Below is a picture of Slater trimming. He may not actually be trimming, but given his position on the wave, and given the way he’s using his body my guess is that he’s trimming. Insert A shows how I believe the water is flowing across the tail of his board. The direction tends to be from right rail to left and back a little. I have not included the fins in the insert. The fins will of course interact with the flow, redirecting it. The point of the diagram is to illustrate how, in this case, I believe the flow is moving across the tail of his board. There are many trim positions that a surfer can take. The illustration I have given here, in my opinion, though perhaps untypical (Kelly’s getting shacked) does a good job of demonstrating the dynamics of the flow, especially during trim situations - the board being angled with respect to the wave, and the flow in the tail is moving from rail to rail and back a little. If I am correct, then consider the consequences of a tail bottom contour. First, since the water is not really moving in the direction from nose to tail, but across and back, the interaction of the flow with the contour would tend to be as indicated in insert B. (I’ve indicate a concave contour with a green line in the insert. I don’t know what the contours are of the board Slater is surfing.) The arrow in B is meant to serve only as an indication of general direction of flow. In insert C I’ve included the wave form, the blue line and arrow showing the flow. The dash blue line showing where the flow would be if the board was not there to alter it. I may not have got the angles exactly right, but hopefully the point is clear. (The tail in this situation has a low tau, especially at the rail, see past Rocket Science threads.) If this is a correct interpretation, then the lift experienced with a concave would tend to be more do to the cross-sectional profile of concave’s vault and trailing side or wall of the contour. The water flow being redirected back into the wave face, the resultant force (equal and opposite), or the force experienced by the surfboard being a up and out of the wave face. I believe that the walls of the concave, the portion of the concave closest to the rails would also tend to redirect some of the flow back towards the tail. That is, they likely have some ‘fin function.’ In the above I assumed that the concave is not very deep, or that the concave offers a smooth transition for the flow. If the bottom contours are too abrupt then you may wind up with turbulence, and energy loss as a result. But a fin is a pretty abrupt bottom contour so depending on the nature of the contour you may likely wind up with added fin function. But then, additional fin function can be, in my opinion very beneficial, under some circumstances. Buts that’s another thread. Kevin 
Below is a picture of Slater trimming. He may not actually be trimming, > but given his position on the wave, and given the way he’s using his body > my guess is that he’s trimming.>>> Insert A shows how I believe the water is flowing across the tail of his > board. The direction tends to be from right rail to left and back a > little.>>> I have not included the fins in the insert. The fins will of course > interact with the flow, redirecting it. The point of the diagram is to > illustrate how, in this case, I believe the flow is moving across the tail > of his board.>>> There are many trim positions that a surfer can take. The illustration I > have given here, in my opinion, though perhaps untypical (Kelly’s getting > shacked) does a good job of demonstrating the dynamics of the flow, > especially during trim situations - the board being angled with respect to > the wave, and the flow in the tail is moving from rail to rail and back a > little.>>> If I am correct, then consider the consequences of a tail bottom contour.>>> First, since the water is not really moving in the direction from nose to > tail, but across and back, the interaction of the flow with the contour > would tend to be as indicated in insert B. (I’ve indicate a concave > contour with a green line in the insert. I don’t know what the contours > are of the board Slater is surfing.) The arrow in B is meant to serve only > as an indication of general direction of flow.>>> In insert C I’ve included the wave form, the blue line and arrow showing > the flow. The dash blue line showing where the flow would be if the board > was not there to alter it. I may not have got the angles exactly right, > but hopefully the point is clear. (The tail in this situation has a low > tau, especially at the rail, see past Rocket Science threads.)>>> If this is a correct interpretation, then the lift experienced with a > concave would tend to be more do to the cross-sectional profile of > concave’s vault and trailing side or wall of the contour. The water flow > being redirected back into the wave face, the resultant force (equal and > opposite), or the force experienced by the surfboard being a up and out of > the wave face.>>> I believe that the walls of the concave, the portion of the concave > closest to the rails would also tend to redirect some of the flow back > towards the tail. That is, they likely have some ‘fin function.’