The venturi effect appears to have a wide application in fluids (at least that appears to be my impression) but there seems to be one problem which is never addressed when the effect is offered as explanation in surfboard design, and that’s the need for a constrained flow. What one can expect from water flowing in pipes is often very different from what can be expected when its flowing around in the ocean. Basically because it doesn’t suffer the same constraints. In particular Venturi relies on the presence of constraints in order to achieve the kind of pressure/kinetic conversions which are key to its application. Think about it. Put a funnel under a surfboard and start surfing. Got that jet feeling yet? In order to constrict the flow, the walls of the funnel must supply a force which is in the opposite direction to the flow, (also downward) otherwise the funnel is just going to be carried along with the flow. So, the argument is that the small amount of water that makes it to the funnel’s small hole will be like a jet, and eventually the funnel will start moving faster than the flow? Wow, get that to happen and we all get to say good-bye to fossil fuels. But what about redirection, that is, water flowing in one direction is redirected to flow in another direction, sure that’s how you get to go down the line. And the more efficiently that’s done, the fast you go. But that’s not Venturi, Venturi is about constriction and increasing velocity, under highly constraint conditions. Redirection of flow, and what you can achieve from it requires a different take. (Constricting a flow of water (alone) will increase the velocity of the flowing water, not increase its flow. Flow is the total volume (or mass) times speed. Also, an increase in the fluid velocity will reduce the fluid’s hydrostatic pressure, as observed in the classical Venturi Tube. The effect is explained using conservation theories, what goes in must come out. But, this is not the case under a surfboard, well what goes in does come out, its just not going to behave like what’s observed in a pipe.) Under a surfboard, at best you’ve got half the constraints in place for a Venturi effect, that is the required constraints for an reasonable expectation of the effect aren’t in place. The principle of Conservation of mass and energy are, of course always present, but the consequences of the principle take other forms. Consider. Move you hand back and forth in a bath tub filled with water, … a lot happens, the water swirls around, little waves are formed, maybe you get a splash or two, your duck moves around a little. Now blow some water through a straw. In the straw, we can almost be quaranteed that the water is moving quite uniformly from one end to the other, the walls of the straw are at work here. You may be able to apply the Venturi effect to whats going on in the straw, but forget it when it comes to the bath tub experiment. It simply isn’t applicable in any general sense, (with respect to its initial development.) (Also, try blowing something through a straw without holding on to the straw, if you can, does the straw start to move towards you? …If it does keep blowing because its not real, and can’t hurt you…) I am not suggesting however, that you can not change the speed of water under a surfboard with bottom contours, I believe you can. And you can change the flow too, but if you have somehow managed to increase the velocity, your flow will have either decreased or at best remained constaint, and that’s unlikely. Thrust through redirection? Sure. But its unlikely to be achieved because of a bottom contour’s ‘constriction’ of the flow. Thrust arises, (gravity aside) from the relationship between the pitch (the angle the line from the tail to the nose makes with the horizontal) and the upward flow of water on a wave. You can redirect it a little, more at a greater cost. You can be even be more efficent. But not without cost. (The kinetic energy of a fluid is in the flow, and flow is volume (or mass, as the difference here is a constant, i.e. density) times velocity. If you increase the velocity and proportionately decrease the volume, the energy doesn’t change, the point being, just increasing the velocity isn’t enough, you have got worry about what’s else is happening.) In the bath tub, the when you move your hand, a lot of the water simply moved around your hand, or maybe you saw a little bow wave, maybe a little wake wave behind your hand. The point is that unconstrained water will do what it wants. It will dispipate energy in a way most convenient to its nature. This is true, even if the flow is only partially constrained, and the bottom of a surfboard offers little constraint to flowing water, well relatively little, apparently enough to have some fun with. The Fin as a Bottom Contour… Fins are fins, why confuse this simple fact? Because I suggest that more often than not, bottom contours, particularly rear contours offer addition ‘fin function’ rather than anything else. A simple somewhat deep concave in the rear of surfboard even begins to look like a set of twins (when taken to the extreme.) A triple concave in tail, looks suspiciously like a morphed and well blended tri-fin. A vee bottom, well that’s a givee. Fins translate upward flow into sideways flow. (Their foil and angle of attack translate into both surfboard lift and drag. By the way these are just names given to the forces generated relative to the foil plane under consideration and don’t necessarily mean up or back.) This is not to say that I believe that this (fin function) is bottom contours soul role - bottom contours change rocker, and rocker is critical during planing. Consider a scooped nose on a noserider, the scoop changes the angle of attack (see past thread, Rocket Science series) and in addition redirects the ‘spray’ (see Rocket Science… planing… thread), but in addition I would suggest that it does offer some forward fin functionality (the lateral walls of the concave.) Actually, after writing that, maybe it is bottom contours sole role? Anyway, perhaps someone has a different take or some argument in favor of of an application of Venturi under a surfboard or the role of bottom contour. There’s definately a lot going on, that I’m missing something is a given. 
