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 Kevin, I think the biggest difference of how we look at these theories is one of dynamics. You look at the pictures and attempt to draw static conclusions. I look at them and think that many of the vector influences are constantly changing. You mentioned earlier that “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.” But that’s a matter of choice of direction by Kelly. If he needed to out run the wave towards the beach, he would be able to for as long as his momentum was able to keep him on a plane. On a short board this is not very far unless you have alot of speed. But, it’s usually far enough not to take the lip on your head. This leads into waterline length argument. A longboard will glide further than shortboard. Meaning if you have a longboard and a shortboard with equal weight optimally placed on both propelled by an equal force across a flat body of water, the longboard will go further due to increased waterline length. But, since a longboard resists direction change relative to a shortboard, it is more difficult to take advantage of pumping. Pumping is the act of connecting bottom turns and top turns in an efficient series to exploit the the tau and gravity vectors optimally. Which leads to another point, surfing is not about out running the wave. It’s about utilizing the force of the wave in a creative manner. This quite often means staying in the most powerful portion of the wave and not out running it until you need to. If you look at Kelly in your latest picture you will note that his weight is positioned over his inside rail. He has chosen to engage more tau. Consequently, the spray is coming off his outside rail more obliquely than in the previous picture. He is starting a gradual climb back up the face. In a second or two he will transition, by shifting his weight towards the stringer. The wake will take on more of an appearance to the first picture you posted and Kelly will have completed one subtle pump.
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, In studying the photo of Kelly, my initial response is to question the assertion that he
s even trimming at all... in full trim, no matter what the surfcrafts length, a rider`s position on the deck (and therefore his weight) will not be shifted/angled as far towards the back of the board and inside rail. It appears that Kelly is merely in transition… Dale
Kevin,>>> I think the biggest difference of how we look at these theories is one of > dynamics. You look at the pictures and attempt to draw static conclusions. > I look at them and think that many of the vector influences are constantly > changing. You mentioned earlier that “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.” But that’s a matter of choice of > direction by Kelly. If he needed to out run the wave towards the beach, he > would be able to for as long as his momentum was able to keep him on a > plane. On a short board this is not very far unless you have alot of > speed. But, it’s usually far enough not to take the lip on your head.>>> This leads into waterline length argument. A longboard will glide further > than shortboard. Meaning if you have a longboard and a shortboard with > equal weight optimally placed on both propelled by an equal force across a > flat body of water, the longboard will go further due to increased > waterline length. But, since a longboard resists direction change relative > to a shortboard, it is more difficult to take advantage of pumping. > Pumping is the act of connecting bottom turns and top turns in an > efficient series to exploit the the tau and gravity vectors optimally.>>> Which leads to another point, surfing is not about out running the wave. > It’s about utilizing the force of the wave in a creative manner. This > quite often means staying in the most powerful portion of the wave and not > out running it until you need to. If you look at Kelly in your latest > picture you will note that his weight is positioned over his inside rail. > He has chosen to engage more tau. Consequently, the spray is coming off > his outside rail more obliquely than in the previous picture. He is > starting a gradual climb back up the face. In a second or two he will > transition, by shifting his weight towards the stringer. The wake will > take on more of an appearance to the first picture you posted and Kelly > will have completed one subtle pump. It sounds like you got a lot of developed ideas on these topics. Perhaps you might consider starting some threads and sharing them with us. Kevin
Kevin,>>> In studying the photo of Kelly, my initial response is to question the > assertion that he
s even trimming at all... in full trim, no matter what > the surfcrafts length, a rider`s position on the deck (and therefore his > weight) will not be shifted/angled as far towards the back of the board > and inside rail. It appears that Kelly is merely in transition…>>> Dale There’s no doubt that you might be right. My interpretation is subjective, but I do give my reasons. For me the real klincher is Slater’s body posture, it appears to be intentionally(?) contorted. Admittedly, I’ve interpreted the significance of his posture using my own experience (Not that my surfing should even be mentioned in the same breath as Slater’s. In fact, if we define what Slater does for a living as Surfing, what I do is likely to be considered another sport. Slater is a Master.) In the past however, on occasion, I have often find myself making the same face that Slater is making in the photo, especially when fine tuning. But its not just his face, his chest is pushed forward his head tucked back, his arms tense and for me at least, deliberately held in position to balance something? Also his feet seem to be squarely planted on the deck, his weight neither shifted to his heels or toes. To make any other manuver, it seems to me he’d have to uncontort first. In the picture he seems pretty committed, at least to me. But your right, I might be dreaming. So can I take it that you’re inclined to disagree with the rail to rail flow notion? Kevin
There’s no doubt that you might be right.>>> My interpretation is subjective, but I do give my reasons. For me the real > klincher is Slater’s body posture, it appears to be intentionally(?) > contorted.>>> Admittedly, I’ve interpreted the significance of his posture using my own > experience (Not that my surfing should even be mentioned in the same > breath as Slater’s. In fact, if we define what Slater does for a living as > Surfing, what I do is likely to be considered another sport. Slater is a > Master.) In the past however, on occasion, I have often find myself making > the same face that Slater is making in the photo, especially when fine > tuning. But its not just his face, his chest is pushed forward his head > tucked back, his arms tense and for me at least, deliberately held in > position to balance something? Also his feet seem to be squarely planted > on the deck, his weight neither shifted to his heels or toes. To make any > other manuver, it seems to me he’d have to uncontort first. In the picture > he seems pretty committed, at least to me.>>> But your right, I might be dreaming. So can I take it that you’re inclined > to disagree with the rail to rail flow notion?>>> Kevin Gee Kevin, I thought I was sharing. But, getting back to your interpretation. I don’t think that either Dale nor I am disagreeing that to some degree water does flow across the tail. We just don’t believe that the degree of influence you give to the tau vector in these examples. Kelly or any competent surfer descides how much influence they want from the power rising up the face, by very subtle changes in weight placement. As for his feet being squarely planted, please look more closely at his back foot. He’s on the ball and toes of his back foot. His body is definitely weighting on his inside rail. He was on a down slope track and he’s in the process of transitioning to a climbing path. But, it’s all in subtle moves.
