# Rocket Science: Planing Dynamics in Surfboard Design *PIC*

This new thread is an off-shoot of the Flex thread of a few days ago. Its basically a repeat of my comments to Havard (modified a little.) I see surfing as planing, and for the most part the lessons learned by marine engineers about planing can be applied to surfboard design. This is actually very important when considering flex. Not to mention, design in general, from noseriders, to template, to overall rocker considerations, in general. The fact is that for a flat planing surface (with a small angle of attack) the pressure distribution under the surface is not uniform (see diagram below, ref. Hydrodynamics of High-Speed Small Craft, by Lawrence J Doctors), it has a well defined maximum, which can be adjusted by both static design (when the board is made), and say, by surfing technique - by changing parameters, like angle of attack. The diagram below is not a surfboard, and really doesn’t reflect the extremes of attack angle that arise during surfing, and they can be quite dramatic. Still the diagram does serve to aid in understanding some of the effects we seem to deal with, in particular those of nose rocker. And even more fun, the classical noserider arguments. (My favorite is flipping the tail for that ‘leveraging effect.’) If, for the moment, you accept the diagram as an an aid to explore what might be going on under a surfboard, then consider where the refered to stagnation line (the point of maximum pressure) might be when a fellow is noseriding. Or how it might move as he starts walking around on his board (especially if his board has a pronounced rocker), or shifting his weight (or does a fellow move , or shift his weight as to be just a little behind it at any time? A better position to control what’s going on?) Analyzing longboard design using these arguments is great fun because there is so much board to analyze, but similar arguments apply to shortboards, especially in tail rocker and over-all foil considerations, i.e. from what position the board will likely be surfed. Flex? Chances are that a structure with too much flex would actually work against the noserider or surfer in general, as the pressure exterted by the stagnation line would tend to result in an even greater angle of attack (a negative feedback, the board yielding to the pressure), and in addition increasing drag, (something not really dealt with in the diagram). It would be real balancing act to get it right, especially if it was passive flex, i.e. with no way for the surfer to control it, other than by weight position. Given our current tools, I don’t think such a board is possible. (Prone and kneeboarders are different, the coupling is greater between the surfer and board.) Strap the surfer in, and things do change though, maybe… Spray Root… By the way, the term Spray Root in the diagram, refers to where the white water (or spray) shoots out from under the board when surfing. This water flow is critical, in both its volume and direction. A surfer noseriding often has a whole lot of it tumbling out from under the nose, which makes sense given where the stagnation line is likely to be. (Changing the direction of water reguires force, i.e. its where the energy to noseride in part at least, comes from, Newton’s Third law kind of thing.) At the same time a fellow going straight down the line often has a neat spray eminating from a large portion of the bottom - in the direction out of the wave, indicating a more distributed stagnation line. (The resultant force on the board being up and into the wave.) Concave Nose? My take on concave noses is perhaps too simplistic for most. Basically, nose concavity increases nose rocker, (I believe this is well understood) but with a slight twist, the direction of the spray root is altered, downward to be precise. So the net effect is to increase the spray root and shoot it down, or at a more downward angle, when your on the nose, otherwise it plays a mimimal role, since its not likely to be interacting with the water. The Spray Root is the critical thing here, its volume and direction reflects the energy exchange between the surfboard and water. So what of the Venturi effect? I don’t know, I never really understood how it was possible to get something for nothing, which when taken to conclusion the Venturi effect seems to suggest. Here, the payment is real - an increase in the volume of the Spray root and drag - the trade off for getting to the nose.

Flex?>>> Chances are that a structure with too much flex would actually work > against the noserider or surfer in general, as the pressure exterted by > the stagnation line would tend to result in an even greater angle of > attack (a negative feedback, the board yielding to the pressure), and in > addition increasing drag, (something not really dealt with in the > diagram). It would be real balancing act to get it right, especially if it > was passive flex, i.e. with no way for the surfer to control it, other > than by weight position. Given our current tools, I don’t think such a > board is possible. (Prone and kneeboarders are different, the coupling is > greater between the surfer and board.) Strap the surfer in, and things do > change though, maybe… Flex is for turning, not for planing (Dale’s mats excepted). A good flexi-board is flat when planing and begins to flex into a curve when loaded up in a turn. Once unloaded, they return to a flat configuration. The better performing flexi’s restrict the flex to the tail where the turn is controlled. The theory is that you have the best of both worlds - flat for planing and curves for turning. Sometimes, if you get it just right, the rebound will propel the board and rider forward or, in technical terms; bawang. Great thesis! QED Newbs