The venturi effect appears to have a wide application in fluids (at least > that appears to be my impression) but there seems to be one problem which > is never addressed when the effect is offered as explanation in surfboard > design, and that’s the need for a constrained flow.>>> What one can expect from water flowing in pipes is often very different > from what can be expected when its flowing around in the ocean. Basically > because it doesn’t suffer the same constraints. In particular Venturi > relies on the presence of constraints in order to achieve the kind of > pressure/kinetic conversions which are key to its application.>>> Think about it. Put a funnel under a surfboard and start surfing. Got that > jet feeling yet? In order to constrict the flow, the walls of the funnel > must supply a force which is in the opposite direction to the flow, (also > downward) otherwise the funnel is just going to be carried along with the > flow. So, the argument is that the small amount of water that makes it to > the funnel’s small hole will be like a jet, and eventually the funnel will > start moving faster than the flow? Wow, get that to happen and we all get > to say good-bye to fossil fuels.>>> But what about redirection, that is, water flowing in one direction is > redirected to flow in another direction, sure that’s how you get to go > down the line. And the more efficiently that’s done, the fast you go. But > that’s not Venturi, Venturi is about constriction and increasing velocity, > under highly constraint conditions. Redirection of flow, and what you can > achieve from it requires a different take.>>> (Constricting a flow of water (alone) will increase the velocity of the > flowing water, not increase its flow. Flow is the total volume (or mass) > times speed. Also, an increase in the fluid velocity will reduce the > fluid’s hydrostatic pressure, as observed in the classical Venturi Tube. > The effect is explained using conservation theories, what goes in must > come out. But, this is not the case under a surfboard, well what goes in > does come out, its just not going to behave like what’s observed in a > pipe.)>>> Under a surfboard, at best you’ve got half the constraints in place for a > Venturi effect, that is the required constraints for an reasonable > expectation of the effect aren’t in place. The principle of Conservation > of mass and energy are, of course always present, but the consequences of > the principle take other forms.>>> Consider. Move you hand back and forth in a bath tub filled with water, > … a lot happens, the water swirls around, little waves are formed, maybe > you get a splash or two, your duck moves around a little. Now blow some > water through a straw. In the straw, we can almost be quaranteed that the > water is moving quite uniformly from one end to the other, the walls of > the straw are at work here. You may be able to apply the Venturi effect to > whats going on in the straw, but forget it when it comes to the bath tub > experiment. It simply isn’t applicable in any general sense, (with respect > to its initial development.) (Also, try blowing something through a straw > without holding on to the straw, if you can, does the straw start to move > towards you? …If it does keep blowing because its not real, and can’t > hurt you…)>>> I am not suggesting however, that you can not change the speed of water > under a surfboard with bottom contours, I believe you can. And you can > change the flow too, but if you have somehow managed to increase the > velocity, your flow will have either decreased or at best remained > constaint, and that’s unlikely.>>> Thrust through redirection? Sure. But its unlikely to be achieved because > of a bottom contour’s ‘constriction’ of the flow. Thrust arises, (gravity > aside) from the relationship between the pitch (the angle the line from > the tail to the nose makes with the horizontal) and the upward flow of > water on a wave. You can redirect it a little, more at a greater cost. You > can be even be more efficent. But not without cost.>>> (The kinetic energy of a fluid is in the flow, and flow is volume (or > mass, as the difference here is a constant, i.e. density) times velocity. > If you increase the velocity and proportionately decrease the volume, the > energy doesn’t change, the point being, just increasing the velocity isn’t > enough, you have got worry about what’s else is happening.)>>> In the bath tub, the when you move your hand, a lot of the water simply > moved around your hand, or maybe you saw a little bow wave, maybe a little > wake wave behind your hand. The point is that unconstrained water will do > what it wants. It will dispipate energy in a way most convenient to its > nature. This is true, even if the flow is only partially constrained, and > the bottom of a surfboard offers little constraint to flowing water, well > relatively little, apparently enough to have some fun with.