Forces in Surfing

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Still Dr. Al I think you have provided a clear enough summary.

Cheers steve, I must just remind everyone that those summary points in my previous post are KCasey’s words copied from the thread ‘surfing the force’.

You seem to have a good grasp of things and i like your “gravs v flows” comment.

I’m a ‘grav’ at the top of the wave, and mostly a ‘flow’ when i’m trimming across the wave or bottom turning out on the flats.

So I guess I’m a ‘grow’ or a ‘flav’.

Ok, I’m a ‘flav’.

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at the right time there is an incredible burst of acceleration from the bottom turn up the wave away from gravity.

Sure. This is just the “wave lifting you up the wave” bit. You’re just placing your board at such an angle that it can travel up the wave, just as on the drop you’re placing it at such an angle that it’ll go down the wave. Sure, some water is “flowing” up towards the lip, but mainly the wave is welling up from the bottom and taking the board up with it. Add to that assistance from the surfer’s muscles and his mastering of weighting and unweighting and he can shoot right off the top of the wave if he wants, just how a skateboarder can drop in on one side of a ramp but fly way above the other side (or use a big drainage ditch and drop down to a bottom turn and come back up the same side and accomplish the same thing).

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Gravity does provide the propulsion if you just surf straight down the wave, but most of us don't. If we talk about a surfboard trimming across a wave there must be a propulsive force pushing us across the wave.

I don’t see the difference. When i carve back and forth across a road on a skateboard or mountain on a ski/snowboard so that my angle of attack is always “down the line” instead of straight down the hill, I’m still using gravity. The reason your surfboard goes across the wave instead of straight down it is that you’re presenting an angle of board that wants to fall down the wave sideways.

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The reason your surfboard goes across the wave instead of straight down it is that you're presenting an angle of board that wants to fall down the wave sideways.

respectfully, this doesn’t make much sense perhaps you could try again?


let’s say you’re trimming across the wave and therefore you’re maintaining your vertical height in space. My view is then that gravity wants to pull you down the wave but because your rails and fins are in the wave and the water rushing up the face is acting on them, you generate lift.

By controlling the attitude of your board to the wave you can direct this lift which then drives you across the face of the wave. KCasey is right with the ‘flow’ providing propulsion but the reaction force allowing this propulsive force is gravity and cannot be neglected.

Gravity cannot pull you horizontally across a wave.

You are not going horizontal. You’re going down, at an angle. The wave is just also lifting you (back) up again.

respectfully, please think about what you are saying.

if you are going down the wave and wave is pulling you back up (and you’re in trim like i said) then you have no vertical velcity - you are not moving up or down.

You are moving across the wave, in trim, and you are therefore moving in the horizontal plane.

What part of this do you not understand? or are you talking about velocity relative to the water flow?

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1 The flow in the front of a wave is in the ‘up-and-forward’ direction.

I do not agree with this point. The “flow”, is different on different parts of the wave. In the flats there is often very little movement. At the trough the flow is moving back toward the wave as the water in the middle of the wave, which is moving up is pulling the water from the trough. The water in the middle of the wave face is moving mostly up but also a bit forward. And the water at the crest is moving mostly forward when the wave is breaking.

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2 Propulsion in surfing comes from the up-and-forward flow of the front of the wave impacting the bottom of the surfboard.

IMHO, the “propulsion” in surfing comes from all three forces. It comes from the momentum that the surfer gains from moving (falling) from the crest of the wave to the trough, also called gravity. It comes from the forward motion of the wave impacting the force of the water on the surfboard (most of this energy is gained when the surfer is at the crest. It comes from the “flow” of the water “up” the wave, and how the “flow” impacts the bottom, rails, and fins.

After much reading and thinking about this topic over the last several weeks, I think we (you all) are finally getting somewhere. OB, your description of all 3 forces working to provide “propulsion” (which is a term I don’t like here) is adequate…the interaction between all of these ‘forces’ is what creates the ability for us to ‘traverse’ a wave on a 3 dimensional axis…

One word of caution, semantics in this discussion are VERY important…I think a lot of us have been arguing the same point but are confused by mis-leading/mis-understood verbage…

‘Flow’ being one, ‘propulsion’ being another, and there are more examples.

I don’t think water ‘flows’ up the face of a wave…this description conjures an image in my head of a river ‘flowing’ downstream, pulled by gravitational forces from a higher elevation to a lower elevation. IMHO, water does NOT flow up the face of a wave, but it definately moves up the face. More like a group of water molecules being ‘lifted’ up the face of the wave by the energy moving through it. Lifting the molecules up and setting them back down in roughly the same place in an open ocean swell, lifting the molecules up and then projecting them beachward and downward in a breaking wave. Steeper waves like Gnaraloo DO have more water MOVEMENT up the face of the wave, but that water is NOT FLOWING up the face.

