Forces in Surfing

General Discussion
241

It is flow relative to the surfboard. The water is moving relative to the surfboard.

Key word: relative.

I have been casualy reading this thread in amazement. You guys cannot even agree on a reference point!

You cannot do dynamic type physics without a reference, at least not in this case.

When you guys agree to the proper reference you may agree more than you have been (altho I dont think a final agreement on this topic is possible…like trying to agree on history’s greatest rock band)

Here are your choices:

a. the moving board/rider

b. the moving swell

c. the relatively still water

d. the still earth

With a valid simplifying assumption, you can combine two of the four and have a reference point that will allow you guys to agree more, but of course, not 100% agreement.

At minimum, only when ya’ll agree on a reference, will this discussion go anywhere productive…or ya’ll can continue to go in circles as you have for the last two weeks.

Everything is relative…Einstein-esque.

242

By that line of argument, you could say the road is flowing relative to my car, or my skateboard, or the snow sitting on a mountain is “flowing” relative to my snowboard.

If we’re talking about an alleged current flowing up the face, let’s get to it.

If you’re trying to call your planing speed something that the water on the face is doing, that’s just relative perception being sold as physcial fact, and any intrument-rated pilot will tell you that the way things feel to you in the cockpit is not the physical facts of your situation relative to the airmass or landmass.

If it’s just the slope progressing forward toward the beach, Rich and I covered it, but here’s another attempt to illustrate:

You plane down a face using gravitational acceleration, you bottom-turn, and now you’re planing up a face using some of the speed you built in on the way down with gravity, combined with the speed that the crest is moving toward you, and the slope is gaining vertical height as the crest moves toward you.

Someone I like and respect very very much once said he thought maybe KCasey’s “flow” and my “renewing grade” were perhaps the same thing, but if you look at that statement, while it has a neat effect, it’s imparting a flexibility to terminology that is more appeasement than anything else.

Let’s get to this real argument, which I now see is what I thought it was all along, but there have been attempts to avoid calling it what it is, and this is what it is: I argue the other guys are really trying to say there is a current flowing up the face, while avoiding saying that. And if they’re not saying that, they’re really talking about renewing grade.

243
Quote:
It is flow relative to the surfboard. The water is moving relative to the surfboard.

If we’re talking about surfboard movement, I don’t know why anyone would prefer to picture the surfboard as fixed and the water moving beneath it instead of the other way around.

I also think these guys actually believe there is a strong river of water on the surface moving out towards sea as waves come in. And then they picture that as a headwind, and the surfboard as a sail boat, sailing upwind. That actually think that it is this “headwind” of water that makes the surfboard move down the line and towards shore!

Of course sailboats wouldn’t need sails or a keels if they had a wave pushing it up and down the line towards shore like we surfers do…

244

You get the clearest picture when you can understand what is happening from all reference points.

IMHO, the most important, from a design perspective, is understanding what is happening relative to the board/rider.

Lots of people seem to think that all of the movement of the water relative to the rider comes from the board moving over the water, but this is not the whole story. The water is also moving up the wave surface.

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What he said!

246

well as I have not read any but the last four posta on this extensive thread I feel

taht I must contribute to this relativity resolution.

today after a stellar slow

moving excercise

with three friends

I see clearly .

the movement is primarily

relative to the mass of the rider.

the board is also a medium to the energy of the wave.

the weight of the rider is able and liable to exert far to many influences to deny.

the base line is the mass/weight of the rider…

the physio-centric center of the universe is…me.

and or you,on your day off

or when you are at work

if you are self employed

or the C.E.O.

…ambrose…

please consider this the comic relief

happy june 1st.

247

Care to tell us about how the ramp effect, and surface only carpet effect, can explain how Laird Hamiltons hydrofoil ski can manage to ride open ocean, non shoaling swells? He is not riding the bottom of the swell, but at, or very near the crest. And he is not on the surface, but about 3 to 4 feet in the air, and the foil he is riding is one to two feet below the surface. Thick carpet maybe? Please go back to the basic movement of water molecules in a wave, for the insight to the answer to the question. Your model seems to require that gravity is turned on going down the wave, (agree) and is turned off when the rider turns up the face of the wave. (don’t agree) You need to work on that.

