Forces in Surfing

Mr Van has brought up a very important point–relative speed. It is relative to planing, control inputs, and as planing is what you are doing over the moving face, I see his point. Gravity is always a prime mover, especially when accelerating, but flow giving you more relative planing speed when in trim increases progress down the line–it would do to watch some standing wave river surfing on this point.

Lee - I was answering the questions in order.

Mostly, I’m changing my plea to ignorance, at least until I get to pick Bill T.'s brain, as I’ve based my designs simply off the balance between speed and control, and the way pressure differences may equate to force.

In the end I’m more than anything with myself: Just try to learn what works for me, and maybe try to understand why it works for me.

Thanks for playing. I’m sure I’ll find myself drawn in again.

Hi Lee,

Yep, I know exactly what you mean…I shouldn’ta dunnit, I shouldn’ta dunnit…

You’re exactly right with the ‘relativity’ angle, kinda like how, say, you feel lighter or heavier in an elevator.

Okay,now, when you’re paddling for a wave and actually catching it, there’s two things happening.

The first is, you’re not on the flat any more. Instead, you’re heading down an inclined concave surface, the wave face. Not free-falling, but you are accellerated some by gravity - think ‘skateboard on a hill’ and you have it -

Lets see that graphicly:

Think about when you catch a wave. You’re paddling towards shore, and paddling through the water and through the water as the wave catches up to you until the wave is inclined enough so that ‘fwoosh’, you catch the wave. You don’t catch it on the flat out in front of the wave, you catch it on the face. Or, think about ‘one-paddle-takeoffs’: you gotta do it when you’re in a very steep part of the wave. .

The other thing is, you are falling to some extent. Accellerating towards the earth’s core. So, like you would on an elevator, you weigh less. Not a lot less, but less. So the weight that the planing area has to support is diminished a little and in turn it takes less force to lift the board to a plane.

Okay, in trim -

Can we agree that this guy is in trim? That is, he’s gonna proceed along the wave at pretty much the same height on said wave and his speed will remain pretty much constant?

Okay, his path is partly downwards, still. But his speed is staying the same- he is neither accellerating nor declerating. So the forces involved are:

Accelleration force F is equal in magnitude but in the opposite direction to the drag: trim position. Turn right a little and take a path where F< drag, you slow down, turn left a little ( F > drag ) and you speed up. .

that help any?

doc…

Here’s a bunch of thoughts:

Your points, doc, are all true, but the wave as it propagates toward shore–never mind the net zeroing out–you only ride the face, the upward half of the water particles’ total motion in a wave.

So as you ride in relation to the crest, you might at any moment turn toward the crest and that upwelling would help carry you toward the crest and over it.

If it’s true that that propagation force is doing work when you do that, it’s true that it’s doing work when you are in trim, and as you surf.

Such that Lee’s point, and all the other old guys’, would hold. Relative speed of your board (compared to a static hill) over the water surface would be seem to increased if you point straight down, or any degrees less than 90 to the flow.

The wave is propagating, so there is an upwelling of the wave. That upwelling is vertical movement. If you stay ahead of the crest, you are riding that upwelling face, not just a static version of it.

All that said, increased relative speed means increased drag, and resistance increases with the square of speed, so that can be our next discussion. Whether we net out any benefit except in short bursts of increased planing. Which makes me think of pumping shortboarders.

Going surfing now.

The preceding was fueled by coffee.

and I told myself I wasn’t going to get sucked into this topic… although important, I think gravity is not the “prime mover” force in surfing. I think it’s the amount of wave energy in the wave that matters - things tied back to the original fetch, and the wave period and size that result. Otherwise, we could surf a crappy 3 foot mushball wave the same way we could surf a perfect 3 foot barrel - and that just isn’t true. They both have the same amount of gravity “effect” - ie. a vertical drop of 3 feet. But the potential to translate wave energy into surf speed, with enough energy to burn off in cutbacks (or launch vertically) is vastly different.

(To my way of thinking whether that useful wave energy comes from “water moving forward” or “water moving up” is form over substance, as it is moving in many directions at different points in time).

Oh yeah, I love watching dolphins surf (underwater), puts the whole “we must be planing” thing into a different perspective for me.

Doc, with the pipe analogy, when the wave hits the bottom, the circular flow is broken and there remains only the back section of the arc of the circle in the water. The arc eventually moves forward and becomes the face. The face is rising so there is a sensation of movement up the face. I have only had three cups of coffee so what am I missing?

Good to see the old janklow back, hope you got a few waves today…

And on that last comment about pumping shortboarders, if you look at slow-mo vid of pumping in small waves, you’ll

see the whole board almost come out of the water on the un-weight, so that water resistance is almost eliminated

as the board is stomped back down, combining gravity and the extension of the rider’s legs to increase board velocity.

Drive the board through another turn, un-weight at the end, and start all over.

Lots of good input on this thread and no arguments. Good on all o’ ya.

This important point seems to be conveniently forgotten in the classical scientific, model diagramm way of explaining things…thank-you for bringing to the table what we have all experienced and know empirically.

LeeV’s point about relative and apparent water movement also brings important illumination to this subject which I believe is the “great Unknown” which when fully understood will allow huge advancments in surfboard design…ie what actually propels a surfboard along…

Of course it’s gravity but anyone who has bottom-turned at the base of a sucking square reef wave knows there is more to it.

