Fins are not required nor are they critical for surfing. (That’s not to say they haven’t added lot to the sport.)
Wave machines like those on cruise ships operate under the same principles that I have described, but here the wall of water is not decelerating, etc. see “The Decelerating Wave-Form” (and to my knowledge the boards generally used in these wave rides don’t have fins either.)
Never said the fin was necessary, just improves tracking.
If you do not see it, I cannot offer any further conceptual insight.
Maybe Meagain can incorporate the mathematics for you. Too much for me to put together without more effort than I have to give.
If you re-read your own recent posts, what you are beginning to describe is planing.
The principal angle and direction of travel are now sideways along the x axis rather than forward along the z axis. But there is still forward movement along the z axis. There are now, overgeneralized, two angles of descent, forward and sideways.
I don’t know much, but I do know from bitter experience that if you try to tube ride anywhere on the face where the wave face approaches 90 degrees, you are on your way to a toss over the falls unless you are driving down pretty hard and have enough release designed into the board. Maybe 40 degrees wave face slope is where you trim to make deep barrels, but I’ve never bothered to drag out the protractor. A surfmat automatically takes the perfect line, and it ain’t half way up the face.
Of course, if you can sit atop the foam ball then everything changes because the foam ball is being squeezed out the cylinder and carrying you with it.
KC persist with your model. As the wise saying* goes,
“All models are wrong, some are useful.”
*George Box, who understood mathematics, but, like Bo, didn’t surf.
It’s the motion along the wave-face that counts. Exactly which way your board is pointing or its orientation in general, will depend on a lot of other factors.
Also, the line of white water that’s you see traveling after a wave has broken, is not the same water or the same “foam” moving towards, say the shore. The foam is continuously generated by the upper layers of the wave-form moving past the lower layers as the lower layers continue to decelerate. Unless there’s an awful lot it, and on bigger waves there can be, when you ride the white-water to the beach you are actually planing on the remaining wave-form underneath all that foam. (That’s why you appear to sink back into the foam on smaller to mid-sized lines of foam.) You can however be moved by the ‘foam’ if there’s enough of it and there’s enough of you to be moved by it. If the foam ball or line of white water hits a sufficient deeper region (deeper as in water depth) what remains of the wave reappears, and the foaming stops. Its obvious why people might say “ride the foam ball in” or “ride the white water in”, but they are rarely doing so - they are surfing in that they are riding the wave-form underneath the foam.
As mentioned if the white water is massive enough, it can exert sufficient pressure to literally push an object. This is relative. For example, seeing beach balls etc. being seeming push by the foam is pretty commonplace; can also be true for surferboards, usually without surfers on them. Also, surfers, on or off their surfboards, under big conditions can be bounced around quite a bit by big white water.
For me the best analogy so far ( I prefer them to graphs charts or formulas) has been the watermelon seed being squeezed between two fingers…
Surfing wouldn’t be possible without both gravity and wave energy forces being present,
So if you are looking for a one word answer to the question of what makes a surfboard go the only correct answer would be PHYSICS…
But on the philosophical question of what is the “primary” force,IMO gravity wins, and for this reason…
EVERYTHING in this situation is being effected by gravity…
Even the shape of the face of the wave is a product of gravity pulling (pushing) the water down ( it would be interesting to see what would happen to a wave if gravity suddenly stopped working,what shape might it take as it floated off into space?)
And everything in this situation (except the surfer) is trying to find equalibrium …
But for me the clincher is this,
Even the way that we describe a wave,(the angle of the face is 45 degrees or 90 degrees) we are describing it relative to what??
Relative to level right?,which is a function of GRAVITY…
It is really just semantics though…
Gravity is not only constant and inescapable but is also a limiting factor…
That is why I use that as my primary force or reference point when I am trying to visualize the forces at work when designing my foil …
Stoneburner’s analogy of the mountainface moving up as the skiier tries to slide down is also pretty close to the way I see it ( which also sounds a lot like Roy’s conveyor belt analogy except for the voodoo part)…
I am tying to create something that has the best glide ratio possible, I think it is the same challenge for conventional surfboards also…
Below are two figures.
The first is a board in the path of a jet of water, and lets say you are holding the board in place (your hand is not drawn). The board’s major plane is perpendicular to the direction of the flow from the jet. The force generated by the jet on the board is also in the same direction as the flow from the jet.
The second is a board at some different orientation to the jet’s direction, and you are still holding the board in place (but now at angle). The force on the board is still modelled as perpendicular to the major plane of the board, but now the force on the board is at an angle to the jet’s direction. Here, slightly off to the right, and forward.
This is the essence of planing, but you wouldn’t call it planing, in fact I know of no special name that’s given to this kind of situation (actually in the first case, the term “stagnation pressure” is often applied.) The term ‘planing’ has its origin in boat design, but its the same phenomena - an exchange of momentum between the flow and the object.
