As for higher velocities flattening the curve? Geee, thats interesting, my > sense is that it does, but I should be clear as to my interpretation of > ‘flatten’; Flatten means to reduce the peak height, distribute the pressure more equally across the length, L.>>> When you shape a board, the temptation is to visualize water flowing from > nose to tail, past the bottom, along the rails. In fact, though this > definately happens a lot during surfing, I tend to think that during those > moments when you’re trimming, even making those big turns, that the rail > becomes an extension of the bottom, given the water flow. (This may not be > the obvious statement it seems to be, please continue…) To understand your last drawing, you have to ignore forward motion (motion toward the viewer). Look at it as a snapshot of trimming rather than a movie. At least that’s how I’m doing it, and it all makes sense.>>> Under most circumstances, the water under the board moves, from one side > to the other, not really from nose to tail. Actually, this is not strictly > so, but the water definately moves across the bottom at a angle. (In a net > sense, a large component of it does move from one side to the other > though.) The function of the rail then becomes what we generally attribute > to the ‘tail’ I think this might be where you were headed above… > maybe… please continue Exactly! You have to break it all down into pieces to understand how it all fits together in the end (or tail).>>> Before getting into rails, some definitions are in order. (Perhaps, > someone will come forward with some terminology that we’ll all agree on.) > For now, I’ll just state what I need. A rail very broadly speaking has > three features; there’s the apex, there’s the rail bit above the apex, and > then there’s the bit below the apex. In the above, given the way the board > is angled on the face (see my final diagram), I’m suggesting that the rail > below the apex is an extension of the bottom, the apex functioning as a > ‘tail’. What’s important is that the water doesn’t really flow parallel to > the rail, but across the bottom, up the rail (the bit below the apex) and > off at the apex. As for the bit above the apex, at the moment I seem to be > in the camp that doesn’t much care what shape it has, as long as it > doesn’t get in the way. Your definitions are fairly standard. I used to think the curve above the apex did not matter until I began to ride a board with highly pinched (small apex angle) rails.>>> So, with this interpretation, my answer to what you were addressing above > is, I agree - the rail can be sticky, in as much as the bottom is sticky. > And like in marine architecture, a clean break at the tail is prefered for > high speed craft in order to avoid any unecessary drag and interaction > with the wake. (Consider the design of fast boats, or even modern sailing > boat sterns.)>>> But nowadays, the standard approach is to sharpen up the rear rails and > soften them up as you move forward. (This leads me back to considerations > of the flow differentials that a surfboard can experience do to its > position with respect to the curl, a term which I use to refer to the zone > where the water has its greatest upward velocity. But it would be too much > of a degression.) But given what I’ve just said, soft forward rails would > therefore have the effect of turning you into and up the wave, by shear > stickiness, drag and interaction with the wake. (A soft forward rail also > avoids a lot other kinds of trouble in general too.) That likely sounds > pretty wild to some. The fact is, I have no references to back me up. I > suspect I may be alone here. This might be true if you engaged the entire rail during trim. However, the eliptical outline keeps the softened forward rails disengaged to a large degree. In a rail buried bottom turn or cutback, the effect will contribute to completing the turn.>>> One more point, when the water does happen to be flowing from nose to > tail, say during those big drops, rails, especially soft ones tend to > function as fins - they help you track.>>> Curiously, the equations developed for flat surface planing were done by > only considering two dimensions, the plan, or craft, is not considered to > have a side, i.e. its doesn’t have a thickness. That the equations are > more or less verified by experiment, in my opinion says a lot about rail > design. In an earlier post I discussed rail function by spending a good portion of a session looking hard at my rails during different portions of a ride. The rails were rounded, 60/40 shape with a tucked under edge. At no time did I see water ride over the rail above the apex. I was convinced that water only came in contact with the lower surface. I then rode a board with severly pinched rails (small apex and large radius curves above and below; more "
I’ve never used a surf mat… after reading this post, it would appear > that I have something to look forward to. Thank you. Kevin, Thank you… but don`t forget to consider about the other portion of my previous email, for which I made reference… displacement hull surfboards such as those created by Greg Liddle www.liddlesurfboards.com/ Dale
