The Theory of Balance This is a paper on two of the basic balances that effect the way surfboards ride. These balances pertain to all boards regardless of shape, size or style of surfboard. The original theory came about 20 years ago when comparing twin fins to single fins. The basis of this theory is that most of the performance aspects of any board are effected by the balance of pitch and resistance between the nose and tail of the board. Imbalance’s hurt the performance of the board. These balances are effected by: a. the board shape b. fin size, shape and placement c. rider size, style, stance and ability d. wave size, shape and speed Small changes in any aspect of the board design can make a great deal of difference in these balances and therefore the ride of the board, positive or negative. Once one understands balance it is easy to tell what is wrong with a board just by watching it while being surfed. It is then just as easy to correct the balance by adjusting fin sizes and/or shapes. In the 70’s the single fin box gave us control of pitch. A large back fin would (in effect) pull the tail of the board down, keeping the front edges free especially in large or hollow surf. In smaller waves we would use a smaller fin to reduce drag and increase speed as the tail of the board was allowed to ride higher. Resistance had to all be built into the rails and outline curves of the board. The magic board was pretty rare. In the late 70’s and early 80’s the twin fin boxes of the day allowed us to change resistance. A larger side fin would resist the wave and transfer the balance of the board when “on rail”, forward. This would cause more rail length and less bottom surface to be engaged during a turn. A smaller side fin would allow less rail length to be engaged and more bottom surface, as the balance of resistance shifted aft. But with no control of the balance of pitch the magic board was still very rare. We did find out at that time that we could control pitch to some extent by softening or hardening the edges in the tail of the board. Of course, deep vee was very popular in these boards. That was also a way of lowering the finless tail of the board into the water. This couldn’t be changed after the board was finished though. In the remainder of the 80’s and all of the 90’s the 3 fin has dominated without an interchangeable fin system. Most of the boards from this era have less than optimum fin set ups, but in many ways the 3 fin is a bit more fool proof. By optimum I mean the board develops excellent drive, glide and speed without hang ups. Generally, for most surfers, the three fin boards of this era were over finned (to much fin). This creates a board that will work in a wide variety of conditions but, because of excessive drag, is rarely optimum. The change to thrusters with fin boxes has, for the first time, given us control of both pitch and resistance. What this should mean is the highest percentage of excellent riding boards ever but this is all dependent on the understanding of the balances of pitch and resistance. The quest for one perfect board has been an incredible drain of time and energy within the sport. It has also narrowed our perspective, keeping us away from potentially revolutionary advancements. When I first discovered the theory, the simplicity of it took me by surprise. To find out that as a surfboard designer the best that I or anyone else could ever achieve in this custom shaping world was a good guess was hard to accept. In fact, all this paper discusses is pitch and resistance. It doesn’t discuss total lift, total resistance, weight, balance of weight, flotation, balance of flotation or any of a million other variables that the interrelationship these and other design aspects contribute to. Consider for a minute that the rocker, outline, rail thickness, rail contour, edges, bottom and deck contours and a myriad of other subtleties all have an effect on overall pitch and resistance balance. Any change in one effects the others. It is easy to see that striking a perfect balance in every board would be quite impossible. Even machine shaped boards have variables that can’t be controlled. If you add to this the fact that there are different balance considerations in different surf conditions, then consistent perfect balance becomes even more improbable. Finally add different surfers of different ability, style, stance, weight, height and another myriad of variables and consistent balance becomes totally impossible. For me personally the theory did free my mind from many things that confused me before and allowed me to concentrate on things that I felt WOULD make a difference. Things like a lighter weight surfboard technology, a usable volume rating system for shapers, an effective scaling system for different size and ability surfers, and a more ecologically responsible board building system. It also made my personal boards a lot more fun. With the existence of surfboard balance proven, any question of the existence of a single perfect surfboard shape can now be put to rest. In short it doesn’t exist. It can’t. The real beauty here is that we can get close and balance can be had in todays board with a few sets of fins and a knowledge of balance.
Whoa! That’s some profound stuff there folks…
Greg, I could not agree more. As a matter of fact about four years ago I wrote an article along similar lines: FINS GET NO RESPECT When you walk into your favorite surf shop and get all goggle eyed over the masterpieces in the racks, it’s not the fins you’re drooling over. Think about it for a minute though. What is a surfboard, but a summation of hydrodynamic surfaces. Just as an airplane generates lift from its wings and control from its tail; your board’s bottom generates lift and affects speed. But, it’s your fins working together with rail and bottom contour that most influence the feel of your board when turning. And let’s face it how many short boarders straight-line the waves they ride. Few surfers really understand how fins affect the way their board rides and leave all the specifics to someone else. Rail and bottom contour assist fins in performing their function. But, what really influences the way your board turns is the combination of several important fin factors. No wonder so few surfers pay attention to the “Fin Affect”. It’s f’n complicated! But, today, as the form shape of surfboards goes through finer and finer adjustments, the biggest gains you can make to your board’s performance have to do with your fins. The predominant factors that influence your “Fin Affect” are: 1) Foil Shape- the curvature from leading edge to trailing edge as it changes from base to tip. 2) Template shape- The combination of depth, width, and rake that make up the profile outline of the fin. 3) Placement- which incorporates how far the fins are from the back of the board, how far apart they are from one another, toe and camber. 