In the first case, you would sense, given the force you would have to use to hold the board in place, that the force is in the direction as shown - the board just wants to go ‘upward’, and you would counter balance that force by applying a downward force to hold it in place. In the second example, the board wants to move upward but also off the right and forward, and you would counter balance that force by applying a force which is downward and slightly off to the left and back to hold it in place.
But say in the second case all you did was provide a downward force to stop the board from moving upward, then the board and your hand would be moved off to the right and forward.
Now think of a situation where there is a stationary row of jets parallel to the picture plane, in this case, as above, you’re only preventing the board from moving upward. The board would move to the right, moving from jet to jet, but also forward, so eventually the board would move right out the row of jets. But you would like it to continue to move along the row of jets, so you adjust the angle so there’s no tendency to move forward. Now your board just moves off the right.
But say the row of jets starts to move as a whole out of the paper, to keep the board in the jet flow, you have to adjust its angle again, given it a slightly forward angle (like the original angle, but maybe more or maybe less in order to follow the movement of the line of jets.)
Preventing the board from moving upwards is similar to the major role gravity plays in surfing (see original post), the rest is just you controlling in which direction the board will move by changing its orientation. Surfing is about getting in the way of the flow, its about exchanging momentum with the flow. This, of course does not rule the possiblity of ‘sliding’ down the face occasionally (angling the board appropriately) to pick up some extra kinetic energy, see original post. But again, what makes surfing different is what I’ve described above, it allows the surfer to control the forces of planing, in particular it allows him to move transversely across the wave-face.
This is an overly simpified example, but hopefully it is enough to allow you to make the leap to Savistsky’s diagram, (Please also consider reading “The Decelerating Wave-Form” and “Rails Plane”, as the flow up the face is not a simple jet, the flow varies along the face.)
It is called hydrodynamic lift (I think).
Also, for the model… I think it is more acurate to think of matrix of jets as turning on successively and in a sequence, with increasing then decreasing pressure, rather than moving. (Note: this model is still does not completely represent the situation) Similar to how the pins move if you run your finger along the bottom of this toy:
With this model it is easy to conceptualize how the upward flow of the water can counterbalance the riders weight (when the board is in the correct orientation) AND the wave form moving forward can continue to push the rider forward and stay in trim.
Great theories: Consider this for a simplistic explanation.
Wave energy is generated over a long distance and moves through the water ( unless it is a tsunami, which is the water moving) this is evidenced in a stationary and bobbing surfer being pushed upwards and over a swell, but not being dragged forward by It. Surfers paddle into the wave by paddling to catch up to the energy, aided by the gravity induced by a breaking waves changing shape. If the surfer does not paddle hard enough or too early, they get left behind. Too late and gravity owns you. So thus gravity assists, but is not the driving force.
Once the bottom turn is complete and the surfer holds a line ( picture a trim at Bruce’s) gravitational forces are balanced by the water moving up the wave face. No turns are necessary, and as gravity is at a balance, it can’t be providing the drive. This is coming from the vector between the energy heading towards the beach and the board angled at about 45degrees, all other forces assumed to be in balance.
thanks, this seems obvious to me, but obviously it isn’t to everyone. The wave provides the main source of the energy that powers a surfboard in motion. Nothing anyone has said has convinced me otherwise. Even tho yeah, gravity holds the universe together.
I strongly feel you need to factor in the lateral movement of the curl, the place where surfing takes place, also.
if we caught a wave at the top of the face, and rode straight down to the bottom of the face, and jumped off, then yes, gravity was the main force in that scenario.
Agreed. To complicate the matter is easy. All forces need to be considered- wave energy (kinetic) and height (potential). Gravity, air pressure( drag, can largely be ignored as it is minimal), viscous friction, etc. at its simplest level the surfer harnessing the wave energy is what makes the surfboard go. Everything else mentioned in the thread is valid and fins, contours, plan shapes, foil, rail shapes are all measures to improve how we harness that energy.
Excellent MP_surf.
Though you didn’t write as much (and I don’t wish to imply you did), speaking in terms of energy tells you nothing about why a fishes have the design they do, nor why guns have the design they do. It tells you nothing about rails, bottom contours, fins, etc… But forces might. But even then not all of the forces at play stand to tell much about design either, in particular the gravitational force, which will never tell you how to design a surfboard.
The question then becomes “which forces do?”. And the major player among those forces in surfing are those that arise from the exchange of momentum between the flow and the surfboard/surfer – planing forces - as they are, especially during ‘trim’, the ‘driving force’ - it is the force that “make surfboards go”. But even when dropping down the face, when the planing forces are no longer the primary motive force, they are still critical in design -i.e. as they will determine how efficiently you can drop down the face.