Well, a portion of my post got deleted so: Water flowed over the pinched rails and onto the deck. The smaller apex angle appears to significantly affect the hydrodynamics of the rail the therefore its performance. The two boards perform compelely differently. This stuff may be boring and useless to others, but I’m fascinated. Newbs
Well, a portion of my post got deleted so: Water flowed over the pinched > rails and onto the deck. The smaller apex angle appears to significantly > affect the hydrodynamics of the rail the therefore its performance. The > two boards perform compelely differently.>>> This stuff may be boring and useless to others, but I’m fascinated.>>> Newbs Newbs If you can, draw a picture of the rails and if possible the way the water flowed and post it Please? Thanks Kevin
(This is a long reply, my apologies… but I couldn’t resist.)>>> Yes? Tau, all else being constant, will likely decrease with forward > speed, well at least eventually, but not at first it seems. The diagram, > and the principle it represents assumes a steady state planing, screw with > it and it will likely enter into various other irreversible states until a > new steady state is achieved, but in general, yeah, I agree.>>> As for higher velocities flattening the curve? Geee, thats interesting, my > sense is that it does, but I should be clear as to my interpretation of > ‘flatten’; its in reference to the over shape, independent of the numbers. > (Nice. I’ll be checking this, and get back to you if I find something > interesting.)>>> Rails! Here’s a short story made long…>>> When you shape a board, the temptation is to visualize water flowing from > nose to tail, past the bottom, along the rails. In fact, though this > definately happens a lot during surfing, I tend to think that during those > moments when you’re trimming, even making those big turns, that the rail > becomes an extension of the bottom, given the water flow. (This may not be > the obvious statement it seems to be, please continue…)>>> Under most circumstances, the water under the board moves, from one side > to the other, not really from nose to tail. Actually, this is not strictly > so, but the water definately moves across the bottom at a angle. (In a net > sense, a large component of it does move from one side to the other > though.) The function of the rail then becomes what we generally attribute > to the ‘tail’ I think this might be where you were headed above… > maybe… please continue>>> Before getting into rails, some definitions are in order. (Perhaps, > someone will come forward with some terminology that we’ll all agree on.) > For now, I’ll just state what I need. A rail very broadly speaking has > three features; there’s the apex, there’s the rail bit above the apex, and > then there’s the bit below the apex. In the above, given the way the board > is angled on the face (see my final diagram), I’m suggesting that the rail > below the apex is an extension of the bottom, the apex functioning as a > ‘tail’. What’s important is that the water doesn’t really flow parallel to > the rail, but across the bottom, up the rail (the bit below the apex) and > off at the apex. As for the bit above the apex, at the moment I seem to be > in the camp that doesn’t much care what shape it has, as long as it > doesn’t get in the way.>>> So, with this interpretation, my answer to what you were addressing above > is, I agree - the rail can be sticky, in as much as the bottom is sticky. > And like in marine architecture, a clean break at the tail is prefered for > high speed craft in order to avoid any unecessary drag and interaction > with the wake. (Consider the design of fast boats, or even modern sailing > boat sterns.)>>> But nowadays, the standard approach is to sharpen up the rear rails and > soften them up as you move forward. (This leads me back to considerations > of the flow differentials that a surfboard can experience do to its > position with respect to the curl, a term which I use to refer to the zone > where the water has its greatest upward velocity. But it would be too much > of a degression.) But given what I’ve just said, soft forward rails would > therefore have the effect of turning you into and up the wave, by shear > stickiness, drag and interaction with the wake. (A soft forward rail also > avoids a lot other kinds of trouble in general too.) That likely sounds > pretty wild to some. The fact is, I have no references to back me up. I > suspect I may be alone here.>>> One more point, when the water does happen to be flowing from nose to > tail, say during those big drops, rails, especially soft ones tend to > function as fins - they help you track.>>> Here’s the last bit…>>> Curiously, the equations developed for flat surface planing were done by > only considering two dimensions, the plan, or craft, is not considered to > have a side, i.e. its doesn’t have a thickness. That the equations are > more or less verified by experiment, in my opinion says a lot about rail > design. in regards to rails-it sounds like you are referring to the type of rails Simmons,Greenough, Liddle, Frye and Gross have utilized for years, so I don’t think you are alone on this.(although I will admit there is only a handful of guys committed to riding those type of boards-so in a sense we are “alone”)