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Quote:
The water is also moving up the wave surface.

But this is not what makes the surfboard surf. A headwind, alone, is what makes a sailboat move upwind. Therefore it is not analogous to surfing and they should abandon trying to think of surfing in the same way.

Also, when you say the water is moving “up the wave surface” do you picture it like the flowrider hose, shooting water towards the “crest” of that wave? I.e. moving basically horizontally at the base and then following the slope of the wave up to the top?

I think of this water being pushed up vertically, not moving horizontally. That’s because on a typical wave when you watch a swell go by, there is no net horizontal movement of the water at all. It just goes up, and then it comes back down. If you ignore the movement of the wave, it will look like that water must have moved out to sea to get to the top of the wave. But it doesn’t. The wave moves towards it.

Yes, i’m not accurately describing Pipeline or Teahupoo. But I am describing what most people experience and see when they go to the beach and ride waves. Since people ride waves without a big hollow face with water being sucked up and projected out into a tube over them, then clearly those forces just aren’t necessary for surfing to take place. So we’re back to a continual slope lifting up behind us and gravity pulling us down it.

249
Quote:

By that line of argument, you could say the road is flowing relative to my car, or my skateboard, or the snow sitting on a mountain is “flowing” relative to my snowboard.

If we’re talking about an alleged current flowing up the face, let’s get to it.

When talking about flow I don’t think that it matters if the fluid is moving over the object or if the object is moving through the fluid. What is important is that the fluid is moving relative to the object. Creating lift, drag, turbulence, etc.

You are right that surfing is like skateboarding in that the water is moving relative to the surfboard just as the ground is moving relative to the skateboard. But unlike in skateboarding, where the ground or ramp is static, in surfing the water is moving.

The water is moving in different directions on different parts of the wave. At the crest of a breaking wave most of the movement is forward. At the center of breaking wave most of the water is moving up, as MaraboutSlim describes, and at the bottom the water is beginning to move up, as MaraboutSlim describes, but it is also being pulled back toward the wave by the water that has already moved up.

It is almost like you have a skateboard ramp with a moving surface, like an escalator (or like the roller under the carpet analogy). There is not doubt that the movement of the surface would impart energy on the rider.

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Care to tell us about how the ramp effect, and surface only carpet effect, can explain how Laird Hamiltons hydrofoil ski can manage to ride open ocean, non shoaling swells? He is not riding the bottom of the swell, but at, or very near the crest.

the pulse of the “ramp” lifting up is where he places his foil and that is trying to push him up. meanwhile gravity is trying to pull him back down. the “ramp” moves and he stays with it. The same thing people do when they noseride, basically.

Quote:
Please go back to the basic movement of water molecules in a wave, for the insight to the answer to the question. Your model seems to require that gravity is turned on going down the wave, (agree) and is turned off when the rider turns up the face of the wave. (don't agree) You need to work on that.

When they say that water molecules move in a circular fashion, they mean each individual molecule. I think you are making assumptions about larger water movement from that molecular level movement.

Our “model” doesn’t require gravity to be turned off to get back to the top of the wave. The welling up of the wave “ramp” itself is of course powerful enough to lift the surfer to the crest again. If we were just floating there on the surface as the wave approached, we would go up with the wave and back down again without moving horizontally at all. Why you are puzzled by our ability to get back up to the top of the wave when riding it?

251

The argument doesn’t exclude gravity at any point. You injected that.

Laird Hamilton or anyone riding a slope is riding a slope. You would find, if you could see his foil element, that it is inclined downward when he is riding downward. And otherwise when he is riding other directions. You can ride a moving slope in several directions.

All due respect, I’m not sure what you find troubling to the point of being so condescending and abrasive, other than people younger than you telling you something you disagree with. You’ll have to work on that, and I’ll have to work on whatever I have to work on. But I digress, perhaps.