>>> The Fin as a Bottom Contour…>>> Fins are fins, why confuse this simple fact? Because I suggest that more > often than not, bottom contours, particularly rear contours offer addition > ‘fin function’ rather than anything else. A simple somewhat deep concave > in the rear of surfboard even begins to look like a set of twins (when > taken to the extreme.) A triple concave in tail, looks suspiciously like a > morphed and well blended tri-fin. A vee bottom, well that’s a givee.>>> Fins translate upward flow into sideways flow. (Their foil and angle of > attack translate into both surfboard lift and drag. By the way these are > just names given to the forces generated relative to the foil plane under > consideration and don’t necessarily mean up or back.)>>> This is not to say that I believe that this (fin function) is bottom > contours soul role - bottom contours change rocker, and rocker is critical > during planing. Consider a scooped nose on a noserider, the scoop changes > the angle of attack (see past thread, Rocket Science series) and in > addition redirects the ‘spray’ (see Rocket Science… planing… thread), > but in addition I would suggest that it does offer some forward fin > functionality (the lateral walls of the concave.) Actually, after writing > that, maybe it is bottom contours sole role?>>> Anyway, perhaps someone has a different take or some argument in favor of > of an application of Venturi under a surfboard or the role of bottom > contour. There’s definately a lot going on, that I’m missing something is > a given. A venturi shaped bottom directs more water at the base of the foil of your working fin. The theory is that the foil of the working side fin will be more effective if the bottom contours aides in directing the flow of water over the outer foil’s base. The increased velocity of the water is significant only to the degree that pressing hard against the planing surface creates compression. It’s not water running through a pipe. It’s a system of planing surfaces moving across the surface of the water. The faster the board moves across the water, the more the water underneath resists the board being forced down into the water. Once a board is planing and the rider barrying a rail, then the contours begin to become effective i.e. they redirect the flow of water that is being compressed.
The venturi effect appears to have a wide application in fluids (at least > that appears to be my impression) but there seems to be one problem which > is never addressed when the effect is offered as explanation in surfboard > design, and that’s the need for a constrained flow.>>> What one can expect from water flowing in pipes is often very different > from what can be expected when its flowing around in the ocean. Basically > because it doesn’t suffer the same constraints. In particular Venturi > relies on the presence of constraints in order to achieve the kind of > pressure/kinetic conversions which are key to its application.>>> Think about it. Put a funnel under a surfboard and start surfing. Got that > jet feeling yet? In order to constrict the flow, the walls of the funnel > must supply a force which is in the opposite direction to the flow, (also > downward) otherwise the funnel is just going to be carried along with the > flow. So, the argument is that the small amount of water that makes it to > the funnel’s small hole will be like a jet, and eventually the funnel will > start moving faster than the flow? Wow, get that to happen and we all get > to say good-bye to fossil fuels.>>> But what about redirection, that is, water flowing in one direction is > redirected to flow in another direction, sure that’s how you get to go > down the line. And the more efficiently that’s done, the fast you go. But > that’s not Venturi, Venturi is about constriction and increasing velocity, > under highly constraint conditions. Redirection of flow, and what you can > achieve from it requires a different take.>>> (Constricting a flow of water (alone) will increase the velocity of the > flowing water, not increase its flow. Flow is the total volume (or mass) > times speed. Also, an increase in the fluid velocity will reduce the > fluid’s hydrostatic pressure, as observed in the classical Venturi Tube. > The effect is explained using conservation theories, what goes in must > come out. But, this is not the case under a surfboard, well what goes in > does come out, its just not going to behave like what’s observed in a > pipe.)>>> Under a surfboard, at best you’ve got half the constraints in place for a > Venturi effect, that is the required constraints for an reasonable > expectation of the effect aren’t in place. The principle of Conservation > of mass and energy are, of course always present, but the consequences of > the principle take other forms.