom, you can get back to the top, losing some kinetic and again maximizing potential and with any hope, more kinetic than at the start of the process, or not, depending on the type of move at the top.

My favorite top move, which I have ok, not great, ability to do, is after a quick bottom turn, throwing my board on the pitching lip and letting it catapult me forward onto the bottom half of the face, hopefully getting a little airborne in the process. That my friends, is capturing concentrated energy that a supercomputer would have trouble analyzing and quantifying…much less a bunch of online forum participants. But it IS there…take it or leave it.

edit: check out the two moves starting at 0:43

http://www.youtube.com/watch?v=IdfWtqmZUt8

89

I agree with what you’re saying.

the point i am trying to make is that ‘a wave’ is not a discrete body of water moving towards the land, i.e. it is not a packet of water that has travelled from a far off distance place to land on the beach. A water wave is a disturbance moving through a body of water. And what i am trying to get at is that water is moving up the face of a wave.

Do you agree?

beach erosion is a whole different thing - nearshore coastal dynamics - which i am not going to get into.

Again I am trying to simply the subject so that we can determine the ‘forces in surfing’.

Quote:

Welcome to Sways

thanks

90
Quote:

Yes, when you are at the trough of the wave the water flow is moving away from the beach, that is why bottom turns fling you back to the top of the wave.

And when you are at the crest of the wave the forward motion energy of the wave is transfered to down the line, or off the bottom projection.

Hmm, not exactly what I had in mind. In progressive top to bottom surfing, you have maximum potential energy at the top, with some kinetic. After a fast drop to the absolute bottom, you have zero potential and theoretically, maximum kinetic. It is this kinetic which allows good bottom turns.

With enough kinetic at the bottom, you can get back to the top, losing some kinetic and again maximizing potential and with any hope, more kinetic than at the start of the process, or not, depending on the type of move at the top.

That is part of it, the water flow and forward movement of the wave also provide energy to the surfer/board system.

You cannot deny the effect that the movement of the water has on the surfer/board.

91
Quote:

I agree with what you’re saying.

the point i am trying to make is that ‘a wave’ is not a discrete body of water moving towards the land, i.e. it is not a packet of water that has travelled from a far off distance place to land on the beach. A water wave is a disturbance moving through a body of water. And what i am trying to get at is that water is moving up the face of a wave.

Do you agree?

I agree. The wave is the manifestation of the energy that is traveling through the sea. Similar to how air doesn’t move very far in sound waves, but the sound travels over relatively long distances.

When the wave energy approaches the shore the sea floor (often including a shelf or sand bar) it is projected back upward and the wave rises out of the sea. As it further progresses the crest is moving faster (forward) than the trough and the wave breaks.

The water doesn’t move that far relative to the shore. But is does (especially after the wave energy has begun to make contact with the bottom of the sea) move relative to the surfboard and rider.

92

Aaaahhhh, concensis at last! Yes, water IS moving up the face of a wave, (briskly) as it advances, and there IS way more energy for the rider to tap into in the upper third of a wave, than the bottom third of the wave, as pointed out in prior posts. When you understand the movement of a single molecule of water in a wave, you understand the movement of ALL the water in the wave. In general, the larger the wave, the more noticable those energy differences are. Now, consider this, we ride on the SURFACE of the air/water boundry layer, what happens when we drop a fin into the wave, below the surface? Remember ALL the water molecules are in motion, in the same way. Think it through. It gets interesting, eh? An important clue is that you must consider the RELATIVE flow of water, in relation to the centerline of the board. It IS independant of the motion of the water in the wave. What is the fin doing? Why? How?

93

That is part of it, the water flow and forward movement of the wave also provide energy to the surfer/board system.

You cannot deny the effect that the movement of the water has on the surfer/board.

I have never denied this. Years ago here, I may have been the first one to say that the ramp is moving towards the beach. Search the archives using “the ramp is moving”.

Let me connect the dots, as I see it…

The “in-your-head” analysis of the movement you refer to works at its finest when the board/surfer are in balanced trim on the face. There is little energy transfer going on, OF THE SURFER. So in perfect trim, where the surfer’s elevation relative to earth does not change, the surfer is in static energy transfer mode, at a point in time.

In steep fast top to bottom surfing, there is far faster exchanges in energy transfer, from potential to kinetic and back.

I am not saying that the ‘movement’ is not part of it, its just that at some point, the common analysis standard of a “static point in time” works best in balance trim, than in dynamic energy exchanges of the surfer.

Have you ever seen a radical surfer drop down a wave in the air? It happens and since the board isnt touching the water, why is the surfer still moving? (And probably moving FASTER doing so).

Almost every free body analysis requires simplifying assumptions and a point in time, and so you see, one would have to analyze several points in time individually to get the entire picture.

Which reminds me, did you know that one of the most common simplifying assumptions in Newtonian analysis is the elimination of frictional affects? How relevant is that in surfing?!