>>> In the above I assumed that the concave is not very deep, or that the > concave offers a smooth transition for the flow. If the bottom contours > are too abrupt then you may wind up with turbulence, and energy loss as a > result. But a fin is a pretty abrupt bottom contour so depending on the > nature of the contour you may likely wind up with added fin function. But > then, additional fin function can be, in my opinion very beneficial, under > some circumstances. Buts that’s another thread.>>> Kevin Kevin, Nice graphics over lay. Using static 2-D diagrams to explain what is going on under Kelly’s feet only gives a very small part of the equation. A more accurate formula would require a multi-directional matrix integration. One formula would describe the vector flow of water rising up the face of the wave integrated over the X,Y,Z & T of the changing wave face . Another formula would have to be the planing formula you refered to earlier on how the previous “wave vector formula” integration interacts with the “flat” plane surface that you previously presented. Then you would need to modify that formula for concavity feature of Kelly’s specific board, integrated on how this contour changes relative to it’s angle of attack. Then you would need a couple formulas to describe how Kelly’s fins in conjunction with their foils and angle of attack would influence the vector flow and assist the previous planear formula. Each of the formulas would need to be integrated in a matrix as previously described. Or you could go with the tried and true method of if it feels good do it. A couple of years ago I wrote a layman’s article which addresses some of the “fin function” factors that would have to be considered in the fin formula of the matrix. http://www.wetsand.com/article-email.asp?ID=21&CatID=102 But, the bottom line is there is an awful lot to consider when mathematically attempting to describe what is actually going on in this picture.
“But, the bottom line is there is an awful lot to consider when mathematically attempting to describe what is actually going on in this picture.” … and the fluid dynamics are constantly changing!
…But, the > bottom line is there is an awful lot to consider when mathematically > attempting to describe what is actually going on in this picture. True??? But here I offered a view as to what I believe is going on in general (picturesque?) terms with respect to the direction of the flow under the tail in certain trim situations. And I used the particular case, illustrated by the Slater photo, in the hopes of making it clear. And if one accepts it, I then go on to suggest how a tail bottom contour (here a concave), subject to such a flow, might be operating. My major point being that if the flow is as I have suggested, then the bottom contour is interacting with the flow in a manner, and this may just be my impression, that is not generally held by most - across from rail to rail and back a little, rather than from nose to tail. Your suggestion that there is a need for a lot more Math to take the analysis further, is true, and appreciated. However, though admittedly, its actually not that clear to me exactly what your point was, if you disagree because things are far to complicated to ever understand without the aid of a lot more Math, or that the flow is in general not in the direction which I have indicated, I would tend to disagree (especially give the objective of the thread.) … thanks for the comments on the diagrams. Kevin
“But, the bottom line is there is an awful lot to consider when > mathematically attempting to describe what is actually going on in this > picture.”>>> … and the fluid dynamics are constantly changing! I agree Dale, but given the situation that I described, do you disagree with the flows? And if you do, how? And how might your take on the flow be interacting with a tail bottom contour? Or maybe you think I’ve just got it all wrong. If you do, perhaps you might be willing to suggest an alternative view. Kevin
True???>>> But here I offered a view as to what I believe is going on in general > (picturesque?) terms with respect to the direction of the flow under the > tail in certain trim situations. And I used the particular case, > illustrated by the Slater photo, in the hopes of making it clear. And if > one accepts it, I then go on to suggest how a tail bottom contour (here a > concave), subject to such a flow, might be operating.>>> My major point being that if the flow is as I have suggested, then the > bottom contour is interacting with the flow in a manner, and this may just > be my impression, that is not generally held by most - across from rail to > rail and back a little, rather than from nose to tail.>>> Your suggestion that there is a need for a lot more Math to take the > analysis further, is true, and appreciated.>>> However, though admittedly, its actually not that clear to me exactly what > your point was, if you disagree because things are far to complicated to > ever understand without the aid of a lot more Math, or that the flow is in > general not in the direction which I have indicated, I would tend to > disagree (especially give the objective of the thread.)>>> … thanks for the comments on the diagrams.>>> Kevin I agree that the vectors would be oblique to the direction of the path of the board in that scenario. But, I think they would not be as greta an angle as you represented. I also believe that since all of this needs to be described as an integral (i.e. constantly changing) the vectors would be better represented as multiple vectors forming an arc path with greater angle to the stringer at the point of entry and lessor angles as they interact with the plane surface.