>>> Consider. Move you hand back and forth in a bath tub filled with water, > … a lot happens, the water swirls around, little waves are formed, maybe > you get a splash or two, your duck moves around a little. Now blow some > water through a straw. In the straw, we can almost be quaranteed that the > water is moving quite uniformly from one end to the other, the walls of > the straw are at work here. You may be able to apply the Venturi effect to > whats going on in the straw, but forget it when it comes to the bath tub > experiment. It simply isn’t applicable in any general sense, (with respect > to its initial development.) (Also, try blowing something through a straw > without holding on to the straw, if you can, does the straw start to move > towards you? …If it does keep blowing because its not real, and can’t > hurt you…)>>> I am not suggesting however, that you can not change the speed of water > under a surfboard with bottom contours, I believe you can. And you can > change the flow too, but if you have somehow managed to increase the > velocity, your flow will have either decreased or at best remained > constaint, and that’s unlikely.>>> Thrust through redirection? Sure. But its unlikely to be achieved because > of a bottom contour’s ‘constriction’ of the flow. Thrust arises, (gravity > aside) from the relationship between the pitch (the angle the line from > the tail to the nose makes with the horizontal) and the upward flow of > water on a wave. You can redirect it a little, more at a greater cost. You > can be even be more efficent. But not without cost.>>> (The kinetic energy of a fluid is in the flow, and flow is volume (or > mass, as the difference here is a constant, i.e. density) times velocity. > If you increase the velocity and proportionately decrease the volume, the > energy doesn’t change, the point being, just increasing the velocity isn’t > enough, you have got worry about what’s else is happening.)>>> In the bath tub, the when you move your hand, a lot of the water simply > moved around your hand, or maybe you saw a little bow wave, maybe a little > wake wave behind your hand. The point is that unconstrained water will do > what it wants. It will dispipate energy in a way most convenient to its > nature. This is true, even if the flow is only partially constrained, and > the bottom of a surfboard offers little constraint to flowing water, well > relatively little, apparently enough to have some fun with.>>> The Fin as a Bottom Contour…>>> Fins are fins, why confuse this simple fact? Because I suggest that more > often than not, bottom contours, particularly rear contours offer addition > ‘fin function’ rather than anything else. A simple somewhat deep concave > in the rear of surfboard even begins to look like a set of twins (when > taken to the extreme.) A triple concave in tail, looks suspiciously like a > morphed and well blended tri-fin. A vee bottom, well that’s a givee.>>> Fins translate upward flow into sideways flow. (Their foil and angle of > attack translate into both surfboard lift and drag. By the way these are > just names given to the forces generated relative to the foil plane under > consideration and don’t necessarily mean up or back.)>>> This is not to say that I believe that this (fin function) is bottom > contours soul role - bottom contours change rocker, and rocker is critical > during planing. Consider a scooped nose on a noserider, the scoop changes > the angle of attack (see past thread, Rocket Science series) and in > addition redirects the ‘spray’ (see Rocket Science… planing… thread), > but in addition I would suggest that it does offer some forward fin > functionality (the lateral walls of the concave.) Actually, after writing > that, maybe it is bottom contours sole role?>>> Anyway, perhaps someone has a different take or some argument in favor of > of an application of Venturi under a surfboard or the role of bottom > contour. There’s definately a lot going on, that I’m missing something is > a given. It’s not necessary to have a totally constrained flow to increase fluid velocity - simply a reduction in one dimension is enough. I would suggest that you think of it this way. Picture a slow-moving river entering a narrow gorge. Ever been white-water rafting or kayaking? The result is faster flowing water, aka “rapids”. However, the water has not been constrained into a tube or funnel, and yet clearly the velocity increases, as it must to push the same volume of water through a narrower space. Now turn that gorge upside down in your mind and pretend its the bottom of your surfboard, perhaps one with a really nice channel (or set of channels) through the tail. The same principles apply. The water that would have spread over a flat bottom is forced up into the channel(s) (by the rider’s weight/gravity plus other forces) where it HAS to move faster due to the same one-dimensional volume constraints that cause rapids in rivers. That water also produces side forces on the more vertical portions of the channel(s), thus reducing side-slippage and changing the handling of the board – similar to having fins with more surface area. That’s how I look at it anyway…
Ya, thats what I think!