I’m glad you threw in, Keith–gravity is the prime mover in surfing for sure–look at skateboarding down a hill–the difference is not that huge – one of the biggest differences between the 8 second period slopes I was surfing today, and a 13 second period barrel, it seems to me, is the latter one has vertical and over-vertical face to ride, and steeper slope everywhere. For sure, what I was lacking today for the most part were those bits–got a few good late drops on some mini-pitchers though–was going fast by the time I got the bottom, I tell you what. Longer period is good and such a wave pulse has more speed and energy and mass when shoaling and so you get waves that have enough energy to pump up into steep faces

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and I told myself I wasn't going to get sucked into this topic... although important, I think gravity is not the "prime mover" force in surfing. I think it's the amount of wave energy in the wave that matters - things tied back to the original fetch, and the wave period and size that result. Otherwise, we could surf a crappy 3 foot mushball wave the same way we could surf a perfect 3 foot barrel - and that just isn't true. They both have the same amount of gravity "effect" - ie. a vertical drop of 3 feet. But the potential to translate wave energy into surf speed, with enough energy to burn off in cutbacks (or launch vertically) is vastly different.

I figure the difference is in how the two different waves lift the surfer back up. One is much steeper and has more energy and is moving faster towards the shore and thus rising up in front of the surfer faster than the mushball wave.

But it’s still all about gravity - without it, no surfing would take place of course. I think the story is that the wave energy is lifting the surfboard up and gravity is pulling it down the wave face. In trim the board is in balance and you just move down the line as the wave rises up there.

Once you do understand the forces involved in surfing (wouldn’t really have a clue myself) you then would want to incorporate this knowledge into your next shape correct? Which is why I think understanding the forces involved is only half the battle. Would probably make for a great phsyics doctorate. Would also be great fun too probably on a limitless budget. But I would certainly agree on gravity and drag.

Interesting conversation…I really appreciate the input in relation to relevance. I have always thought about waves and energy and movement this way in my head, but had a hard tme explaining my thoughts (like to OBProud after a couple Belgian Strong Ales…). It is all relevant, especially to your perspective as to what is moving (i.e. water) and where it is going, or if it is even moving at all.

In my head I see a tennis ball bobbing outside the surfline…as swell passes under the ball, it moves up and down, and forward and backward in relation to the circular movement of the water molecules it is sitting in. The water molecules are in fact moving, but only in that limited eliptical pattern being imparted to those molecules by the energy of the wave passing through/beneath…the tennis ball IS moving, but it isn’t going anywhere.

As the ball moves closer to the surfline, by say the action of a slight onshore wind or current upon it…things change…and things stay the same. A steeper cresting wave will act differently upon the ball…it will rise further as the wave begins to crest, but then drop back down the back of the wave and not really be propelled towards shore…agian moving yes, but not really going anywhere…but it WILL be moving inshore slightly now due to the different pattern of eliptical water movement as the bottom acts upon the wave…creating a steeper vertical elipse that drops the ball a little farther inshore than the simple swell did outside the surf line.

A breaking wave will act differently again upon the tennis ball…lifting the ball up even further, and then capturing the ball in the projecting lip and moving it inshore at an even more accelerated clip than the cresting wave did…now gravity has entered the picture, and along with the viscosity of the water, has trapped the tennis ball and propelled it on its merry way back to my Viszlas’ waiting mouth on the beach, where a lifetime of cat poop tainted slobber and unmercifull chewing awaits…

What…what were we talking about…hello? Did any of that even make sense to anyone but me…cause I understand perfectly!

Without gravity there’d be no surfing… no, we’d all have drifted off into outer space. The thing is that gravity is always there, while the wave energy is NOT always there. But for the wave’s energy we couldn’t surf. I don’t think it’s just “upward” wave energy (or motion) though, it is also “beachward” that I think is important. In rough seas its hard to surf because the swell, even if vertically large, is moving many directions. On the other hand a very small wave can be surfed if it is moving the right direction, over a bottom shallow enough. And even on a big wave, what’s easier to surf, the front of the wave, or the back? While it’s possible to coast off the back of a wave, you don’t go very far, for very long.

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And even on a big wave, what's easier to surf, the front of the wave, or the back? While it's possible to coast off the back of a wave, you don't go very far, for very long.

Of course not: Because once you hit the bottom, the wave doesn’t pick you up again. If you’re not picked up, you can’t fall back down again…

The place at which the wave energy is going vertical is progressively closer and closer to shore as the wave comes in. Surfers stick to that spot and so I guess you could look at it as the wave “pushing” the surfer towards shore: but the force isn’t horizontal towards shore - it’s just lifting the surfer up, where gravity lets him fall back down, and then repeat again. Or at least that’s what it feels like to me.

This is what I think happens.

dot(A) starts behind the plane at which the wave breaks, it mostly moves up and toward shore.

dot(B) starts close to the plane at which the wave breaks, it move back and up, straight up and then forward a bit.

dot(C) starts in front of the plane at which the wave breaks, and it moves away from shore, then up and forward.

and in all of that, the wave energy is moving, left to right… can’t ignore the one, regardless of the other.

Yeah, but ripples do that too.

Exactly - you can’t surf a ripple, because there is not enough energy in it. My point is this… picture a bowling ball on the ground next to a U shaped ramp. Gravity exists, right? The ball is not flying up? Now, pick up the bowling ball and put it on the ramp. You’ve just added energy to the system, potential energy. Put it on the ramp and let it go, it will roll down the ramp and then fly up in the air. What caused it to go back up in the air? As far as I know, physics would not say “gravity” but “stored energy”. Yes, gravity caused the ball to move down, but only because it had stored energy to start with.

Waves are similar, they are energy. What causes us to surf is not gravity, it is the energy in waves. That energy may interact with gravity, but without the wave energy, we wouldn’t be going anywhere. It’s a “but for” thing.

The stored energy is gravitational energy. You can’t ride the crest for very long.

No, the stored energy is originally solar energy, resulting in thermal differences which cause wind; thus the energy is delivered via wind over long periods of time and distance to the water.

The only gravitational energy in the ocean is tidal, and it is too dispersed over time to surf on.