Flex is for turning, not for planing (Dale’s mats excepted). A good > flexi-board is flat when planing and begins to flex into a curve when > loaded up in a turn. Once unloaded, they return to a flat configuration. > The better performing flexi’s restrict the flex to the tail where the turn > is controlled. The theory is that you have the best of both worlds - flat > for planing and curves for turning. Sometimes, if you get it just right, > the rebound will propel the board and rider forward or, in technical > terms; bawang. Great thesis!>>> QED Newbs which is exactly what Greenough’s spoons are all about.

Flex is for turning, not for planing (Dale’s mats excepted). A good > flexi-board is flat when planing and begins to flex into a curve when > loaded up in a turn. Once unloaded, they return to a flat configuration. > The better performing flexi’s restrict the flex to the tail where the turn > is controlled. The theory is that you have the best of both worlds - flat > for planing and curves for turning. Sometimes, if you get it just right, > the rebound will propel the board and rider forward or, in technical > terms; bawang. Great thesis!>>> QED Newbs Good Point Newbs. I was caught in the same place. Question to Kevin. What if the potential energy in the planing surface increases with torsional load? Does the planing surface still inevitably bog down? or does it snap back and lead to an acceleration? In other words can a planing surface in an ideal situation behave like a ski or a snowboard? This is outta sight stuff! Thanks Kevin.

Good Point Newbs. I was caught in the same place. Question to Kevin. What > if the potential energy in the planing surface increases with torsional > load? Does the planing surface still inevitably bog down? or does it snap > back and lead to an acceleration? In other words can a planing surface in > an ideal situation behave like a ski or a snowboard? This is outta sight > stuff! Thanks Kevin. “Flex is for turning, not for planing (Dale’s mats excepted)…” With the right design, and under reasonable circumstances, tuned flex is integral to both high speed planing, as well as turning… bawang!

Flex is for turning, not for planing (Dale’s mats excepted). A good > flexi-board is flat when planing and begins to flex into a curve when > loaded up in a turn. Once unloaded, they return to a flat configuration. > The better performing flexi’s restrict the flex to the tail where the turn > is controlled. The theory is that you have the best of both worlds - flat > for planing and curves for turning. Sometimes, if you get it just right, > the rebound will propel the board and rider forward or, in technical > terms; bawang. Great thesis! Well put. I do believe a very limited flex in the nose would provide a nice rocker for turning also. Too much is always bad tho. regards, Håvard

Well put.>>> I do believe a very limited flex in the nose would provide a nice rocker > for turning also. Too much is always bad tho.>>> regards,>>> Håvard Interesting stuff here.As far as old style cross stepping longboards go I wonder if flex is a good thing?I am leaning toward stiffer is better.I just figured out how to put an aluminum stringer into a longboard to really make it rigid.I would like to hear what you guys think about this theory.Nice thread.

