Forces in Surfing

General Discussion
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But to a surfboard (and rider) paddling to catch a wave, something in excess of paddle power is causing the board to achieve enough velocity to allow you to abandon bouyancy and drop in using planing forces only. What is doing that?

In a trim, where the board is neither being paddled nor is it falling (due to gravity), the board is still planing. What is keeping the board planing?

Lee, have you tried my yardstick example? How do you feel it does in answering your questions above? What do you think the force is that causes the yardstick to move? Thanks, Rich.

p.s. personally, i ride boards that are nearly always planing. I don’t require any forward paddling speed towards shore to make the board plane and I don’t think this would matter to the ability to drop in or not anyway. To drop in to a wave, all I have to do is allow the wave to lift the tail of my board up towards the sky, which tips the nose down, and then by keeping my weight balance on the nose side of this teeter totter where there is less force pushing it up towards the sky, my board tries to fall back down to sea level. There just happens to be a slope of water in front of me and so instead of falling all the way down, I gradually slide down this slope. The slope is pretty slippery, as is my surfboard, so it works out pretty darn well. While this is happening, the wave has picked a new spot to push up towards the sky at and if I position myself at that spot, and get the balance just right, I can move with the wave as long as it allows me to.

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MaraboutSlim,

I agree with you that gravity IS a major force in surfing, but the water movement, or ‘flow’, depending on how you use that word, cannot be discounted in helping your board achieve a planing state faster than if it did not exist.

Did you ever jump on a large trampoline as a kid with a friend, at the same time? Remember when your friend jumped slightly before you, depressing the trampoline, so that it was already fully extended and stretched towards the ground when you landed? The trampoline then launches you much higher in the air than if you had jumped down, stretching it down yourself. This extra force often launched you right off the trampoline, sometimes resulting in serious injury for some I knew!

I am starting to believe that as you make your bottom turn at the base of the wave, not out in the flats, as you are using your kinetic energy, pushing against this water moving up the face of the wave, that this results in a much higher energy return than if you were pushing against water that was not moving at all.

While, logically, this extra bit of energy contained in the moving water seems insignificant when compared to your gravitational energy…might it not somehow exponentially increase your energy return out of a bottom turn?

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I’m inclined (heh) to throw a word in again. Rich, this roller/carpet or sheet-steel or whatever model does beg a bit of consideration thus: if you have a roller and youre pushing it under a rug, and you place your finger on the forward slope at say, the 10 o’clock position and keep it at the 10 o’clock position as the roller progresses forward, propagating the “wave,” you do end up with a relative “flow.” But, that’s given a static position, like say, high-line trim. And gravity does accelerate you if you tip your nose down… The “flow” would be a resistant force to any motion you are doing except if you aimed to go over the top. And the thing is, as per the model I put forward a while back (page 3), it’s not necessary in other slope-riding sports, and the argument is that for some reason it is totally necessary and fundamental here. Why this is such a point of pride, or why Occams’ Razor doesn’t apply, given the demonstrated reality of the other board sports…

Given what surfing is most of the time, then, your (Rich) questions and mine are: “Is that relative movement under the board doing any real work toward moving the rider down the line? Especially given the relative work value of gravity and its rate of acceleration? Or could surfing be accomplished by gravity, if you could isolate the wave propagation element of the forward slope out altogether? And does the work alleged by some actually get accomplished not by “flow,” but simply by the forward movement of the energy-pulse-propagated slope you’re riding?” Like if you turn toward the crest of the wave pulse, and you’re planing on the surface of it, it will move under you, and the “flow” doesn’t have anything to do with it. It’s just that you’re riding on the surface, and the wave pulse is progressing under the surface, so it’s a simple matter of planing/skating/boarding on a surface, and a wave pulse rolling under that surface. The same thing could be and is accomplished all the time by snowboarders and skateboarders riding bowls, where the curve of the surface toward you helps you ascend the slope again.

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Hi to everybody pursuing the forces behind!

In my opinion it might at this point be a good thing to try answering LeeV´s question about the extra push during the takeoff.

Whenever thinking about this, I imagined two effects doing that work:

  1. The oncoming face moves me and the board upwards, which is an acceleration of my mass due to my and the board´s buoyancy.

(except for the buoyancy itself, there´s no gravitation involved, just mass acceleration, like in any direction)

The acceleration has its maximum, where the face is steepest , and will result in forward propulsion because of the angled position of my board on the face of the wave.

Also this push forward will be there any time I´m rapidly moved up the face during cutbacks etc.

2.This is more of a complicated idea and it´s significance for this discussion might be little. still I´d like to share it:

When the upward acceleration on the face takes place, the board will sink in a little, depending on the volume of the board in relation to mine. Of course a moment later I pop up again. The timing of this depends on volume/weight - ratio and could ideally, so my idea, be adjusted to help taking off in the pop up - moment, because to get from floating to planning one has to rise a bit higher over the waterline.