A venturi shaped bottom directs more water at the base of the foil of your > working fin. The theory is that the foil of the working side fin will be > more effective if the bottom contours aides in directing the flow of water > over the outer foil’s base. The increased velocity of the water is > significant only to the degree that pressing hard against the planing > surface creates compression. It’s not water running through a pipe. It’s a > system of planing surfaces moving across the surface of the water. The > faster the board moves across the water, the more the water underneath > resists the board being forced down into the water. Once a board is > planing and the rider barrying a rail, then the contours begin to become > effective i.e. they redirect the flow of water that is being compressed. That’s not the Venturi effect, that’s planing theory and a bit of redirection. I keep checking the literature on this, please put me on to where the Venturi effect is described in this manner. Also, water is incompressible. If your suggesting that the board is itself compressed of that the fin is bent that also a different twist. I may me ignorant here.
It’s not necessary to have a totally constrained flow to increase fluid > velocity - simply a reduction in one dimension is enough. I would suggest > that you think of it this way. Picture a slow-moving river entering a > narrow gorge. Ever been white-water rafting or kayaking? The result is > faster flowing water, aka “rapids”. However, the water has not > been constrained into a tube or funnel, and yet clearly the velocity > increases, as it must to push the same volume of water through a narrower > space. Now turn that gorge upside down in your mind and pretend its the > bottom of your surfboard, perhaps one with a really nice channel (or set > of channels) through the tail. The same principles apply. The water that > would have spread over a flat bottom is forced up into the channel(s) (by > the rider’s weight/gravity plus other forces) where it HAS to move faster > due to the same one-dimensional volume constraints that cause rapids in > rivers. That water also produces side forces on the more vertical portions > of the channel(s), thus reducing side-slippage and changing the handling > of the board – similar to having fins with more surface area. That’s how > I look at it anyway… I agree about velocity and say so, but as for the river analogy, you’ve forgetten about the forces on the walls and floor of the gorge, are you suggesting that the gorge wants to move in the other direction than that of the rapids… natural erosion will contradict this. The gorge pays a price for creating the rapids, and eventually it comes up short, i.e. the river wins. A surfboard design which constricts the flow of water, will pay a price. All else held constant, it will either rise out of the water (assuming that its not a total funneling) or slow down in the direction, if not start to eventually move with the flow. But thats where gravity comes in. Its used by the surfer to conteract these contrary forces (the upward flow of the water being manipulated by the surfboards bottom) and provides a means for redirection. But this ain’t Venturi, this is planing theory and redirection of flow. The ‘Venturi effect’ has a far more narrow application, at least my incounters with reference to it in the literature would suggest that is the case. And a constrained flow, i.e. the presence of constrains to counteract the forces generated by making the fluid do things is doesn’t necessarily want to do is important in its application.
Ya, thats what I think! Sushi!
That’s not the Venturi effect, that’s planing theory and a bit of > redirection. I keep checking the literature on this, please put me on to > where the Venturi effect is described in this manner.>>> Also, water is incompressible. If your suggesting that the board is itself > compressed of that the fin is bent that also a different twist. I may me > ignorant here. Venturi affect as described by various surfboard manufacturers is not the same scientific principle as described by Bernouli. When I said compress I didn’t think that I was going to be held to the same standards as my college physics professor. As a planing surface moves along the surface of a “non-compressable fluid” the surface of the fluid resists penatration and “displacment” to greater degrees as the velocity increases. Alot of the water that becomes displaced will have a tendency to spread out in vectors that are askew to the vector direction the board is moving in. The marketing discription of a “venturi” contour bottom is to redirect some of the water that is being lost out the sides of the rail and directs it across the fins. Flex characteristics are an entirely different topic.