Good Point Newbs. I was caught in the same place. Question to Kevin. What > if the potential energy in the planing surface increases with torsional > load? Does the planing surface still inevitably bog down? or does it snap > back and lead to an acceleration? In other words can a planing surface in > an ideal situation behave like a ski or a snowboard? This is outta sight > stuff! Thanks Kevin. I agree with your answer, well the later one, you can recover it; if the board is constructed with the right materials, and with the right design. (I suspect you knew that, given your comments.) The guestion is can you control it, and that’s where my issues lie with flex. (By the way, mats and body boards are not good examples here. You can bring your whole body to bear on the mat or board, and hence control a hell of a lot more than with just two feet, two feet which are very loosely coupled to the board during surfing.) The problem is that unloading will not likely be ‘all up to the surfer’ unless you are able to somehow capture the stored elastic energy for later ‘controled’ release (like a spring in a clock.) I’m going to backup a moment because this is leading to some interesting bits. Remember that the definition of a fluid is an incompressible material which, when at rest cannot support a stress. That is, under any stress, water tends to flow. This is not really the case for snow. I agree that carving up powder is very close to carving up water, but that might just be one of sensation, not Physics - snow ain’t a fluid - you can make a snowman, you can’t make a waterman… (no pun?) Snow can support stress! (Snow is a weird material, and when viewed as a continuum is likely to get even weirder. Admittedly, I haven’t really researched snow though; I’m guessing here, using the little experience I have with the substance.) Consider applying some pressure to snow - it may flow a bit, it may flow a lot at first, but with the slightest pressure it begins to compact and change its physics, its density (near and close by, the compacting surface) increases. Not so with water, water simply moves out of the way, i.e. it flows. Also, water does not change its density (much), even under great pressure. The amount of density change for water is very very small - water is considered to be incompressible. Under the slightest pressure water begins to flow, and will continue to flow. Water at rest cannot support a stress, but moving water apparently can. And this brings me to the comments made by Havard in another thread, see ‘Passive Flex.’ At first, the following rant may appear unrelated, hopefully, when finished you will agree its not. (My apologies if I seem to be changing the thread line a bit too much.) The comment was made that the dynamics of boats are different as they have an active propulsion mechanism, surfers don’t, surfers use gravity. (Havard, if I’ve done an injustice to your comments, let me, actually, let everyone know. Its important, because I’m about to disagree with them, somewhat.) The water on a wave has velocity. You can reslove it into two three different directions, there’s an upward motion, theres a forward motion, and luckily there is no sideward motion to speak of. Its that upward and forward motion that surfing uses, and gravity is what we use to harvest it, well, gravity and good surfboard design (good technique too.) The surfer and surfboard can also create a sideward flow, more on that later. Its all relative. For boats, the view is taken of an object being propelled through a stationary medium, for surfing it is almost the opposite, a stationary object being subjected to a moving medium. And that’s the flow under the board. We use Gravity, and good board design, to trap the upward flowing water. (By the way the Physics is the same for either view - moving medium, stationary object, or stationary medium, moving object.) Yes, its possible to literally slide down a wave, much like what someone on a sled going down a hill might do. And yes, after achieving all that kinetic energy from the drop, make some big bottom turn, but it ain’t going to get you far. The aquired energy is burnt up faster than it was aquired, and the whole cycle has to be repeated (I’ll resist a diversion into Pumping.) Surfing is not just repeated free lifts and drops - but it can be. I’ve seen some top surfers, take the technique to its limits and its impressive. But its not the whole picture. Consider, trimming; trimming when in the barrel, or trimming when on the nose, or ripping down the line to beat some section, or even just an angled drop. Unless your pumping, you’re more or less staying put in terms of height (the angled drop being an exception), so if its not gravity what is it? I contend that its that interaction between upward flow of water and the bottom of the surfboard, i.e. planing, but at an angle of attack which is fairly extreme. In order to see this you’ve got to see things from a slightly different view. Oh boy, more pictures… The first picture (aside from a shameless display of my mediocre talent) is a set up for the next more mechanical view. And since the posting software doesn’t allow more than one picture per post, I’ll just start another post.

Here’s the second drawing, hopefully getting me closer to the final drawing… There’s a couple points to make. See angles F and D, there’s one more angle not detailed in the drawing, and that’s the (direction) angle that the board is actually pointing, i.e. if the surfer looked down his stringer, the direction he’d be looking. Also note (and I realize that I drew the picture so I guess this is my invention) that the upward water velocity of the water increases the closer you get to the to the curl. In fact for longboards the differential from tip to tail can be critical. (That you can have a object experiencing this is in part of what all the magic of surfing is about. Well, at least in my view.)

Here’s a more abstracted view of the last picture. In particular notice where I’ve drawn the spray root, where the ‘stagnation line’ meets the surfboard, the point of highest upward pressure. I may not have done it justice, but I’ve tried to show how the surfer is angling his board, in particular, out of the wave, in order to balance the upward force (see angle F). Same for angle D, he’s angling his board to balance or counteract against the upward force on the tail.

