Would like to relaunch the discussion on the application of various theories to bottom contour design.
From reading various threads posted on this forum, it appears that there is diverse opinion as to whether the Venturi effect is applicable in surfboard design. Those that think that bottom concaves do harness this effect seem to be split as to the precise nature of the benefits; greater speed, increased lift, greater speed leading to increased lift, increased lift leading to greater speed, greater control, more loosness etc. etc. You're all familiar with these and other claims.
Now, I accept that concaves will not lead to a plaining hull accelerating without the application of additional force (just as dents in a car roof do not cause a car to accelerate, the engine does). This is why cars and aircraft need engines. Indeed, displacement of water resulting in anything other than straight down the line lamiminar flow can only increase drag in the direction of travel. That, we can all agree on.
However, let us imagine a hypothetical surfboard that converts force applied in the forward direction into force applied downwards into the vertical direction in order to generate lift. This, of course, is what a wing of a powered aricraft does. The engines provide force in the horizontal plane and the wings convert some of this into lift. The reverse can also occur. The application of downward (vertical) force on the surfboard, say by extending our bent legs out of a turn, can be converted into force in the horizontal plane, causing acceleration. This is what the wing of an unpowered glider does when the glider is dropped vertically from a height; the vertical force applied to the wing by air resistance is converted to force in the horizontal plane, and the glider begins to accelerate in the forward direction.
This, I believe, is the principal which underpins the claim that the right configuration of bottom concaves can produce forward acceleration. The claim is not that the concaves will do this on there own, but that the concaves can do this when vertical pressure is applied by our legs.
Shoot me down. If we can go firm on this, then we can further focus the discussion.
... The claim is not that the concaves will do this on there own, but that the concaves can do this when vertical pressure is applied by our legs.
Shoot me down. If we can go firm on this, then we can further focus the discussion.
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ok, but gravity is constant, so then as long as the board is moving forward, and we're standing on our legs, then we must be applying vertical pressure, no?
Quite right. Let me clarify. I should have said, 'The claim is not that the concaves will do this on there own, but that the concaves can do this when additional vertical pressure is applied by our extending legs.
Think of a car 'planing' along a flat road. Does the downward force exerted by the mass of the car cause the car to accelerate? No.
Now consider a surfboard just planing along, rider just standing there, not moving up or down, legs locked, static (as per the premise of your question). It's just the same. Here's the reason....
Even though he is not moving, his mass provides a vertical force downwards. However, the surfboard doesn't sink into the water because the upwards force (the sum of the static lift (buoyancy) + dynamic lift from the planing hull) is equal to and therefore in equilibrium with the vertical force downwards. They cancel each other out and so the board just planes along not going up or down, neither vertical force (up or down) overcoming the other. Just like a car on a flat road. The weight of the car pushes down and is met by an equal and opposite force from the road's surface which pushes back up. The car doesn't sink into the tarmac or float away because the vertical forces are in equilibrium.
This is basic physics. Good old Newton.
However, what I want to focus on, is the ability of the bottom design of a surfboard to convert additional vertical force applied through one's extending legs (ie 'weighting') into horizontal force (ie acceleration).
Does the Venturi effect really help us with bottom shape design?
Further to my last. Does anyone out there have any actual scientific evidence (beyond the 'it felt real good' anecdote) of concaves having any actual benefit to speed, manouvreablility or any other aspect of wave riding? I've looked everywhere; here, boat design forums, physics forums etc.etc.
Lets nail this. It seems to me that there's a lot of BS out there. Lots of very rich and famous shapers trying to blind us with science without actually showing us the scientific evidence. I want to be convinced. I really do.
Come on chaps! I know we're all anti-establishment types who regard science as the soul preserve of those nasty weapons manufacturers. But we can do better than this.
Another thought. I think the most convincing theory for the benefits of concaves is the one that says the concave allows water to flow along a straighter line by avoiding the rocker, and therefore reduces drag through turbulence.
The whole 'concaves produce lift' theory is dubious I reckon. Anyone with evidence to the contrary? Mr Preisendorfer? Mr Carper? Anyone?
It is pretty simple, really, once you understand how the water in the wave is moving. It MOVES UP!
Just like the wind at the edge of a cliff, that gliders or birds ride.
So, when the board is in trim the water is moving UP, like you said, creating upward pressure that counteracts the weight (gravity) of the rider.
Lift is created when an object changes the direction flow of the fluid. The more the direction of the fluid is changed, the more lift is created. So, if you imagine a surfboard stuck in the upward flow of the wave, it will be deflecting the flow. A board with vee will deflect the flow less i.e. turn the direction of the flow less, than a board with concave. Boards with concave have a higher “natural” angle of attack (AoA) than those without.
It is almost like the board with concave is already on its rail, while in a neutral position, compared to a flat one.
Have yet to read a proof that the venturi effect works in a non-closed system, and the bottom of a board is an open system. Seems to be about change in flow and its relation to direction of flow.
So the lift that you talk of is provided by the water moving up the face of the wave. And the extra lift provided by concaves is the result of extra 'bite' of the board buried in the face of the wave. Now that makes eminent sense.
But what some people claim is that the effect of concaves is to compress the water and 'squirt' it out the back when extra downward pressure is applied (by extending one's legs out of a turn for example) and generating thrust. Rusty produces a board called the 'Venturi Compressor', which claims to have a 'Venturi' concave. The Venturi effect is all about channelling a fluid through a choke point to increase fluid velocity and decrease static pressure (although quite how this is meant to influence board performance I've never managed to work out).