Again, the physics of the wave happen whether the surfer is there or not. But the surfer can perform with a certain chosen style, and that style is either closer to static, or radically dynamic.

Its the radically dynamic that is far more difficult to analyze. And to me, it makes very little sense to try and quantify it…the things man has been able to achive is remarkable. Sometimes, its better to observe, than to try and attach numbers.

If you want to be a better surfer or have more fun doing it, surf more often, ride better quality surf, in preferrably uncrowded conditions, ride better boards or one’s that match your style or waves.

In the end, knowing the surf physics answer(s) wont do much in your/our understanding of surfboard design. This is still best done using a trail and error approach as has been done for decades.

But I guess it makes for interesting online discussion, even about matters of little consequence.

Onwards…

94
Quote:
In the end, knowing the surf physics answer(s) wont do much in your/our understanding of surfboard design. This is still best done using a trail and error approach as has been done for decades.

I have to disagree here.

I think this attitude and lack of understanding of the physics has led to the invention of such silliness as the bullet fin (i’m sorry but i’m a nonbeliever)

http://www.bulletfins.com/how-it-works.htm

amongst other things.

of course the surfer feels his ride and provides ‘empirical’ feedback, but understanding the physics can surely lead on to the design of more radical concepts and hopefully a break away from such ‘accepted’ setups such as the thruster.

there is more development of the surfboard to come and I’m guessing fins and their setup is where its at…

95

I guess my point there was badly worded.

The point is, if you want to make better boards, knowing how the board/fins interacts with the water, its flow, attachment/release could make you a better board designer. This stuff happens similarly whether youre riding a wave or being pulled behind a boat.

If you choose to ignore the above, and instead focus your intellectual energy and knowledge towards wave physical phenomenon and energy analysis of wave or surfer, then your chances of making better boards, based on this knowledge, while ignoring the other, decrease significantly. IMO.

As far as your bullet fin reference, I think youre confusing science with free enterprise. That fin may not work better than conventional fins, but if he sells $100k+ of them this year, then I would consider him successfull. More power to him. Markets have a way of sorting these things out on their own.

there is more development of the surfboard to come and I’m guessing fins and their setup is where its at….

Good guess. Im looking forward to any contributions you or anyone else on Sways can make towards this.

96

Good post, D

97

craftee you’re right, there’s no substitute for getting out there and doing it. But sometimes, when there’s no swell and the resin is going off, it seems a good idea to throw some ideas around and try to get a better grasp of things…

And as for bullet fins… (and you must understand this is one of my pet hates)

You see while I’m not a capitalist at heart I do believe in free enterprise which leads to development. But Mr bullet fins made the mistake of putting a science slant on his sales pitch, and science is based on critical analysis of itself. Therefore I can criticise the idea.

If he had have just taken the usual ‘fin marketing approach’ based on reviews by surfers and a brief wishy washy explanation of how it works, I wouldn’t mind, but once he includes ‘the science’ he’s heading for trouble.

And also, does ‘free enterprise’ mean that it is perfectly acceptable to sell people crap products as long as you’re making a profit? This is shameful and I can only assume that he believes in his product.

Sorry, rant over.

Hey, anyone know what the predominant forces in surfing are?

Here’s what I got

  1. Acceleration due to gravity

  2. Drag force (on board and fin) proportional to the velocity

  3. Inertial forces associated with the acceleration of water providing lift (planing)

  4. Horizontal acceleration relative to the beach, associated with the change in horizontal water particle motion on the face of the wave (bit of a mouthful)

also

  1. lift from the fin (and rails)

  2. bouyancy (when not planing), but this may push the rail out of the wave?

98
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On this site, in discussing surfing and surfboard dynamics people often have disagreements about what forces are acting upon the surfer/surfboard. I think that there are three forces.

Gravity, Water Flow up the Wave, and Forward Wave Motion.

It seems that lots of people think that there are only two forces (pick any two).

If anyone out there thinks that there are more or less or different forces acting upon the surfer I would like to have this discussion.

Whats rolling around in someone’s head has about as much to do with it as the forces. I have seen really good surfers surf like crap because they were too concerned with whose watching them or etc. I think state of mind is a force.

99

Sorry to jump back in in the middle, but it is gravity which pulls the water down, after the wind blows it up, and thus the wind energy is transfered to the waves in the water because gravity pulls it down, and like the bowling ball example, the energy put in by the wind is transfered out in waves…

But as I write, I think who cares? I am still looking for proof of the moving water theory.

100

“Magic does exist in surfing. Evidenced by: When a board reaches 37 mph, it becomes frictionless, and proceeds to INCREASE speed exponentialy, regardless of wave size, and ONLY the force of gravity keeps the board and rider in contact with the earth. (using your earth reference) However, I’m not sure if the phenomenon occurs in the northern hemisphere.” Hahahahahhahah!!!

What? I gotta go “down under” to experience the magic?!?!!!