Newbs>>> If you can, draw a picture of the rails and if possible the way the water > flowed and post it Please?>>> Thanks>>> Kevin Man, I knew I’d get busted one of these days…I don’t know how. Let me try this; pinched look like
I rushed that last diagram. Here’s what it should looked like. Its a slice through a surfer and surfboard at about the center of the board. Notice that I’ve now indicated that the water is moving with velocity V, not the board. (It amounts to the same thing, it is relative.) The motion of the surfer/surfboard is out of the paper towards the viewer. Notice the wake coming off the rail. This, I believe is important in considering rail design. (I’m not sure that this interpretation has been given to the effect, but its likely somebody may have considered it and called it something else. If this interpretation is correct then the existance of a wake has real design consequences. Its a strange place for a wake, but I believe its there under most conditions.) I’ve also indicated the ‘down’ direction, i.e. the direction of Gravity. Remember that the surfer/surfboard has an angle D with respect to the flow. This is important since its where he gets his drive, or lift (in hydrodynamic terms), or I guess you could even call it thrust. Hopefully we will get to that shortly. Sometime this weekend, I will try and get around to a diagraming sections further back, showing the fins, and further forward. My apologies for the delay. 
Kevin, I think there is a flaw with the adaption of the first equation you posted. First of all it’s related to a flat plane. A surfboard is not. Thus you’d end up with alot of vectors which are perpedicular(sp?) to the bottom of the board where it’s in contact with water. Basically you end up with an integral to find the total amount of force applied to the surfboard and it’s direction. Thus the 1/2pvsquared with curve only apply to a very small part of the board IMHO. regards, Håvard
All sorts of weird things have been found to function on a basic level in > deep powder that simply will not work at all in harder snow… other than > just sliding downhill at high speed. In addition, the relevance of precise > leverage via firm, positive boots and bindings is of much higher > importance in hard pack than in deep powder. One snowboard movie had someone riding a surfboard in deep powder. I don’t remember the name of the movie nor the rider. It did seem to work. regards, Håvard
Well, a portion of my post got deleted so: Water flowed over the pinched > rails and onto the deck. The smaller apex angle appears to significantly > affect the hydrodynamics of the rail the therefore its performance. The > two boards perform compelely differently. How? regards, Håvard
Well, a portion of my post got deleted so: Water flowed over the pinched > rails and onto the deck. The smaller apex angle appears to significantly > affect the hydrodynamics of the rail the therefore its performance. The > two boards perform compelely differently. The fact that the softer/thicker rail didn’t penetrate had me thinking about some other thread related to the swizzle where someone said something along the lines that ‘Y’ had only removed what was not neccesary. Maybe that’s it. Since the rail does not get wrapped in water, it would seem like you have to use alot of force to engage it(the water has to go somewhere) So, if you remove the part of the board that doesn’t penetrate the water anyway, what do you have to loose? Gains? Less leverage needed and possibly more rail engaged thus also more of the rocker is engaged leading to better turning. Think about a thruster where for the most part only the rear half of the board is in the water. Maybe that’s what the swizzle is all about.>>> This stuff may be boring and useless to others, but I’m fascinated. Me too! regards, Håvard
Kevin,>>> I think there is a flaw with the adaption of the first equation you > posted. First of all it’s related to a flat plane. A surfboard is not. > Thus you’d end up with alot of vectors which are perpedicular(sp?) to the > bottom of the board where it’s in contact with water. Basically you end up > with an integral to find the total amount of force applied to the > surfboard and it’s direction. Thus the 1/2pvsquared with curve only apply > to a very small part of the board IMHO.