I agree that the vectors would be oblique to the direction of the path of > the board in that scenario. But, I think they would not be as greta an > angle as you represented. I also believe that since all of this needs to > be described as an integral (i.e. constantly changing) the vectors would > be better represented as multiple vectors forming an arc path with greater > angle to the stringer at the point of entry and lessor angles as they > interact with the plane surface. ‘… not be as great an angle?’ Are you measuring with respect to the stringer line, the angle increasing towards the rail? That is, you see the flow having a greater nose-tail contribution?
‘… not be as great an angle?’ Are you measuring with respect to the > stringer line, the angle increasing towards the rail? That is, you see the > flow having a greater nose-tail contribution? Yes, I believe that (depending upon the size & power of the wave)that the gross vectors affecting the planing surface are two. The first is the flow of water rising up the face and second the momentum of the board and rider over coming the vector #1. The faster the board travels the more the momentum vector overcomes the wave’s water flow vector. But, since I have just introduced two new variables wave power and board/rider momentum that angle can range from the angle you indicated to almost parrallel to the stringer. I would prefer to ride a wave with that minimized. i.e hauling ass down the line kind of like you see Kelly doing. Albeit, that wave probably does have a hell of a flow vector to over come.
Yes,>>> I believe that (depending upon the size & power of the wave)that the > gross vectors affecting the planing surface are two. The first is the flow > of water rising up the face and second the momentum of the board and rider > over coming the vector #1. The faster the board travels the more the > momentum vector overcomes the wave’s water flow vector. But, since I have > just introduced two new variables wave power and board/rider momentum that > angle can range from the angle you indicated to almost parrallel to the > stringer. I would prefer to ride a wave with that minimized. i.e hauling > ass down the line kind of like you see Kelly doing. Albeit, that wave > probably does have a hell of a flow vector to over come. So, in this case, you see the flow under Slater’s board as being more, if not almost completely in nose to tail direction? (Relative to Slater’s board.)
Yes,>>> I believe that (depending upon the size & power of the wave)that the > gross vectors affecting the planing surface are two. The first is the flow > of water rising up the face and second the momentum of the board and rider > over coming the vector #1. The faster the board travels the more the > momentum vector overcomes the wave’s water flow vector. But, since I have > just introduced two new variables wave power and board/rider momentum that > angle can range from the angle you indicated to almost parrallel to the > stringer. I would prefer to ride a wave with that minimized. i.e hauling > ass down the line kind of like you see Kelly doing. Albeit, that wave > probably does have a hell of a flow vector to over come. If you look at the spray of a tow board (going very fast) and a regular board (going relatively slow) there is no difference in the direction of spray. I think the flow vector does not change significantly with forward motion or momentum. It may be directed slightly toward the stinger by drag between the fluid and the board, but that would remain a relative constant. Redirection of the flow by fins or bottom contours remain, in my mind, the only way to change the flow vector across any portion of the board.
So, in this case, you see the flow under Slater’s board as being more, if > not almost completely in nose to tail direction? (Relative to Slater’s > board.) If you look closely at the wake coming off his board you will note two things happening in this specific frame. First there is a slight arc towards the bottom of the wave. This would indicate that Kelly began to feel to much of the wave flow vector and is angling in a direction that is less than perpendicular to the wave flow vector. In order to turn down Kelly has to unweight his inside rail and decrease the angle between the boards bottom (in the athwart shipdirection or across) and face of the wave. Second the amount of wake that is on either side of the board is not heavily favored to the outside rail side. So, yes in this picture I do believe that the resulting combination of vectors would be less than say 10 degrees askew from the stringer.