So here’s the final diagram. Here, I’ve rotated things so that the face of the wave is across the page. Hopefully, those that viewed my original diagram of planing will see the similarity. (The motion of the surfer is out of the page toward the viewer.) But the view in the diagram below is only a slice, its not an complete view. For unlike planing boats, the water and boat moving in opposite directions along a single line, in surfing the motion is angled, almost as if you had a boat that could somehow move through the water in a crab style. The boat not necessarily point exactly in the direction opposite that of flow. Also, the red dot in the diagram is the stagnation point. My apologies, but life calls… I will finish this later, perhaps tonight.

The guestion is can you control it, and that’s where my issues lie with > flex. (By the way, mats and body boards are not good examples here. You > can bring your whole body to bear on the mat or board, and hence control a > hell of a lot more than with just two feet, two feet which are very > loosely coupled to the board during surfing.) The problem is that > unloading will not likely be ‘all up to the surfer’ unless you are able to > somehow capture the stored elastic energy for later ‘controled’ release > (like a spring in a clock.) This is exactly what you do in a turn on a snowboard. You load it in the turn and control the release by unloading your weight. Also keep in mind that the buckles on the bindings on a snowboard only keep you from sliding off the board, it does not give you any way to flex the board(it does, but it neglectable(sp?))>>> I agree that carving up powder is very close to carving up water, but that > might just be one of sensation, not Physics - snow ain’t a fluid. Yes and no. In very fluffy powder it should and would behave much like a fluid, the kind of powder that doesn’t support you, but support your board. Remember that a fluids have a density and a viscosity. Thus powder could possibly be considered a fluid. Powder sort of behaves like something in between a fluid and a plastic.>>> Consider applying some pressure to snow - it may flow a bit, it may flow a > lot at first, but with the slightest pressure it begins to compact and > change its physics, its density (near and close by, the compacting > surface) increases. Not so with water, water simply moves out of the way, > i.e. it flows. Also, water does not change its density (much), even under > great pressure. The amount of density change for water is very very small > - water is considered to be incompressible. Under the slightest pressure > water begins to flow, and will continue to flow. However, because of the density of water and possibly surface tension it doesn’t move out of the way fast enough. For anything hitting the water at high speed the fluid feels pretty solid. Same forces apply to a surfcraft or a boat, the major flow or movment is in reference to the surfcraft and thus it can support the surfcraft(ie. planning). Thus at extreme speed you could consider a fluid a solid and get away with it. (thinking barefoot waterskiing kind of speeds here). So at moderate speed, the two mediums might be compareable.>>> Water at rest cannot support a stress, but moving water apparently can. > And this brings me to the comments made by Havard in another thread, see > ‘Passive Flex.’ At first, the following rant may appear unrelated, > hopefully, when finished you will agree its not. (My apologies if I seem > to be changing the thread line a bit too much.)>>> The comment was made that the dynamics of boats are different as they have > an active propulsion mechanism, surfers don’t, surfers use gravity. > (Havard, if I’ve done an injustice to your comments, let me, actually, let > everyone know. Its important, because I’m about to disagree with them, > somewhat.) [snipped alot] Let’s not call it gravity then, let’s call it wavepower. I’m still uncertain if there really is any other force giving propulsion on a surfboard(oversimplifing of course, not considering venturi effects and the effect of a thruster). I’m actually not sure that the water is flowing upward on most waves, my guess is it’s standing still. The wave is moving through the medium. It seems to flow upward and creates a force that can keep us in the wall. The boat reference was ment to explain that we cannot change the propulsion of the board like you can with a boat. The propulsion stays the same on a surfboard(roughly). Specifically you cannot redirect the direction of the propulsion to make a turn. With the requirement of a modern, rigid surfboard you have to make a compromise between planing, turnability and control. With a boat you adjust propulsion and turnability on the fly, on a modern surfboard you cannot adjust any of the above in an active way on the surfboard. Everything is rigid(not only talking as apposed to flex, but as in none adjustable when riding). All you can do is use technique. regards, Håvard

“I contend that its that interaction between upward flow of water and the bottom of the surfboard, i.e. planing, but at an angle of attack which is fairly extreme.” Kevin - I appreciate your analysis, diagrams and drawings as well as the overall concept of your theories. If I find fault, it’s in the statement above. I’ve heard it said before (by famous shaper) that aerodynamic principles (foils specifically) don’t jive with surfboard principles because aero foils slice through the air while surfboards plane on the surface. This seems over simplistic as it ignores the design of rails and fins which do penetrate the surface of the water rather than plane on top. It would also discount the effects of thickness taper and how a surfer’s shifting weight affects the angle of plane. For an extreme example of what I’m referring to, take your outline and put whatever planing surface you wish on the bottom, leaving the rails square, the thickness constant and leave off the fin.