Venturi affect as described by various surfboard manufacturers is not the > same scientific principle as described by Bernouli. When I said compress I > didn’t think that I was going to be held to the same standards as my > college physics professor. As a planing surface moves along the surface of > a “non-compressable fluid” the surface of the fluid resists > penatration and “displacment” to greater degrees as the velocity > increases. Alot of the water that becomes displaced will have a tendency > to spread out in vectors that are askew to the vector direction the board > is moving in. The marketing discription of a “venturi” contour > bottom is to redirect some of the water that is being lost out the sides > of the rail and directs it across the fins. Flex characteristics are an > entirely different topic. I’m not a Professor. I am struggling with this stuff, and I’m shamelessly using this forum to listen to myself reason. I’m also hoping others might step in and correct my thinking. If somebody in the process goes away with some new insight, that’s great too. As for the marketing description, something that actually amounts to the ‘spin’ of a scientific term, I make no apologies for not accepting their definition. (That works in politics.) If the use of the term is dishonest, or it is used to imply something more than whats there, or its sprinkled into a description to hype something … actually I don’t get your point about this. Are we now to accept the redefinition of a scientific term because some marketing strategy calls for it. That’s insane. And by the way, my apologies but you are still describing planing theory with some discussion of redirection of flow. Venturi describes something else, and that was my point.
I’m not a Professor.>>> I am struggling with this stuff, and I’m shamelessly using this forum to > listen to myself reason. I’m also hoping others might step in and correct > my thinking. If somebody in the process goes away with some new insight, > that’s great too.>>> As for the marketing description, something that actually amounts to the > ‘spin’ of a scientific term, I make no apologies for not accepting their > definition. (That works in politics.) If the use of the term is dishonest, > or it is used to imply something more than whats there, or its sprinkled > into a description to hype something … actually I don’t get your point > about this. Are we now to accept the redefinition of a scientific term > because some marketing strategy calls for it. That’s insane.>>> And by the way, my apologies but you are still describing planing theory > with some discussion of redirection of flow. Venturi describes something > else, and that was my point. Hey, It’s not my marketing strategy. I completely understand that this is plane theory. Your acting as though someone submitted a thesis to the scientific community to redefine a scientific term. We are talking Surfing here. People verify if a new concept works intuitively. If it feels good it works. You can get all huffy about “truth in advertising” if you want. But, your bitching at the wrong person. I have only described what I have seen of boards that are marketed under the “venturi” design claim and how they appear to work from a macro perspective.
Misunderstandings. Kevin – Tom was just pointing out that “venturi” is an incorrect term to apply to the situation and that it came about as a marketing term. Bad on the marketers. But thats another topic isn’t it? Kevin’s question still stands: Does the venturi effect, as in Bernoulli’s college phyisics textbook prinicple, ever take place on the bottom of a surfboard?
I feel totally outgunned trying to deal with all this. You and Tom@Daum are on a whole different plane when it comes to theory and ability to articulate. I agree that “Venturi Principle” usually pertains to a restricted tube… in a nutshell, a given volume of fluid at a given pressure will increase in velocity when compressed through a narrower passage. That much I think I understand. I don’t know if a concave actually qualifies as a “Venturi” but I do know that when you dangle a spoon in water coming full blast out of a faucet that the concave side pushes out of and the convex side sucks into the water flow. I’m thinking that a concave on the bottom of a surfboard (when designed and placed correctly) maybe qualifies as a “semi-Venturi” and creates lift… which maybe reduces bottom drag, which maybe enhances surfboard performance? I think that sometimes what makes sense on paper or drawing board doesn’t necessarily translate to actual performance in the water. By the way, nice board you posted - it looks like it’ll work great!