If you look closely at the wake coming off his board you will note two > things happening in this specific frame. First there is a slight arc > towards the bottom of the wave. This would indicate that Kelly began to > feel to much of the wave flow vector and is angling in a direction that is > less than perpendicular to the wave flow vector. In order to turn down > Kelly has to unweight his inside rail and decrease the angle between the > boards bottom (in the athwart shipdirection or across) and face of the > wave. Second the amount of wake that is on either side of the board is not > heavily favored to the outside rail side. So, yes in this picture I do > believe that the resulting combination of vectors would be less than say > 10 degrees askew from the stringer. Interesting. Thank you.
If you look at the spray of a tow board (going very fast) and a regular > board (going relatively slow) there is no difference in the direction of > spray. I think the flow vector does not change significantly with forward > motion or momentum. It may be directed slightly toward the stinger by drag > between the fluid and the board, but that would remain a relative > constant. Redirection of the flow by fins or bottom contours remain, in my > mind, the only way to change the flow vector across any portion of the > board. newbalonie, You bring up an interesting observation. But, with tow board the momentum vector is so much greater that many of the other influencing vectors can be reduced. Tow boards have much smaller fins, typically vee bottoms in the tail and much less buoyancy. Consequently, the wake of a tow board does not necessarily need to fan out as much. But, it will be much longer trailing behind. And that gets minimized because the extra velocity prevents the board from displacing as much water in depth. The faster the boards go the less they penetrate the surface.
You bring up an interesting observation. But, with tow board the momentum > vector is so much greater that many of the other influencing vectors can > be reduced. Tow boards have much smaller fins, typically vee bottoms in > the tail and much less buoyancy. Consequently, the wake of a tow board > does not necessarily need to fan out as much. But, it will be much longer > trailing behind. And that gets minimized because the extra velocity > prevents the board from displacing as much water in depth. The faster the > boards go the less they penetrate the surface. Forget the tow board - go back and look at photographs in any Surfer Magazine and you will see that the direction of spray appears consistent from board to board regardless of speed and shape. Cutbacks, bottom turns, trim, it all looks the same to me. Look at Kelly’s board in Kevin’s photo: the first bit of spray on the outside rail appears to come off the board at a high angle to the stringer and the hull speed is probably near maximum for that design. On a sligthtly different tack, the AMOUNT of spray is probably controlled by tau; more tau, more spray…maybe the groms should start saying “throwing tau”… Newbs
Forget the tow board - go back and look at photographs in any Surfer > Magazine and you will see that the direction of spray appears consistent > from board to board regardless of speed and shape. Cutbacks, bottom turns, > trim, it all looks the same to me. Look at Kelly’s board in Kevin’s photo: > the first bit of spray on the outside rail appears to come off the board > at a high angle to the stringer and the hull speed is probably near > maximum for that design. On a sligthtly different tack, the AMOUNT of > spray is probably controlled by tau; more tau, more spray…maybe the > groms should start saying “throwing tau”… Newbs Newbaloney, Could it be that the amount of spray is relative to the amount of tau? Surfing is a balancing act. In that, for roller coaster type moves, a surfer engages more inside rail to get more tau to rise up the wave and disengages or unweights the rail to drop back in. So, even if one board has a greater ability than another board to produce more tau, the rider would only adjust their weight to produce the tau they need.