In my opinion those two effects are additional reasons for the easier take off on bigger boards, aside from the faster paddling

Cheers - Detlef!

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Hey,

I think Maraboutslim is right about gravity being the force that makes the board move in conjunction with water moving up the face. Even if you are not moving up or down the face! If there was no gravity, you would shoot right up the face of the wave; gravity keeps you from doing that.

Same as a sailplane that is "motionless’ on a thermal. The air moving up balances the plane “falling down”; net result is that the plane is staying at the same altitude.

Michael

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I think Maraboutslim is right about gravity being the force that makes the board move in conjunction with water moving up the face. Even if you are not moving up or down the face! If there was no gravity, you would shoot right up the face of the wave; gravity keeps you from doing that.

ah so we need both forces!!!

gravity and the lift force from the ‘flow’ and surfboard motion, to produce a resultant force which moves us down the line (wet bar of soap)

you must be a ‘flav’ Michael.

The way i see it, the main forces in surfing are:

(and i must define ‘surfing’ as simply trimming straight down the line)

  1. Acceleration of surfer/board mass due to gravity

2 Hydrodynamic lift force acting on the board surfaces and on the fins.

  1. Drag force

By doing a vector summation of these forces we get a resultant force which drives us, or propels us, or whatever.

Now we can be sticklers and add

2a. Buoyancy uplift (when the board is being paddled) but we’re not talking about that situation, and once the board is planing buoyancy become irrelevant.

And we could also start talking about the torque and loads generated by the surfer himself but why complicate things?

Somebody also mentioned momentum of the mass (surfer/board), well momentum isn’t really a force, although it does take a force to change momentum… drag.

I also think that, under my assumption of trimming down the line on the perfect wave, the forward motion of the wave is irrelevant and can be safely neglected.

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the main force in surfing is huey

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DrAL,

As it is a just approach to simplify things for start, I doubt a rigorous simplification can offer much insight into the original topic of the thread.

Let me quote Einstein, whom you also seem to be quite fond of:

(a theory to be): “as simple as possible, but not simpler”

that said, I´ll try to argue, why, in my opinion, the beach-ward speed of the wave makes a difference for the forces in surfing (trimmed and down the line, as you wish to discuss).

If the wave-progress is rather fast, the board will be depressing the oncoming water-surface very little to generate a weight/lift balance, with the result of little water flowing over the rear deck of the board.

But this seems to be different in slower everyday-surf, as the rear end of the board sinks into the water, is affected by the water-flow around, getting sucked into the crest, until almost sticking out the backside of the wave. In this (trimmed) position there is a balance of downward-forces generated by downward-movement of water behind the crest (and the bernoulliforce, if you wish) and the weight of the surfer, who stands further to the front of the board, and the upward-lift in the planing middle of the board. So this different set of forces could, in my view, be at work the better part of the year, at least, where I live (sigh!).

An extreme example to give a picture is the nose ride. But I believe this is a considerable part of the surfing-forces most of the time.

Anyway late drops , cutbacks and other moves in which the momentum of the surfer´s mass and buoyancy have their part are to me something I´d rather not neglect in this discussion, since they are most important issues for the performance of your personal board-design. So there´s a chance to have a practical use from sorting out the forces involved, as I tried to describe them in my post above.

Cheers - Detlef!

PS: I´d like to add that this thread had a lot of interesting viewpoints from personal experience to offer, especially in the beginning, and i´d really like this mode of discussion to somehow replace harsh words in misunderstanding over terminology.

If you try to dig someones personal experience in the water, as he/she tries to share, what should be wrong with that in the first place.

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Firstly,

my simplification of the problem is only to get the fundamentals straight and then we can add complexity later. At present we don;t seem to be able to agree on how ‘down the line surfing’ works, so talking about cutbacks seems a little premature. Each phase of surfing requires separate analysis (paddling, dropping, trimming, turning, airs etc.)and this is why board design is difficult. Where’s the optimum? Where’s the ‘paddles like a longboard, goes like a shortboard’ optimum? Where’s the board for the people, the Model T-board?

secondly

Quote:

If the wave-progress is rather fast, the board will be depressing the oncoming water-surface very little to generate a weight/lift balance, with the result of little water flowing over the rear deck of the board.

I’m not sure i understand what you mean, but a longer period , fast moving swell will have more ‘flow’ up the face and may allow you to go faster and plane easier than on a smaller, short period, slow moving wave, where your speed may be slow enough that you’re dragging the tail, not fully planing and relying, at least partially, on the buoyancy force to keep you above the water surface.

so if the surfer is moving towards the beach at the same speed as the wave, (back to relative velocity again) then the wave is not accelerating him towards the beach. So with the assumption that he is in trim, the velocity of the wave can be neglected as it does not provide a force towards the beach (at least until you start moving to different parts of the wave, the lip etc.)

nose riding is a departure from my ‘simplification’ and deserves a whole thread of its own.