I agree about velocity and say so, but as for the river analogy, you’ve > forgetten about the forces on the walls and floor of the gorge, are you > suggesting that the gorge wants to move in the other direction than that > of the rapids… natural erosion will contradict this. The gorge pays a > price for creating the rapids, and eventually it comes up short, i.e. the > river wins. Don’t hold your breath. A rock gorge will hold out pretty long.>>> A surfboard design which constricts the flow of water, will pay a price. > All else held constant, it will either rise out of the water (assuming > that its not a total funneling) or slow down in the direction, if not > start to eventually move with the flow. Rising out of the water is what we want. Often known as lift in surfboard manufacturing when describing the effect of concaves. Lift would meen less board in the water and thus less drag? It propably also increases water flow to some extent which would result in planning happening at slower speeds?>>> But thats where gravity comes in. Its used by the surfer to conteract > these contrary forces (the upward flow of the water being manipulated by > the surfboards bottom) and provides a means for redirection. But this > ain’t Venturi, this is planing theory and redirection of flow. The > ‘Venturi effect’ has a far more narrow application, at least my incounters > with reference to it in the literature would suggest that is the case. And > a constrained flow, i.e. the presence of constrains to counteract the > forces generated by making the fluid do things is doesn’t necessarily want > to do is important in its application. I don’t know much about the venturi effect and it’s propably not what’s going on under a surfboard. But it seems to me like both the bottom designs known as concaves and the venturi tubes work on the same principle. When a flow is forced to redirect it generates pressure in one direction or the other. I appreciate your thinking. Alot of people here seem to know what works and in which conditions, but I would really like to know why it works. regards, Håvard
Misunderstandings. Kevin – Tom was just pointing out that > “venturi” is an incorrect term to apply to the situation and > that it came about as a marketing term. Bad on the marketers. But thats > another topic isn’t it?>>> Kevin’s question still stands: Does the venturi effect, as in Bernoulli’s > college phyisics textbook prinicple, ever take place on the bottom of a > surfboard? Understood and appreciated…
I feel totally outgunned trying to deal with all this. You and Tom@Daum > are on a whole different plane when it comes to theory and ability to > articulate. I agree that “Venturi Principle” usually pertains to > a restricted tube… in a nutshell, a given volume of fluid at a given > pressure will increase in velocity when compressed through a narrower > passage. That much I think I understand. I don’t know if a concave > actually qualifies as a “Venturi” but I do know that when you > dangle a spoon in water coming full blast out of a faucet that the concave > side pushes out of and the convex side sucks into the water flow. I’m > thinking that a concave on the bottom of a surfboard (when designed and > placed correctly) maybe qualifies as a “semi-Venturi” and > creates lift… which maybe reduces bottom drag, which maybe enhances > surfboard performance? I think that sometimes what makes sense on paper or > drawing board doesn’t necessarily translate to actual performance in the > water. By the way, nice board you posted - it looks like it’ll work great! First things first… thanks about the board. (Its my Big Boy Ballerino Board, guarenteed to lighten the step of your average 250 pounder, and a real chic magnet too.) Language and terminology can be a real issue. But perhaps you’ll agree, well defined terms do help in understanding and communication. In a old thread there was some question as to the surfing’s source of energy. Actually, we know it comes from the wave and gravity, but what there seems to be some disagreement on is exactly how a surfboard translates whatever is there into kinetic energy. This is actually very important, so I though I’d reuse a few diagrams in an attempt to get some agreement on the way a surfboard translates wave energy into kinetic energy (the energy inherent in motion.) This first diagram is of a (shoaling) wave approaching a reef or beach. Its a pretty standard view of things. I’ve included it to make sure we are on the same path. If you have a different take, please let me know. The next post (sorry there’s only one picture per post) shows my understanding of what might be going on, in particular the source of flow and how its translated into kinetic energy. I seem to have left out gravity, its there and its role is critical, and I will get back to in a moment.