I agree Dale, but given the situation that I described, do you disagree > with the flows? And if you do, how? And how might your take on the flow be > interacting with a tail bottom contour? Or maybe you think I’ve just got > it all wrong. If you do, perhaps you might be willing to suggest an > alternative view.>>> Kevin Kevin, I
ve really been delighted by the focus of your threads, and a good part of that interest has been because my personal attraction to surfing and surfcraft design has always come from another direction... having analyzed and interpreted my experiences and observations by means of introspective analysis, intuition and the senses... along with a great deal of tedious trial and error. Im definitely more a sensualist than a mathematician! Much of what I understand now, was first learned (frequently by accident) when I was much younger, long before I had ever heard specific terms correlating to the movement of an object across the water of a wave, which is also in motion… for example, I would estimate that theres far more going on (underneath a given design) with the actions of a surfer riding a surfboard on a wave, than with an individual being pulled by a boat, across a static body of water (such as a lake) whos riding a wakeboard. IMHO, a fascinating part of what is neccesary to buttress any mathematical theories/explanations of these phenonomena would be to also employ a working scale model, affording the observer an opportunity to carefully document and repeat data gathered from a disconnected and impartial vantage point. As an alternative view, I would offer the wake of a high performance surfmat at speed… their distinctive, very low spray pattern clearly demonstrates an advanced level of planing efficiency, yet with minimal penetration and disruption of the waters surface. As tom@daumtooling so succinctly stated, "the faster the boards go, the less they penetrate the surface"... and with less breaking of the surface, there comes a progessive loss of control in rigid surfcraft designs that mainly rely on penetration of the fin(s) and rails for control. Other means must then be employed to compensate, such as tighter contact between the rider and board via foot straps, increased displacement of the tail area, as well as greater overall weight. In contrast, the best surfmats develop balanced levels of control in response to increasing wave power and velocity through close conformity and adaption to the waters surface, utilizing a continually changing, larger “footprint”, if you will. In addition, rigid surfcraft designs that are dependant upon pure planing become increasingly vulnerable to water surface irregularities. Of the two basic means to travel across the surface of a wave… planing boards and transitional volume displacement hulls, only high performance surfmats blend the wide operating range of planing and displacement with multidirectional, flexural adaption, even going so far as to mimmick the actual surface textures… sort of like a little hovercraft. At some speed, any rigid planing form will become functionally uncontrollable. Therefore, when using rigid surfcraft, the only means of “adapting” to a variety of waves` increased power, surface textures and their resultant velocity, must come from of a “quiver” of compromises… different shapes, fins, weights, etc. Dale
But here I offered a view as to what I believe is going on in general > (picturesque?) terms with respect to the direction of the flow under the > tail in certain trim situations. And I used the particular case, > illustrated by the Slater photo, in the hopes of making it clear. And if > one accepts it, I then go on to suggest how a tail bottom contour (here a > concave), subject to such a flow, might be operating. I think it get’s way to general, just as the anaology with a flat surface planing. It’s just to simple to work on for a surfboard IMHO. On the other hand, I might be wrong. Just thinking about it, compared to the surface of the wave the board might be flat.>>> My major point being that if the flow is as I have suggested, then the > bottom contour is interacting with the flow in a manner, and this may just > be my impression, that is not generally held by most - across from rail to > rail and back a little, rather than from nose to tail. I still think the flow would be more from nose to tail then you suggest. According to the thesis Tom refered to the surfer was moving at 5-10m/s. A 2m@15sec swell has an upward ‘flow’ of 2m/15s/2 = 0.26m/s. Ofcourse this is for a deep ocean swell. Anyone have the formula for how energy conservation of a wave going from deep water to shallow water acts? I suspect the change is not all that radical since the energy is conserved(or even lost to friction with the seabed). regards, Håvard
If you look at the spray of a tow board (going very fast) and a regular > board (going relatively slow) there is no difference in the direction of > spray. I think the flow vector does not change significantly with forward > motion or momentum. It may be directed slightly toward the stinger by drag > between the fluid and the board, but that would remain a relative > constant. Redirection of the flow by fins or bottom contours remain, in my > mind, the only way to change the flow vector across any portion of the > board. If you look at the inside rail spray, it seems to me like the flow is parallel to the stringer, much more so then expected. However, the ‘wake spray’ of a board does not seem very straight when going slow, it seems to spiral more into the wave. This might be due to the way the wave moves(ie. the whitewater and wake moving up on the breaking wave) or indicate an angular flow under the board. My money is on alternative number one. regards, Håvard