Discuss…

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respectfully, please think about what you are saying.

if you are going down the wave and wave is pulling you back up (and you’re in trim like i said) then you have no vertical velcity - you are not moving up or down.

You are moving across the wave, in trim, and you are therefore moving in the horizontal plane.

What part of this do you not understand? or are you talking about velocity relative to the water flow?

I’m not sure that is any part that I don’t understand, ha! FWIW, I can’t be talking about velocity relative to water flow because I simply don’t think the water “flow” has much to do with anything at all.

Think of an angling drop-in: in this case, do you agree that gravity is the “force” that is making the surfboard move? And yet the surfer is moving “horizontally” down the beach as well as moving down from crest to trough. I think you’ll agree to gravity being the force that allows surfing in this scenario. But then you don’t see a way in which a board can move down the line in trim, basically gaining nor losing any vertical distance above sea level without some second force of water flowing up the wave and hitting the board and this somehow pushing the board nearly perpendicular to that water flow force.

Here’s how I see it, instead. The energy of the wave is trying to lift the surfboard up above sea level; gravity is trying to make the surfboard fall back down to sealevel; the reason one can ride down the line instead of just falling straight down is that the tail of his board is being lifted up, but the place in space where the tail is being lifted continually moves horizontally down the line (and towards shore of course: we don’t surf truly horizontally, but at a certain angle towards shore). The board is carried along on this wave of energy. As long as we keep the angle of our board and weight just right (i.e. on the seaward rail), we will not fall off of this energy spot as it moves down the line and towards shore.

Sit at a table. Picture that left breaking waves are coming in from your body towards the table. Place a yardstick sideways on the table. The left side of the yardstick represents the nose and the right side of it is the tail (since we are surfing towards the left). Hold a pencil in your right hand and place it under the yardstick, near the tail and lift straight up. This mimics the upward lifting wave energy hitting the surfboard at the first place it encounters it (near the tail). What happens to the yardstick? It slides off the pencil away from where you are lifting, and goes “down the line” just as a surfboard does when the wave lifts up its tail.

OK, repeat this experiment, this time holding another pencil in your left hand as well, a few inches to the left and a few inches lower than the one in your right. As you lift the pencil in your right hand and the yardstick begins to slide down the line, keep the pencil in your left hand in contact with the yardstick as well. It will be on a lower vertical plane. As the yardstick slides off the pencil in your right hand, raise the one in your left hand up to the same vertical plane the one in your right hand ended up at. What happens to the yardstick? It keeps moving horizontally down the line. If you had a hundred hands, to represent the welling up of the ocean under the board, the yardstick would keep going sideways. This to me, is surfing in trim down the line.

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I don’t think water ‘flows’ up the face of a wave…this description conjures an image in my head of a river ‘flowing’ downstream, pulled by gravitational forces from a higher elevation to a lower elevation. IMHO, water does NOT flow up the face of a wave, but it definately moves up the face. More like a group of water molecules being ‘lifted’ up the face of the wave by the energy moving through it. Lifting the molecules up and setting them back down in roughly the same place in an open ocean swell, lifting the molecules up and then projecting them beachward and downward in a breaking wave. Steeper waves like Gnaraloo DO have more water MOVEMENT up the face of the wave, but that water is NOT FLOWING up the face.

I totally agree with this description. I see the board flowing through/over the water, not the water flowing through the fins or whatever.

Please study the following clip. It will tell you everything you need to know about the dynamics of surfing:

http://youtube.com/watch?v=FdbxLk8_C8c

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‘Flow’ being one, ‘propulsion’ being another, and there are more examples.

I don’t think water ‘flows’ up the face of a wave…this description conjures an image in my head of a river ‘flowing’ downstream, pulled by gravitational forces from a higher elevation to a lower elevation. IMHO, water does NOT flow up the face of a wave, but it definately moves up the face. More like a group of water molecules being ‘lifted’ up the face of the wave by the energy moving through it. Lifting the molecules up and setting them back down in roughly the same place in an open ocean swell, lifting the molecules up and then projecting them beachward and downward in a breaking wave. Steeper waves like Gnaraloo DO have more water MOVEMENT up the face of the wave, but that water is NOT FLOWING up the face.

Discuss…

It may be better if you modify your conception of “flow”. As far as I can tell, flow in Fluid Dynamics Theory does not imply a stream as it does in layman’s terms. Flow simply means that the fluid is moving.