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

part 1 of 4

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I just watched the videos you linked, and enjoyed them for their soul. I don´t know about the date of their making, but the old lift-theory mentioned has been corrected somewhat recently (see “incorrect lift theory” on the nasa-page, I think). But this doesn´t make much difference in the argument in the video.

Now what I tried to say, was that, even though theoretically the speed of the surfer on a wave breaking infinitely close to parallel could be infinitely fast, drag ignored, there is a limit to maximum speed in the everyday surf of, say, a slow left-break. The progression of the left-break is much faster than the wave, but still dependent on the wave-speed itself.

Simply put, in my view there are slow conditions under which, seemingly adversary to your opinion, the “physics” alter somewhat in the direction of nose riding. This might be (but I´m not sure) the reason, why some people here don´t want to exclude the “flow” in this matter.

To be more precise, in addition to the arguments in the video, I figure the downward forces on the tail are also generated by two associated effects in the slower ride:

  1. The water, being pushed aside and upwards by the middle-part of the board, washes back down on the tail in the passage.

2.The wave is altered to a sort of little breakdown after the middle section has passed, and thus the tail gets even more flow from the higher “sidewaterlevels” .

So we seem to picture the slower ride slightly different, me suggesting that there is a little noserider-component in the forces, and you saying that there isn´t anything like that going on, if I´ve got you right.

As to the fundamental process in fast rides I think there is a consensus in the thread, and only differences in terminology, but that´s just my guess.

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I’m not talking about slower rides.

It complicates my simplified summary of the forces and i don’t like complication ;o) This type of surfing requires a separate analysis of its own which would include buoyancy and flow over the deck of the board. - messy.

Perhaps another assumption I could throw in to help strengthen my ‘trimming down the line’ analysis, is that the surfer/board maybe considered small with respect to the wave. Now don’t all run for the pitch forks. This is a common approach to hydrodynamic modelling and if you read the thread

‘Dynamics - surfing the force’

http://www.swaylocks.com/forum/gforum.cgi?do=post_view_flat;post=277120;page=1;sb=post_latest_reply;so=ASC;mh=25;

I think it will at least make the diagrams of board on wave easier to get your head around. What this assumption is trying to do is to make it apparent that the board is in one particular part of the wave rather than spanning a slice of the wave where there would be different water particle velocities - more mess. Of course in reality it is doing this but can we deal with this complication yet? I say no.

PS: the quoted thread is well worth printing and reading as it really precedes this one and offers a much more structured insight into the the ‘forces of surfing’

Again some of you may think, ‘why is he trying to simplify everything and neglect things’ (and you’ll probably try to recomplicate what i have simplified), but it’s common to solve a problem by first simplifying it (within reason) to gain basic understanding and then later develop it further into a more complete model of ‘how the thing works’, which includes the predominant forces.

We will always neglect forces in the analysis but include those that we think have the greatest effect. For example gravitation pull of the moon and sun, Coriolis force and perhaps even aerodynamic drag due to a heavy wind. If we think some of these force are important then we’ll add then later.

and finally

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As to the fundamental process in fast rides I think there is a consensus in the thread, and only differences in terminology, but that�s just my guess.

I think you are right here, which is why i suggested that people reading this thread understand the concept of ‘lift’. This is very important, and as I am a ‘flav’ I must say that it holds equal importance with gravity.

In short gravity pulls us down, lift force is mostly up but has a horizontal component that we can direct to drive us along the wave. And then drag continually tries to slow us down. Easy!

now where did i put my helmet?

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Quote:

In short gravity pulls us down, lift force is mostly up but has a horizontal component that we can direct to drive us along the wave. And then drag continually tries to slow us down. Easy!

I’m still wondering if the reason you are looking for a horizontal component that would allow us to surf down the line is because you think of the wave as moving towards shore and so are puzzled by how we can surf down the line. But the wave does not move straight towards shore. It is coming in at an angle to the sea floor and breaks down the line “horizontally”, from right to left or left to right. When we surf down the line in trim, we are simply with or right in front of the spot that the wave is pushing up to its peak height. So all we need is that wave pulse lifting us up and gravity pulling us back down.

There are other things going on, sure, but I don’t know how anyone can believe that gravity isn’t the main force that makes surfboards move over the water.

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There are other things going on, sure, but I don't know how anyone can believe that gravity isn't the main force that makes surfboards move over the water.

I will concede to this slightly.