Below is a picture of Slater trimming. He may not actually be trimming, > but given his position on the wave, and given the way he’s using his body > my guess is that he’s trimming.>>> … Flow Across the Tail - Rail to Rail and Back? This, notion of a flow being more rail to rail than nose to tail is, I believe key in understanding how tail bottom contours might work. I’d like to provide a little more evidence(?) to support the rail to rail idea. There are some observations about the original Slatter picture which I like to mention. They are of course, IMO. Slatter is not moving any faster towards the beach than the wave is, that is he’s got the same shoaling speed as the wave. Slatter is likely moving at the same speed as the curl. That is, given the wave is breaking left (unless they mirrored the photo) Slater is moving at (or roughly so) the same speed in the down-the-line direction as the breaking portion of the wave. Slatter is likely staying put in terms of height on the wave face, that is he’s not dropping a whole lot, or climbing a whole lot. Slatter body posture suggests he’s ‘fine’ tuning, which not something you generally see when things are changing, say dropping, turning, etc. The above four observations tend to suggest that Slatter is trimming, i.e. Slatter is in a steady state, or about as close to a steady state as there is in surfing. Steady states are useful because the are not just momentary and potentially allow for a greater (observational) analysis. In this case, analysis of the possible evidence which can be gotten from the photo. … fin wakes Nice thing about the standard tri-fin setup, all three fins are out of alignment with each other. Slatters right fin is producing a nice wake, so is his center fin (or maybe its just his leash), however there’s no obvious wake from his left fin. If his left fin was producing a nice wake, it could be obscured by the left rail wake, or maybe even his rear leg, I simply don’t know. The point here is that if his left fin was roughly in alignment with the flow (a flow with a direction closer to Tom’s comments, see Tom’s posts in this thread) then one might not expect very much of a wake at all. At the same time, it may be producing a wake, but because of all thats happening back there (in the tail region), its just isn’t possible to make out the wake, or its form is slightly different that the other two. So, here’s another picture of Slatter trimming (at least I think it is Slatter), a little higher in barrel, and from a different angle. The disturbance from his right or wave-side fin (he’s headed right on the wave) is apparent, or at least I believe it to be. Similarly for his center and left fins. (I’m not completely convinced that Slatter has achieved the same level of steady state that he had in the original photo, which does make a difference.) But if that is a wake from his right fin then it tends to suggest that the flow is indeed from rail to rail and back. Also, it could be argued that this is supported by where the wake first becomes apparent along the rail. This hardly refutes Tom’s agruements. Hell, the reason I’m posting these Rocket Science threads is to gain some insight beyond my own subjective(?) analysis. Perhaps, in the original Slater photo the wave-side fin wake was not as pronounced because of Slater’s low position on the wave, or what I’m doing here is comparing apples to oranges. Still, I tend to think that the rail to rail flow is valid, and not only in the tail. And that it forces one to reconsider how bottom contours might be operating in general. Perhaps, you may not like asymetric boards, but they can make a difference. My point being that, if a large component of the flow is rail to rail than one might consider asymetry in the bottom contour as a viable application. Something which has already been done, and discussed on this site. (Which is not introduced as evidence, just observation.) Water line and speed (Another House of Cards?) In displacement hulls, waterline is king - the more waterline, the higher the crusing speed. There’s a whole theory here and I will not go into it, hopefully most are familar with the principle, at least for sailing boats. A sailing friend, during a discussion of surfboard design said, like sail boats waterline is speed, and that’s why longboards ‘go faster’ than shortboards. At first I bought into this, now I’m not so sure. First off, if the flow is more rail to rail, then what exactly is the actual length of the waterline on a surfboard? For a longboard, it would appear to be a hell of a lot less than board length, same for a shortboard. Actually, it could be argued that shortboards and longboards have pretty similar waterlines, accepting the rail to rail flow notion. (The width of surfboards tending to differ by inches.) The real difference between the two lying in the total (on average) wetted surface. There’s also this idea that what we are dealing with is not really a displacement based application, but some sort of hybrid between a displacement and planing. So a strict application of the waterline principle is missaplied here anyway. I guess, in a nutshell, I just don’t think the standard waterline ‘line?’ applies here, at least it doesn’t apply to boardlength… but this begs the question, why do longboards go faster? Or do they? (By the way, going faster is not the same as acceleration, and in my opinion shortboards accelerate far better than longboards.) But I guess that’s another thread. Kevin
Dale, Tell me where I can go on the Net to see some shots of mat surfing, or better recommend some (pictures) that might reveal a little about what is going on. Kevin 