>>> regards,>>> Håvard The simplified treatment of a flat plane planing, is, well simple(?) Its meant more as a design quide. Even in its application by marine engineers, it seems that they are quick to move to more empirical data, i.e. charts and tables generated by experiments with planes and hulls in water tanks. So, I agree, a surfboard is not flat, and that caution must be applied here in the interpretation, but I believe the basic idea is sound. At the moment, however, I don’t believe I completely understand the problem you’ve raised (see below, … some diagrams would be helpful.) But If you’re looking for a more accurate picture of the pressure distribution or some sense of the numbers, expect to have some problems. My attempt was to find some basic principle upon which some design principles might arise, and if possible to connect it with some observable. I’ve since abandoned all hope is getting an objective evaluation from surfers, or even my own experiences riding my own, or other boards. Conditions and individual technique simply vary too much. But, the ‘spray root’ seemed to offer such an opportunity. I’ve been screwing around in the area fluid dynamics for a quite a while and after being overwhelmed by its complexity, I stepped back to see if there was at least some basic principle connected to some observable present during surfing, that I didn’t need much to observe it with, i.e. that I could see. And I believe the spray root is just that, its always there (virtually)in one form or another, and we now have a crude interpretation of its significance. That it could mean something else is a real possibility though, but I’m convinced its interpretation, as offered in the original diagram regarding planing, is on the right tract. To suggest that there’s a flaw in the application is absolutely correct!!! These ain’t planes (well almost, skim board surfing is, but that’s another thread), but the basic principle is sound, planing is key. (By the way, this suggests another thread, a potentially interesting one regarding bottom contours.) The pronounced spray root which is evident under surfboards when surfing, its form, where it starts and ends, and its volume, its direction, in my opinion all supports the planing hypothesis. The exact profile of the spray will vary continuously, as will the pressure distribution, so it all can get a bit fuzzy, but I suspect that, whatever it’s exact nature, the peak pressure is not far from the spray root. But more importantly, it can be used by the designer to interpret what might be going on under the surfboard, and he only need to ‘look’ to get the information. The argument is basically that the stagnation line, or a kind of ‘flow shed’ is the point of maximum pressure. Crudely, On the tail side of the line, the net flow has some given value, on the other side the net flow is reduced from this value because of the presence of the spray, or water having been “forced” to move contrary to the flow i.e. there’s a energy differential, and the difference more or less shows up in the pressure profile. (The arguments are similar to the Integral arguments used in Reynold Transport Theorem, net flows and masses are consider, its not a Differential approach, which though prefered, is rare.) I can really appreciate your analysis though. I bet we can take this a lot further, but first we have to make sure we’re talking about the same thing; and that’s why I rely heavily on diagrams. Perhaps at some point you’d be kind enough to diagram the kind of flow situation you had in mind when you developed you arguments. (I know, I’d love to see them. I’m ignorant; I know I’m missing a lot, and I can only stand to improve my knowledge in this area.) My hope is that there may be some basic principle that applies here, but to expect that the way it is applied always takes the same form, would be expecting too much, things change, especially under a surfboard. Please consider making some diagrams… and great stuff. Kevin
Here’s the flow I’m refering to. It took me two diagrams. This diagram is pretty much everywhere in the literature, and is in fact easily verfified by observation. Watch surfers waiting for waves, as a wave passes they bob up and down, in fact they make a little circle. next post… 
Here’s the second. 
Here’s the flow I’m refering to. It took me two diagrams.>>> This diagram is pretty much everywhere in the literature, and is in fact > easily verfified by observation. Watch surfers waiting for waves, as a > wave passes they bob up and down, in fact they make a little circle. Doesn’t meen that water moves with the wave. A circle makes perfect sense. They move inward as they lay in the shoreward downhill side and move outward on the seaward downhill side after the wave passes. Maybe something else is going on as well, but it’s not flow. A wave is not a flow. regards, Håvard
The argument is basically that the stagnation line, or a kind of ‘flow > shed’ is the point of maximum pressure. Maybe. But unless the speed is very high 1/2pvsquared doesn’t apply much force. I withdraw the argument that it doesn’t apply to a curved surface since the equation does not include angle or area of any sort. Think about this. A surfer who weight 80 kgs will need a support of 80ksx9.81ms/s2 = 785kgm/s2. 1/2pv2 at 10m/s(36km/h, significant speed) equals only 50kgm/s2. Bouyancy me be able to support much, but not all of the riders weight specially on a shortboard. So where does the rest come from? We need the rest of the equations! Only then we’ll know if the pressure along the stagnation line or spray area or whatever is significant. Also, since we’re working in two dimention(the sprayline is spread out along the length of the board(turn) or across the nose), the actual force is distributed over a larger area, not? regards, Håvard