It is the force which governs the amount of surface you will need under a given set of conditions for a given surfer. Its the force that tells you how to distribute the surface area -i.e. plane-shape. Its the force that governs the design of rails*, bottom contours, rocker, and flex.
But it isn’t the only force that might matter.
There’s also the wave-form itself, as it is important to understand what it is doing (see, “The Decelerating Wave-form”) as that will also have bearing on how the design elements are to be applied.
And of course, the skill of the surfer.
*, see an excellent video of McCoy speaking about rail design. But I don’t think he goes far enough, see thread “Rails Plane” - as the wave-side rail (when you’re not turning) deals with more than just one kind of flow, see thread “The Decelerating Wave-Form.”
It kinda seems to me like “what makes the surfboard go” is the elephant in the room - seldom addressed when discussing surfing, surfboard design,and surfboard performance. It isn’t necessary to any of the aforementioned, i.e. we can surf, design and build surfboards, even refine the shapes, without knowing what exactly the main force is that powers the act of surfing.
We have surfboards that work, and there are certain means and averages and givens to surfboard design around which all surfboards tend to fall. A surfboard has to plane, to hold up the surfer, that’s a given. Standards of planing vehicles, like ratios of width to length, edge shape (rails), etc, while not arrived at with surfcraft in mind, are nevertheless useful to surfboard design (I think Lindsay Lord figures into this part of the story).
The power that moves the surfboard, whatever you percieve it to be, is gonna do its job no matter what. We don’t have to identify it, or agree on what it is, to know it will power the surfboard, without fail. This frees us then to focus on the ride itself. This is probably one reason the subject is viewed with some disdain - it isn’t necessary to what we do. Most all surfboard design, to my knowledge, is based on the given that the surfboard will go, and focuses mainly on engineering the ride, not so much tapping into the forces at work. An exception might be Thrailkill’s theory of fin friction, where a design element is specifically designed to do just that. Another example might be the hydroplane surfboard.
Hydroplane surfcraft are clearly re-writing the givens of surfcraft design, but they generally resemble standard surfboards above the waterline, because of the similar requirements up to the point / speed where hydroplaning takes place. But they should make us aware of the power or the wave energy that they tap into that is separate and distinct from the interaction of water / surfboard at the waterline.
At any rate, I don’t see any harm from analyzing what actually powers the act of surfing, and maybe some good could come from it.
It is clear to me (though that doesn’t mean its correct - ha!) that the power and force of the wave energy is the prevailing force. Gravity is clearly necessary, and when we view the sloped face of the wave, and think of a surfer paddling “downhill” to catch a wave, or heading down a breaking wave face, it seems like gravity is propelling the surfer.
But the real power and energy for surfing comes from the energy of the wave, not from going downhill. Gravity keeps the surfer on the wave face (most of the time), so that the wave energy can be tapped, but the wave energy is the thing. Gravity keeps a surfer in the curl, but the moving curl provides the force and energy that moves the surfer, he doesn’t even have to be pointed or moving downhill. While gravity pulls a surfer downhill, or pulls him earthward keeping him on the wave face, the moving wave and its resultant curl pushes the surfer. When surfing, are we mainly being pulled, or pushed? I say pushed.
It seems to me that if gravity were the main force powering the act of surfing, then heavier would be better.
A feather and a bowling ball fall at the same rate in a vacuum.
Absurdly esoteric or perspective?
It is all about AoP.
Obtuse or acute.
Terse. No numbers. If it fits…
OK, I’ll give you that one. But that was an afterthought anyway, not central to my argument.
AoP has much broader context than bowling balls and feathers in the greater discussion.
I will leave it at that…
Hi guys thanks for taking the time to try to explain further…
It’s not that I don’t get the concepts that you’ve laid out,
I just think that you are giving gravity the short stick…
I believe it is employed to enhance speed more than most may realize…
Kcasey your last waterflow illustration is spot on for a surfer standing lock legged and trimmed out and also for my foil… it’s also a great visual aid…
but I think it’s just a little too simple to be accurate when it comes to what a surfer does to develop speed on a wave…
You guys have contributed some cool charts and drawings to explain your pov.
So I thought I’d try to put together something similar to add to the discussion.
https://www.youtube.com/watch?v=hZ93IkW-zNk
I’d like to hear your take on my assertion…
This thread has gotten some negative responses ,
But the cool thing for me about it is that I’ve spent the last few days trying to understand better what does make a surfboard go,
and how is that a bad thing…
Wood dave,
Great video great explanation.
Not the ONLY force…definitley a tool on tap to the surfer… that no one has bothered to consider yet.
Came to the same conclusion on this topic thinking about how a skateboarder pumps a half pipe and is able to clear his initial starting height.
Or a kid rocking a swing for that matter.
Seems like the transverse angle of the surfboard across the face would matter as much as the angle going forward.