If (that’s if) the flow is rail to rail and back, something close to what I described in the original post, I believe flex, if controled is likely to find some application in stand-up surfing. Control remains key, and my guess is that it will take the form of coupling the surfers feet to the board somehow. (Of course there are other vehicles and other solutions, i.e. surf mats, etc.) This assumes that the board is actually flexible, and not marginally so as with current surfboards using resin-glass composite construction. (Resin-glass can be flexed, but I’m not sure its the best material to be ‘worked’ continuously and deliberately, its bound to crack.) The snowboard shape now begins to make sense (especially for the rail to rail notion of flow.) The diagram below has got the possible flow for two different angles with respect to the wave face, the surfer bring the front radius on and off line at will. How the flex would work likely doesn’t need more explaination other than something similar to a snowboards. (I’m guessing.) Interesting stuff? (IMO, of course.) Kevin 
Gee Kevin,>>> I thought I was sharing. But, getting back to your interpretation. I don’t > think that either Dale nor I am disagreeing that to some degree water does > flow across the tail. We just don’t believe that the degree of influence > you give to the tau vector in these examples. Kelly or any competent > surfer descides how much influence they want from the power rising up the > face, by very subtle changes in weight placement. As for his feet being > squarely planted, please look more closely at his back foot. He’s on the > ball and toes of his back foot. His body is definitely weighting on his > inside rail. He was on a down slope track and he’s in the process of > transitioning to a climbing path. But, it’s all in subtle moves. Which picture? I had assumed Dale was refering to the original Slater photo. In the second photo, I did mention that I did not think that Slater was in a similarly tight trim position (see related post.) Kevin
Which picture? I had assumed Dale was refering to the original Slater > photo. In the second photo, I did mention that I did not think that Slater > was in a similarly tight trim position (see related post.)>>> Kevin I was talking about your last picture. In the first picture Kelly is un-weighting his inside rail by moving the bulk of his mass (thighs through chest) over the center of the board and counter balancing with his head and feet. So, he doesn’t get too far over. Sorry if I was confused. Trying to work and keep track of what we’re discussing can sometimes get a little complicated.
There’s no doubt that you might be right.>>> My interpretation is subjective, but I do give my reasons. For me the real > klincher is Slater’s body posture, it appears to be intentionally(?) > contorted.>>> Admittedly, I’ve interpreted the significance of his posture using my own > experience (Not that my surfing should even be mentioned in the same > breath as Slater’s. In fact, if we define what Slater does for a living as > Surfing, what I do is likely to be considered another sport. Slater is a > Master.) In the past however, on occasion, I have often find myself making > the same face that Slater is making in the photo, especially when fine > tuning. But its not just his face, his chest is pushed forward his head > tucked back, his arms tense and for me at least, deliberately held in > position to balance something? Also his feet seem to be squarely planted > on the deck, his weight neither shifted to his heels or toes. To make any > other manuver, it seems to me he’d have to uncontort first. In the picture > he seems pretty committed, at least to me.>>> But your right, I might be dreaming. So can I take it that you’re inclined > to disagree with the rail to rail flow notion?>>> Kevin Kevin, I was referring to the first photo… the second one is a bit closer to being a full trim position, with most of Kelly
s weight moving to the front foot, accelerating slightly downward. In the first shot, I believe his body posture is simply a split seconds anticipation of an upward movement…so, leading with the head and upper body in the expected direction would be a natural response. Besides, a riders body and board dynamics are very different when surfing frontside, as opposed to backside. Regarding rail to rail flow, it seems that both photos demonstrate the rider/surfboards lateral movement due to momentum and the force of gravity acting off the slope of the wave, and the pentration/displacement of water along the back, inside rail as it opens a track across the wall of the uprushing surface of the wave. The most visible result of that displacement is sheared water or spray, which always tries to exit in the direction of least resistance. Dale