Well, a portion of my post got deleted so: Water flowed over the pinched > rails and onto the deck. The smaller apex angle appears to significantly > affect the hydrodynamics of the rail the therefore its performance. The > two boards perform compelely differently.>>> This stuff may be boring and useless to others, but I’m fascinated.>>> Newbs newbs- i’m fascinated as well. two weeks away from the computer and i’m spending hours catching up on these threads that need to be read and re-read for full comprehension. wish my mind worked at the level yours does. keep up the good work and cheers from us on the sidelines. jim dunlop
newbs- i’m fascinated as well. two weeks away from the computer and i’m > spending hours catching up on these threads that need to be read and > re-read for full comprehension. wish my mind worked at the level yours > does. keep up the good work and cheers from us on the sidelines. jim > dunlop Thanks… my next rant is coming out soon… more diagrams and confusion, here’s my working title, Rocket Science:“I’m deeeewing all I caahhnn, Captain.” Chief Engineer Scotty, USS Enterprise. I attempt to ballpark the flow rate and force on a surfboard during surfing, (real numbers!) including the effects of adjusting the parameters involved, also some of the consequences of surfing technique. Basically its an attempt to estimate what actually is available to play with in terms of acceleration, and maybe suggest how to get the most of what’s there (or maybe just point out those shapers and surfers who apparently discovered it a long time ago.) Its nice to know somebody else also enjoys this stuff. Its also nice to have somebody like Swaylock to provide a forum for it.
Kevin, Someone down the line described the “spoon-in-the-flow” experiment to you a couple of days ago…I may have misunderstood your response, but it seemed that you felt that the hand holding the spoon was applying an unseen force on the spoon. Before you go on to new and different things, go to the sink with a spoon and try it. It is quite an eye opener. It answers your rail and noserider tail questions (drag results in “downward” or “waterward” reaction) and concave nose for a noserider (redirected spray resulting in “lift”). By the way, nice shape in the board archives! Newbalonie
Kevin,>>> Someone down the line described the “spoon-in-the-flow” > experiment to you a couple of days ago…I may have misunderstood your > response, but it seemed that you felt that the hand holding the spoon was > applying an unseen force on the spoon. Before you go on to new and > different things, go to the sink with a spoon and try it. It is quite an > eye opener. It answers your rail and noserider tail questions (drag > results in “downward” or “waterward” reaction) and > concave nose for a noserider (redirected spray resulting in > “lift”).>>> By the way, nice shape in the board archives!>>> Newbalonie Thanks about the board. It wasn’t my intention to minimize the value of the experiment. I just wanted to point out that the spoon had to apply a force on the flow to get the effect described. For me the ‘money for nothing, chics for free’ phase kind of summed it up. (Hence the title.) I am always tripping over Newton’s Third Law, ‘for every action there is an equal and opposite reaction.’ So much so that it now tends to be my almost my first consideration. But the phase ‘Money for nothing…’ is also in reference to a very general principle which seems to hold everywhere in the Cosmos, called Conservation of Energy. So, (and you’re right, I shouldn’t just move on before such comments have been addressed), lets consider the a tail concave, and lets start with Newton’s Third law. Lets use Eaton’s bonzer diagram as starting point (http://www.eatonsurf.com/Bonzer.htm) Eaton’s bonzer has some very pronounced and interesting contours. (His site also has some good pictures.) The diagram below is a more general view of a single tail concave. But it could just as easily have been one of the walls on Eaton’s bonzer contours. (I am, like Eaton, only using two diminensions in the diagram. A more complete analysis would call for three dimensions. The point that I hope to make however can be made in two.) The direction of the flow is given by the blue arrow. The resultant force R on the concave wall by the red vector, R. The walls must apply a force to the flow in order to get it to change direction, but they don’t do it in a ‘vacuum’ so to speak, for the flow is applying an equal and opposite force on the walls. R can be resolved into contributions in two directions, here shown as green vectors; one in the direction towards the rail, the other towards the tail. Basically, it is the force towards the tail that I was refering to in the thread you mentioned. It will actually work to slow the surfboards forward speed down. Moving water around costs. Next thread, Conservation of Energy.