The physics of this are kind of fun to talk about, but the irritating thing to me is people trying to tell me, with such high-horse abrasiveness especially, that something totally explainable by obvious universal causes that translate to other boardriding sports with a high degree of integrity is actually due to something that is completely unnecessary in those sports, not to mention the current you allege flying in the face (heh) of the wave crest itself.

obproud, as far as design goes, sure, relate the board to the flows it sees. But you’ll have the dickens of a time trying to prove this up-face current to yourself for those purposes. Better you just say the board is moving over the water in the directions you understand it to be moving and go with that.

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If we’re talking about surfboard movement, I don’t know why anyone would prefer to picture the surfboard as fixed and the water moving beneath it instead of the other way around.

If you are designing surfboards, and the surfboard is the variable that you can change (you cannot change the waves) why would you think of it any other way?

It seems like a surfboard designer would want to think of how the water is moving relative to the board.

Quote:

I also think these guys actually believe there is a strong river of water on the surface moving out towards sea as waves come in. And then they picture that as a headwind, and the surfboard as a sail boat, sailing upwind. That actually think that it is this “headwind” of water that makes the surfboard move down the line and towards shore!

Of course sailboats wouldn’t need sails or a keels if they had a wave pushing it up and down the line towards shore like we surfers do…

I think you are confusing flow with stream. I don’t think that anyone thinks this (I hope not).

253

if you scan back up the topic you will find reference to sailboats upwind and also riding the river of water up the wave and whatnot.

254
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Quote:

The water is also moving up the wave surface.

But this is not what makes the surfboard surf. A headwind, alone, is what makes a sailboat move upwind. Therefore it is not analogous to surfing and they should abandon trying to think of surfing in the same way.

I agree that it is not what makes surfboards surf. It is only part of it. Neither necessary or sufficient, but definitely significant.

Quote:

Also, when you say the water is moving “up the wave surface” do you picture it like the flowrider hose, shooting water towards the “crest” of that wave? I.e. moving basically horizontally at the base and then following the slope of the wave up to the top?

No, that is not what I mean. I misspoke if I said surface, because all of the water in the wave is moving up (until it is breaking).

Quote:

I think of this water being pushed up vertically, not moving horizontally. That’s because on a typical wave when you watch a swell go by, there is no net horizontal movement of the water at all. It just goes up, and then it comes back down. If you ignore the movement of the wave, it will look like that water must have moved out to sea to get to the top of the wave. But it doesn’t. The wave moves towards it.

There may not be net horizontal movement, but there is movement. And there is more movement as the wave begins to break.

Quote:

Yes, i’m not accurately describing Pipeline or Teahupoo. But I am describing what most people experience and see when they go to the beach and ride waves. Since people ride waves without a big hollow face with water being sucked up and projected out into a tube over them, then clearly those forces just aren’t necessary for surfing to take place. So we’re back to a continual slope lifting up behind us and gravity pulling us down it.

I agree that on smaller weaker waves there is less water movement.

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That waveform is pushing them, as they work to maintain their position relative to the waveform, in order to keep getting pushed. And we do that too, but we slide down the front. Same but different.

257

There´s certainly no gravitation involved while being completely under water. Hard to tell their exact position in relation to the crest, but i´d say they are cruising with their bellies on slightly calmer,deeper water, and are propelled from the down-wash on their rear fins a little behind the crest Nice thing to get the wrinkles on everybody´s forehead for a minute (certainly mine - HA!)

Cheers - Detlef

258

what you have to do is sacrifice a virgin (in this case greg) to the great surf god huey

then huey farts and blows waves to where you are sitting

259

also vibe

you need to have goodvibes is esential ingredient

260

You mean gravity isn’t involved in the dolphins riding in the wave? Gravity must act through water, but in this case it’s not that material to the ‘ride’.

Maybe the Dolphins are riding more directly the energy of the wave than we would standing on a board on top of the water. Although they must still overcome drag and generate lift with their fins (and body?) to ride the wave.

So is part of it how you harness the energy in a wave in relation to the other forces? Are there any analogies with drafting in cycling? Or wind in a sail?