Kind of like how resin flows out when you pour it on your surfboard. It is not moving fast, or shooting like a stream it is simply moving.

http://en.wikipedia.org/wiki/Fluid_flow

Also, as was stated earlier, it is important to think of the motion of the water relative to the surfboard/surfer. It really doesn’t matter if the water is moving past the board or if the board is moving past the water. The water is flowing relative to the board.

I think that it is true that the board moving over the water constitutes the majority of the flow in most situations, but you cannot discount the significance of the water moving on the wave as it effects the rider and the flow relative to the board.

Can one of the grav guys tell me if they really honestly believe that surfing on a wave is analogous in a meaningful way to skateboarding down a hill on concrete?

The big difference is between water and concrete, also the circular motion of water particles in a shoaling wave.

What is the motion of water particles relative to the bottom of a surfboard, say , in the tube at Pipeline ?

Does this really compare to a skateboard rolling down a hill.

A small surfboard at the base of a steeply shoaling wave where the surfer is riding at the base waiting for the right moment to pull in would quickly sink if there weren’t some kind of “propulsive/lifting” force being applied from the water flowing across the board (flowrider effect).

If there were only gravity, low-volume boards would very quickly sink if kept at the base of the wave for too long.

But they don’t because the water flow across the board provides enough lift to keep the board planing.

Try it yourself next time you are surfing a high energy long period groundswell at a shallow reef or sandbar…stand at the base of the wave for almost too long and then just lean into the flow…

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A small surfboard at the base of a steeply shoaling wave where the surfer is riding at the base waiting for the right moment to pull in would quickly sink if there weren’t some kind of “propulsive/lifting” force being applied from the water flowing across the board (flowrider effect).

I’m confused by your scenario. If the wave energy is lifting the board vertically up towards the sky from the bottom, why do we also need that water flow to create lift in the manner you propose? Even at the bottom of the wave, there is this lifting force as the forward part of the wave hits shallower water. Even while riding the soup straight to shore there is the lifting of the tail of the board (it’s just hidden under all that soup).

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A small surfboard at the base of a steeply shoaling wave where the surfer is riding at the base waiting for the right moment to pull in would quickly sink if there weren’t some kind of “propulsive/lifting” force being applied from the water flowing across the board (flowrider effect).

I’m confused by your scenario. If the wave energy is lifting the board vertically up towards the sky from the bottom, why do we also need that water flow to create lift in the manner you propose? Even at the bottom of the wave, there is this lifting force as the forward part of the wave hits shallower water. Even while riding the soup straight to shore there is the lifting of the tail of the board (it’s just hidden under all that soup).

The water isn’t only moving vertically, it is moving (being sucked) back toward the wave. The moving water is enhancing the planing effect. It doesn’t matter if it is the board moving past the water (like wake boarding) or the water moving past the board (like the flow rider), the water is moving relative to the board, and this creates the planing effect (along with creating lift with the fins).

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Here’s how I see it, instead. The energy of the wave is trying to lift the surfboard up above sea level; gravity is trying to make the surfboard fall back down to sealevel; the reason one can ride down the line instead of just falling straight down is that the tail of his board is being lifted up, but the place in space where the tail is being lifted continually moves horizontally down the line (and towards shore of course: we don’t surf truly horizontally, but at a certain angle towards shore). The board is carried along on this wave of energy. As long as we keep the angle of our board and weight just right (i.e. on the seaward rail), we will not fall off of this energy spot as it moves down the line and towards shore.

sorry but this is pure twaddle. what is energy?

What you are trying to do is explain a physical phenomenon, which you have experienced many times, but you are avoiding the use of physics. Please try to understand the concept of lift - we are not talking about lifting something up (lifting the tail of the board), we are talking about hydrodynamic lift.

As I’ve said before, this is technical discussion on the forces of surfing, if you won’t take the time to understand the physics then…


Regards ‘flow’ up the wave. I would use the term flow because it is easier than water particle velocity but i’m open to suggestion. And again if you don’t believe its important, forget about it at your peril, all the info is here, take the time to read it. Open your mind, understand and perhaps accept it.

Gee, it must be nice to be so sure of oneself and therefore free to treat others like shit.

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sorry but this is pure twaddle. what is energy?

I’m sorry it’s such a foreign concept to you. I’m talking about the wave’s energy, which was transfered from sun to wind then from wind to the ocean and then travels thousands of miles through the water in a wave shape, leaving the water right back where it was. Not “flow” that you are describing as a horizontal movement of the surface water out towards sea.

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What you are trying to do is explain a physical phenomenon, which you have experienced many times, but you are avoiding the use of physics.