Gravity is always accelerating us directly downwards, into the earth, so we need some other force to drive us down the line, assuming we’re maintain our vertical height. If we didn;t have lift we would simply slide down the wave into the flats.

So you can look at the forces like this:

Gravity is the ‘main’ force

Lift forces on the board provide direction

Drag slows us down

Regarding the ‘main’ force, we need to start drawing force vector diagrams and for that we need KCasey or another volunteer to take the time to do this. I don;t have the time. But basically what I am trying to say is that we need gravity and lift to surf down the line, so calling one of them the main force seems silly.

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I will concede to this slightly.

Gravity is always accelerating us directly downwards, into the earth, so we need some other force to drive us down the line, assuming we’re maintain our vertical height. If we didn;t have lift we would simply slide down the wave into the flats.

Although the only forces acting on my yardstick were gravity pulling it down and my finger lifting it up like the wave does a surfboard, it still moved at what appeared to be a “down the line” direction. I still think you are viewing the down the line direction to be somehow different than the direction the wave is traveling. But, by definition, “trim” is following the exact direction the wave is traveling. That vertical lifting force of the wave is also moving towards shore at a certain angle. Down the line surfing follows that path. No?

So far, the only critique of my demonstration has been (via pm) the statement “mechanics and fluid mechanics are not the same thing.” Different sure, but in this case it might be a difference without a distinction. (to turn a phrase upside down).

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So far, the only critique of my demonstration has been (via pm) the statement "mechanics and fluid mechanics are not the same thing." Different sure, but in this case it might be a difference without a distinction. (to turn a phrase upside down).

I think the main problem here goes back to language or semantics, you say ‘vertical lifting force’, and I assume you are talking about water lifting the board through the application of buoyancy force.

However, I, and others, are talking about hydrodynamic lift force generated from water velocity relative to the rails and fins.

These are two completely different forces which is why i keep banging on about ‘lift’. Your force could be construed as a mechanical forces (as ‘lift’ in your yardstick analogy) whereas mine is a fluid mechanical force.

Now with out beating eachother with hangbags perhaps we can clear this up. Do you know what hydrodynamic lift is and do you understand the direction in which it is acting?

(hint: it has a ‘down the line’ horizontal component to it, aswell as a vertical component which makes the board plane.)

again in the thread ‘dynamics - surfing the force’ this is explained by KCasey.

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I think the main problem here goes back to language or semantics, you say 'vertical lifting force', and I assume you are talking about water lifting the board through the application of buoyancy force.

I am merely talking about the force that causes the water to well up above sea level. The surfboard is on top (more or less) of that water, and so goes along for the ride. Sure, call it mechanical lift.

Quote:
However, I, and others, are talking about hydrodynamic lift force generated from water velocity relative to the rails and fins.

Sure. We all know this force exists. Some of us just can’t accept a description of surfing which says that it’s the water flowing under the board that makes it move, when it is clearly “falling” that makes surfboards move with the wave.

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How does a ‘‘FALLING’’ board, boost air? Oooops!

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How does ski-jumping work?

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Sure. We all know this force exists. Some of us just can't accept a description of surfing which says that it's the water flowing under the board that makes it move, when it is clearly "falling" that makes surfboards move with the wave.

well this is where the problem lies then.

I’ll have another quick go…

So the water is pulling the board up the face and gravity is pulling it down as you say. but because the board and fins are at an angle to this flow (remember that i’m talking about relative velocity of the water - the board is moving too - in fact its moving at an obtuse angle to this flow). Like KCaseys ‘protesters and police with hoses’ analogy in ‘Dynamics - surfing the force’. (I don’t get paid for advertising the other thread I swear)

So we generate lift from the rails and fins, not just lift for planing but lift acting in the direction of our motion and thus driving us down the line.

pretty much the same way wind flowing over a sail drives a

yacht forward when its sailing upwind or closehauled.

Please don’t read this last bit if you;re confused or know nothing about the ‘forces in sailing’. Its a work in progress.

…and to complete the sailing analogy, the keel or centreboard provides the same ‘reaction’ force (in this case due to hydrodynamic lift acting through the centre of lateral resistance) which gravity provides in surfing. If it wasnlt for gravity we would get pulled over the crest, and if it wasnb;t for the keel the yacht would go sideways.

Can’t wait to get in the water tomorrow, I’m all talk and no action at the minute and that makes Al a very dull boy.

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How does a ''FALLING'' board, boost air? Oooops!

How does a skateboarder catch air when he drops in on one side of the halfpipe and then blasts out the other side? Even at my age I can do it even when the side I drop in on is 6’ high and the side I’m airing out of is 8’ high. In other words, the speed gained by falling down one 6’ side is sufficient to take me back up to at least 10’ (with a little help from my muscles via weighting/unweighting - the same thing we do when we carve turns while surfing).