“I contend that its that interaction between upward flow of water and > the bottom of the surfboard, i.e. planing, but at an angle of attack which > is fairly extreme.”>>> Kevin - I appreciate your analysis, diagrams and drawings as well as the > overall concept of your theories. If I find fault, it’s in the statement > above. I’ve heard it said before (by famous shaper) that aerodynamic > principles (foils specifically) don’t jive with surfboard principles > because aero foils slice through the air while surfboards plane on the > surface. This seems over simplistic as it ignores the design of rails and > fins which do penetrate the surface of the water rather than plane on top. > It would also discount the effects of thickness taper and how a surfer’s > shifting weight affects the angle of plane. For an extreme example of what > I’m referring to, take your outline and put whatever planing surface you > wish on the bottom, leaving the rails square, the thickness constant and > leave off the fin. I’m not sure I understand. Fluid Dynamic principles do apply, and planing is one such area of application of those principles. And that surfing is all about planing is my point. (Aerodynamics is a subset of fluids that applies to the medium of air. So I guess we agree.) I don’t believe I’m mixing my mediums here. I guess I should have indicated where the water was and where the air was, my apologies… I was rushing it a bit. As for rails, yes, this opens up a whole new thread. And I bet we might disagree on the value of rail penetration… start a thread and well get some discussion going. I’ll give you a hint as to where I’ll be coming from on the topic… that rails matter, and more often than not, in the same way fins do. (There are other issues with regards to the water line.) Here’s some more, I love watching skim boarders catch waves and then surf them. Now skim boards come about as close as possible to a surfboard without a rail as there is… they also don’t have fins… but they rip, albeit briefly. Fins allow you to take advantage of the upward flow of water (relative to the board trying to obey the law of gravity), they redirect it and translate the force along the length of the board. Skim board surfers don’t trim very well, and thats where fins come in. … I think a new thread ‘Rocket Science: Part 2, Rail Issues’ would be nice… if you don’t start one, I probably will, at somepoint.

Kevin, How does v (velocity) affect the stagnation point? The higher the velocity the smaller tau? Higher velocity flattens the pressure curve, distributing it over the entire platform? In your last diagram, it appears that the rail “holds in” because of drag formed by a wake as shown by your first diagram, correct? The more curve, the more wake, the more drag, the more hold. All this reminds me of the quote by the Wright Bros. regarding the dynamics of a propellor, to paraphrase; all the actions seem to cancel each other out, yet it works! I think Harvard may have you on his snow analogy. He is talking about powder snow which does act like a viscous liquid (ever see an avalanche?). In that particular case, flex in a snow board does react like flex in a surfboard. The analogy breaks down when applied to packed snow which definately acts as a solid and flex is used in an entirely different manner.

Kevin - Sorry, I’m to blame for mixing up the mediums. I’ve been in discussions before where foil characteristics of rails and fins were compared to bird and airplane wings - that’s why I brought up the aero/hydro comparisons. My point was only that planing theory is one component of board analysis. “And that surfing is all about planing is my point” If you’re trimming or turning on rail’s edge the foil and how it penetrates water becomes more (and planing surface becomes less) of an issue. Finely foiled “hull bottom” boards capitalize on this feature. Thrust from body torque and fin foil creates drive as well. I refer to “Swimming and Flying” link originally provided by Halcyon. A board is very difficult to analyze from an inanimate object being pushed by wave perspective but you’re right - the rails and fins should probably in different chapters! http://www.usm.maine.edu/bio/courses/bio205/swimming_and_flying/swimming_and_flying.html