If you can’t explain your concepts in plain language, then what good are they. What did you not like about my very simple example of the yardstick. It showed how vertical lifting that comes to an object from the side (waves break left to right or vice versa as they also move towards shore), cause the down the line surfing that you couldn’t understand taking place without some sort of water flowing on the surface back out to see that hits the board and makes it squirt sideways or whatever you were imagining. Did you really think about the experiment or try it. Think about which parts of the surfboard are lifted by the wave first and how this causes the board to move.

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Please try to understand the concept of lift - we are not talking about lifting something up (lifting the tail of the board), we are talking about hydrodynamic lift.

You can avoid talking about lifting the tail of the board up if you’d like, but you’ll be avoiding the force of the wave and that’d be kind of silly. The ocean is not a flowrider. A surfboard floats on top of the water, a wave comes along and lifts that water, and thus the board, more or less straight up, gravity tries to get it to fall back down (they are called surface gravity waves after all), and surfing occurs. Do you really think there is a major force missing from that?

Does some water get sucked back up the wave? Sure. Everyone has felt this. But to describe this as the major force in surfing and ignore the vertical force and gravity, is silly.

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Gee, it must be nice to be so sure of oneself and therefore free to treat others like shit.

you are taking me up wrong, tone is unfortunately lost in text based conversations.

I don’t mean to be patronising or derogatory.

I’ll get my coat.

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It may be better if you modify your conception of “flow”. As far as I can tell, flow in Fluid Dynamics Theory does not imply a stream as it does in layman’s terms. Flow simply means that the fluid is moving.

Kind of like how resin flows out when you pour it on your surfboard. It is not moving fast, or shooting like a stream it is simply moving.

http://en.wikipedia.org/wiki/Fluid_flow

I was actually trying to avoid getting too technical as some on this board have done as the majority audience here and in surfing get lost in technicalities. Flow as defined in fluid dynamics is actually much more complicated than you have stated above, and actually should be broken into turbulent and laminar flow, both of which contain more variables than I am willing to get drawn into here…we would need to discuss turbulence, etc and that would go against the spirit of this discussion in attempting to simplify this concept for our mutual understanding.

Therefore I do not think I need to modify my concept of flow, as when discussing waves the flow is relative to the surfboard, as you also stated above. Everything is relative…

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I was actually trying to avoid getting too technical as some on this board have done as the majority audience here and in surfing get lost in technicalities. Flow as defined in fluid dynamics is actually much more complicated than you have stated above, and actually should be broken into turbulent and laminar flow, both of which contain more variables than I am willing to get drawn into here…we would need to discuss turbulence, etc and that would go against the spirit of this discussion in attempting to simplify this concept for our mutual understanding.

Therefore I do not think I need to modify my concept of flow, as when discussing waves the flow is relative to the surfboard, as you also stated above. Everything is relative…

Every time I think that I have it figured it out someone says something that makes me question my conception of what is happening. So you can lump me in with those who are lost.

All I was saying is that flow doesn’t imply a stream all it means is that the fluid is moving.

When talking about the part of the wave that has yet to break I don’t think we have to concern ourselves with turbulence. It is just too damn complicated. I read that Werner Heisenberg when asked about what he would ask God if he got the chance said “When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first.”

DrAl, now that we’ve calmed down a bit, let me get back on topic. You previously wrote:

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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.

Is the water moving up the face or is the face just moving towards the water and the water actually mainly going straight up in space? The latter is how I see it, and this “force” is what I believe lifts the surfboard up above sea level, and then gravity causes it to fall down again. The surfboard in trim follows the spot at which the “disturbance” as you call it (is that the physics language you chastised me for not using?), or the “wave energy” as I call it, are pushing basically straight up towards the sky.

As I previously wrote, the reason we can go down the line is that the “disturbance” is also welling up down the line: it isn’t moving straight towards shore. It’s tracing the exact same path as a down the line surfer in trim. Or more accurately, we are tracing the same path as the “disturbance”. By definition, being in trim means sticking to this spot and being carried along with it!

someone wrote:

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The water isn't only moving vertically, it is moving (being sucked) back toward the wave. The moving water is enhancing the planing effect.

I see this as something that will reduce drag but is not a force that factors much in propelling the board down the line or towards shore. That’s what we’re after here, right? The main forces that allow surfing to be possible? (or does everyone agree this is gravity?).

someone wrote:

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Can one of the grav guys tell me if they really honestly believe that surfing on a wave is analogous in a meaningful way to skateboarding down a hill on concrete?

Yes, very analogous. Did you read Janklow’s idea about the sheetmetal which is being pushed up like a wave (page 5 I think).

Can anyone point out why this is not accurate? Anyone care to comment on my yardstick example, which is basically the same idea?