Is this marketing tosh (as I suspect) or do you think there's anything in this. Anyone like to stand up for Rusty?
You know, as much as I respect the Rusty's of this world, and his boards really are very nice, I do think that some shapers are guilty of believing their own pseudo-scientific hype.
Have you heard Greg Webber talking up his boards on YouTube. What a load of tosh!
You know, as much as I respect the Rustys of this world, and his boards really are very nice, I do think that some shapers are guilty of believing their own pseudo-scientific hype.
Have you heard Greg Webber talking up his boards on YouTube. What a load of tosh!
IMHO this is basically about arguing the validity of the venturi principle in relation to bonzer bottoms-- without regard to years of film documentation from the campbell brothers surfing their bonzers. establishing scientific fact depends on repeatable experiments using the same variables, so if those bonzer home movies don’t quite convince skeptics, no amount of rational analysis will.
Look up a thread about “concave(s) for control,” or something to that effect. It was the turning point in my thinking, or helped me “let go” of the “lift” idea. Anyway…
There, or somewhere around that time, someone pointed out the deep concaves on a bonzer would slow the water down, but cause it to flow out the back - less resistance in that direction. So, more control, may allow one to handle more speed, so the board my feel faster. As I wrote about my latest quad, and my communications w/Robin “Handshaper” Mair, I’m working w/the idea keeping the water flowing back and through/over the fins helps with the action of the fins - which are both control and lift.
I looked at the link - Good on the Campbell’s for not mentioning the venturi effect. Their explanation was nice and simple. Not that I’m anybody, but it sounded reasonable to me.
Well, the water doesn’t really “move up the face of the wave”. A more acurate statement would be “The water moves up, creating the face of the wave”. And ocean wave is not a flow rider, it is more like a wack-a-mole, where the mole holes are aligned and each one successively moves up and down in a wave pattern.
Concaves don’t compress water (I don’t even think water is compressable, at least in surfing applications), they change the direction of the flow.
Think of the board as a deflector, it deflects the flow of the water of the wave. The higher the AoA of the deflector, the more it will change the direction of the flow. The more the direction of the flow is changed, the more lift is created.
If you put a spoon under the faucet, the concave side will change the direction of the flow more than the convex side, and the concave side creates more pressure than the convex side.
So the water is flowing UP, it hits the bottom of the board, tries to take the path of least resistance (off the rail), but if the bottom of the board in concave (in general) the path of least resistance will be more toward the tail.
What taylorO said - open system, incompressible fluid.
Surface tension is ourfriend and foe.
But it worries me when people talk of concaves as flatter rocker aiding water flow. Have I got it wrong? I’ve been taught that rocker in the concave is the rocker you want (I use the same stringer rocker on flat and concave bottoms) - a good rocker is a good rocker. Then you lift or lower the rails to achieve the water shed or capture effects you want. In small, smooth and steep waves you can harness more water. In fat, big and bumpy you want to(depending on your ability) drop the bottom deeper in the water to get some control.
PS As a kneeboarder I don’t have legs to extend. Concaves (including 1/2 deep ones on quads) work finie for me and my peers. - as long as the rocker is good - kneeboards are all about pure water flows unaided by pumping.
So I take it that what concaves (in conjunction with the fins, especially on a Bonzer) achieve is reclamation of energy that would otherwise be lost to water flowing diagonally off the outside rail on a turn. Instead, water is directed by the concaves and the fins towards the tail of the board and the equal and opposite reaction is to project the board forwards.
Makes sense. Although 'energy reclamation' sounds rather less sexy than 'thrust generation'!
So my next question would be.... how does a single to double concave do this better than just a straight forward single concave?
What do you think of this? I've come to think of a board with a single concave as having two rockers; one through the centre and one at the rail (although there is of course a continuum of rockers from the centre to the rail).
The rocker at the rail is normally curvier than the rocker through the middle (because the concave normally fades out at the nose and tail). This means that when you bury a rail for a turn the curvier rocker of the submerged part of the board helps the turn. But at the same time the flatter rocker through the centre means that when the board is level, when out on the flats say, the flatter rocker through the centre allows water to flow in a straighter line than it would if it were curved like the rocker at the rail. So a single concave board shaped like this acts like a heavily rockered board when on its rail and a less rockered board when flat planing.
I believe concaves affect a few things: 1) redirection of flow, 2) alteration of rocker, 3) presentation of rail, and 4) ventilation.
Regarding single to doubles, I believe the single concave does create lift in the way it has been mentioned earlier, but to add to that, the aft section of a single concave, behind the wide point and rocker apex, tends to pull water together from both sides of the board, creating a sense of lift and projection. But at higher speeds, like those generated by larger, more powerful surf, that flow convergence can create a “slippery” or “squirrley” feeling that singles give (and that people complain about on single concaved boards designed for bigger surf without some kind of convex feature to balance it out). This squirrley feeling, IMHO, is caused by the increased pressure and turbulence around the area of the back foot… I think a little ahead of the back foot, slightly between the feet. The double concave minimizes this convergence of flow, and the associated increase in pressure and turbulence, by keeping the incoming flow from both sides of the board separated, at least to some degree.