Kevin, How does v (velocity) affect the stagnation point? The higher the > velocity the smaller tau? Higher velocity flattens the pressure curve, > distributing it over the entire platform? In your last diagram, it appears > that the rail “holds in” because of drag formed by a wake as > shown by your first diagram, correct? The more curve, the more wake, the > more drag, the more hold. All this reminds me of the quote by the Wright > Bros. regarding the dynamics of a propellor, to paraphrase; all the > actions seem to cancel each other out, yet it works!>>> I think Harvard may have you on his snow analogy. He is talking about > powder snow which does act like a viscous liquid (ever see an avalanche?). > In that particular case, flex in a snow board does react like flex in a > surfboard. The analogy breaks down when applied to packed snow which > definately acts as a solid and flex is used in an entirely different > manner. There are huge differences between hard pack snow and powder… on hard pack the nature of a snowboard or ski`s edges, template and structural flex are critical to maintaining control as velocity increases, along with consisitent edge penetration of an often irregular surface. With increased speed, as contact with the running surface decreases, the necessity for control increases. This true in surfing, as well. In powder, with enough surface area and momentum, the rider and skis or snowboard will literally “float” to the upper surface. With the greater degree of penetration, flex, edges and template are less important to overall control than on ice or hard pack snow. 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. Even though the action and practical composition of waves and snow are very different from one another, many lessons can still be gleaned by direct comparison. Dale

Kevin, How does v (velocity) affect the stagnation point? The higher the > velocity the smaller tau? Higher velocity flattens the pressure curve, > distributing it over the entire platform? In your last diagram, it appears > that the rail “holds in” because of drag formed by a wake as > shown by your first diagram, correct? The more curve, the more wake, the > more drag, the more hold. All this reminds me of the quote by the Wright > Bros. regarding the dynamics of a propellor, to paraphrase; all the > actions seem to cancel each other out, yet it works!>>> I think Harvard may have you on his snow analogy. He is talking about > powder snow which does act like a viscous liquid (ever see an avalanche?). > In that particular case, flex in a snow board does react like flex in a > surfboard. The analogy breaks down when applied to packed snow which > definately acts as a solid and flex is used in an entirely different > manner. (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.

Here’s some more, I love watching skim boarders catch waves and then surf > them. Now skim boards come about as close as possible to a surfboard > without a rail as there is… they also don’t have fins… but they rip, > albeit briefly. Fins allow you to take advantage of the upward flow of > water (relative to the board trying to obey the law of gravity), they > redirect it and translate the force along the length of the board. Skim > board surfers don’t trim very well, and thats where fins come in. As a respectful counterpoint, I would add that competantly used, high performance surfmats are the obvious exception to much of what has been previously stated. They efficiently trim, accelerate and carve extremely well, over a very wide range of waves and surface conditions, with the absolute least amount of surface penetration… all without fins. Beyond the issues of a rider weighting and unweighting their surfcraft, the force of gravity, momentum and glide, lies much deeper aspects of the wave`s internal energy that can also be tapped into with a minimum of brute athleticism by the rider. Such surfing relies primarily upon the sensitivity, flex and function of the design itself. In contrast to most surfcraft which are created to take advantage of planing speed… anyone who is familiar with the surprisingly effortless sensations of a finely delineated displacement hull, rapidly accelerating and being mysteriously “pulled forward” or “drawn into and through” critical, unwinding sections of a wave knows just how uniquely satisfying and unforgettable this style of surfing can be. Dale

As a respectful counterpoint, I would add that competantly used, high > performance surfmats are the obvious exception to much of what has been > previously stated. They efficiently trim, accelerate and carve extremely > well, over a very wide range of waves and surface conditions, with the > absolute least amount of surface penetration… all without fins.>>> Beyond the issues of a rider weighting and unweighting their surfcraft, > the force of gravity, momentum and glide, lies much deeper aspects of the > wave`s internal energy that can also be tapped into with a minimum of > brute athleticism by the rider. Such surfing relies primarily upon the > sensitivity, flex and function of the design itself.>>> In contrast to most surfcraft which are created to take advantage of > planing speed… anyone who is familiar with the surprisingly effortless > sensations of a finely delineated displacement hull, rapidly accelerating > and being mysteriously “pulled forward” or “drawn into and > through” critical, unwinding sections of a wave knows just how > uniquely satisfying and unforgettable this style of surfing can be.>>> Dale I’ve never used a surf mat… after reading this post, it would appear that I have something to look forward to. Thank you.