Here’s the second Diagram. I’ve included a (bad) cartoon of a surfer on a wave in order to show the area that the main diagrams addresses. (In the cartoon its the shaded area.) I have not included gravity in the diagrams, perhaps I should have, for its what surfers use to in opposition to forces of flow. That is, (in my opinion) surfing is about a tension between two somewhat opposing forces, that of the upward flow of water on a wave, and that of the downward force of gravity. The surfer uses this tension, changing the balance in favor of gravity to drop a little, or in favor of the upward flow to climb a little. By using gravity against the upward flow, the surfboard/surfer can maintain his position (in particular, in terms of height) on the wave. But, its not just gravity, he needs a surfboard (and whatever else that might include, bottom contours, fins, etc.) The surfboard is basically a way of translating the flow of water on the face of the wave into kinetic energy, or alternatively, the surfer uses his surfboard to redirect flow and translate the forces generated on his surfboard into something more useful, like motion down the line (in the diagram, this would be the motion I’ve shown as forward.) But its important to understand that this flow arises because the surfer using gravity places himself in opposition to the motion of water in the wave. (If you have ever seen a big swell pass under and parially over a pier, you might have observed water shooting out from the cracks. The pier is getting in the way of the motion of the water in the wave, from the piers point of view, the water is moving, its flowing thru its cracks. The dock or pier is held down, possibly by gravity, but more likely by some mechanical means, e.g. posts driven into the sand.) There are other ways I surfer can increase his kinetic energy, in particular, he can use the wave as a big water hill and take a sleigh ride, i.e. simple take the drop. But at the moment, its the former way that is of interest here. So, where is this headed. I’m not there yet, there’s something else which is pretty important and it relates directly to your comments. More diagrams? (Next post.) And by the way, I’m simply assuming we’re in agreement, if you see things from another take, please let it be known. But before that, I should point out that I’ve left another motion out of the diagrams, and it is very important. Its in the direction of the wave’s propagation, possible towards the beach, maybe elsewhere. It is used in surfing and it definately contributes to the ‘flow’, but I’m struggling to make these diagrams comprehensible in two dimensions, so, and I apologize for this, maybe we can restrict the discussion to just those two dimension, upward and forward, the ones that I’ve indicated in the diagram. 
Wow,surfers are a lot smarter these days,I wish I could figure out what the hell you guys are talking about.Pretty interesting though.The best magic board I ever had was twisted like a frigging pretzel, never could duplicate it though.
Your spoon. Which ever way it moved in your experiment, there was a force which you did not mention. You not only had to hold the spoon up (gravity) but you had to change the flow of the water (in order to get the effect.) To change to flow of water requires force, and that force had to come from somewhere. So in order to generate the forces that you believe are there you got to have some other force holding the spoon in place so to create the circumstances that allow these forces (which resulted in some lateral movement) to be generated. “Money for nothing, chics for free.” Throw a surfboard in a river and it will just move with the current, regardless of its bottom contours, fins or whatever (excluding somebody paddling it.) I am not claiming that the circumstances, analogous to your spoon experiment do not arise, its just that how might they and when might they arise is an issue, which in my opinion is often overlooked. But there’s even something more important, … and that’s everything costs something. You can achieve greater lift, but it will cost you, possibly in speed. You can achieve greater speed, but the cost might be stiffness in turning or in maneuverability. Which brings me back to my cartoon about the Venturi effect. The effect assumes a constricted flow, if you constrict the flow, you pay a price. And its likely that price is a force in a direction contrary to the flow moving through the constriction. In the extreme example of a funnel, the price would be the force required to hold it in place. (Claims of a Venturi effect? See http://www.eatonsurf.com/Bonzer.htm I have little doubt that the claims made by surfers who use a bonzer are real. I have problems with the description of what’s going on underneath the board. Eaton’s explaination is interesting, but in my opinion misses an important point, the price paid for constricting the flow,… chicken before the egg?) I do not disagree with the ability of some bottom contours aiding in the redirection of flow. Redirection of flow comes at cost however, and you may see it as being worth the price. (I do.) Under many, if not most circumstances, redirection of flow results in some lift (say in the direction of Upward force in the prior diagram) as well as in the Forward direction (see prior diagram). And under most circumstances, the surfer is able to counterbalance that lift with some change in technique or reposition themselves on the wave (angling the board in some manner) or even changing the center of mass of surfer and surfboard, or use it. Same for the lift in the Forward direction. No pictures this time. I was going to draw your spoon but decided it was unecessary, hopefully my point about the unmentioned forces is clear. I guess I’ll find out soon enough.
Sometimes you just make the time,… if you thinks its important enough.