4) Stability and flex. Let’s discuss each of these and how changing them will affect the way your board performs. Foils are surfaces that affect lift and drag. If you notice an airplane wing has a flat side on the bottom and curved side on the top. The path of least resistance is the bottom side. It takes more effort (drag) to flow around the curved top surface, so more air flows under the wing than over. This creates high pressure under the wing. The air that does flow over the wing separates from the wing at the apex of the leading edge of the wing and creates a low-pressure area. This difference in pressure forms the force (lift) that allows the wing to go up. The more curve a foil has the more drag it induces over the curved surface; which means that a foil with greater curvature will have more lift at lower speeds. The problem is that at higher speeds that additional drag will develop turbulence and stall the flow across the foil. The exact same scenario occurs under water with side fins. The big difference is that instead of lifting your tail out under water; Side fins orient the curved surface so that they actually pull your boards fin and rail down into the water. This gives you hold when cranking a nasty slash. Consequently, thicker more curvy foils for slow waves and flatter more fine foils for high-speed waves. Template shape has to do with how the fin looks in profile. An over simplification would be deeper, rakier & wider fins provide more control. But, the more profile you have the more fin you drag around. So, you have to optimize the combination of the three so that it is loose enough for your conditions. Yet, it is also tight enough to not get too squirrelly on you. Other factors that figure into requiring more or less of these three variables are: 1) Type of wave: steep and heavy or slopey and fun. 2) Surface conditions: Choppy and irregular or clean and smooth. 3) Rider Size: Big and heavy or small and featherweight. 4) Rider Style: Subtle and flowing or Extreme and radical. Each of the first considerations requires more fin template area and each of the second work better with less. Placement has traditionally been left to convention. Simon Anderson set a benchmark twenty years ago for approximate location. Each shaper has their own personal preference for each of the “models” they make. But, there are subtle differences in most boards and in all riders. Otherwise why would custom boards be in such demand? And remarkably as little as an 1/8" movement fore or aft in either or both the center fin or the side fins can have almost as much effect as going from a rakey 4 ¾" fin to a vertical 4 5/8" fin. If you move your fins closer together they act looser and if you spread them further apart they get tighter. Toe is the amount of angle the base of your side fins are pointed in towards the center of the board relative to the leading edge and trailing edge at the base. Camber is the amount of angle the body of your fin is set at relative to an imaginary horizontal plane perpendicular to your stringer. Both affect the angle of attack that your fin foils experience as they flow through the water. More angle forces more water flow around the outside plane at lower speeds. The net affect is that it becomes easier to initiate turns on slower waves. To much angle at higher speeds increases turbulence and drag. Finally, stability and flex are crucial to making this all click. If you have a deeper fin you can get away with more tip-flex and not wash out. The benefit of tip flex is that it dampens or smoothes out some of the bite in direction changes. The down side of tip flex is that if you get too much tip flex it will wash out. Base stability is crucial to a good set of fins. If a fin moves around at the base it will set up turbulence. Turbulence generates drag and disturbs the lift, which keeps you fins holding. So it is slow and out of control. If you like a more pivoty board you would do well to try a stiffer set of smaller more vertical fins. Prior to eight years ago the only options a surfer had to muck with all these variables was to grind his glass-on fins off and install a slightly different set in a slightly different location and record the differences until they found the optimum. Not very likely. So, with the exception of a few elite pros, we all lived with what we were given. Now with the advent of removable fins (i.e. FCS, Future, Lock Box, OAM and O’Fish’l) you can at least muck with the first two “Fin affect factors”. But, if you want the ability to dial in your board with all four factors you have got to try the newest competitor to the fin system market Red X. Tom O’Keefe
VERY well said. This is the type of knowledge that is sorely missing in our sport. Where did this appear? This is the type of information that our media usually doesn’t have the BALLS to print. The surfing media should be ashamed that this kind of information rarely graces their pages!!! They are pathetically inept and irresponsible for NOT printing articles that have basic information about board design with a scientific slant. Did you notice that Tom didn’t once mention cavitation? Do you know why? Because it doesn’t exist in surfboards. It only exists in props and pumps. And yet this “cavatation” has been repeated to me as the cause of spin out for over 35 years. Fins spin out because they either stall or ventilate. Stall happens because the angle of incidence (angle of the water as it meets the leading edge of the fin) becomes to high for the attached water flow on the low pressure side of the fin to remain attached. Ventilation happens when air is sucked down the low pressure side of the fin interrupting attached flow. The low pressure side of the fin is 2/3 the power. In other words the suction side is twice as powerful than as the pressure on the high side of the fin. Tom, I honestly didn’t know anyone in this sport knew this stuff. I am thrilled to find out others do.
Greg: I’ve read this article several times now and I’m not quite sure I understand…Pitch is an attitude (yaw, pitch, roll) and resistance is force (in a direction, vector). There can be no “balance” between them. If you mean that you can control pitch with resistance (fin), then I understand and agree completely. You can also control pitch by redistributing the volume of the board (width, foil) or by moving the rider up and back (your “balances”?). If these assumptions are right, then I think I disagree with your conclusion that since it’s all so complicated that we should forget about trying to use this stuff in design. You probably already use most of these factors already. You just don’t think about them as single items. When you walk you don’t think about “lift my left thigh, lean forward, etc.” You just walk. When you shape a gun, you know what factors need to be shaped into the stick for it to work the way the rider wants, you don’t quantify them, or look them up in a table…you know from experience, but they are there just the same. The place you can use them independently is when a rider comes to you with a board and tells you what they like or dislike about it. If you have to fine tune a design, that’s when you can start dealing with the independent variables specifically and with really good results (I’m pretty sure you already do this). As far as the magic board is concerned, I think they are achievable for very specific conditions. Not because the board’s design characteristics are complicated, its because the medium they are floating/planing on is too complex. The perfect board for an 40-year old, out of shape geezer on gutless, 3 foot slop is perfectly buildable. It won’t be his perfect board if he gets in shape or starts riding head high point surf though. As an aside, you should go to archives and look up “Rocket Science”. A guy named Kevin spent a couple of months (or maybe weeks) here and really revolutionized my understanding of how and why a surfboard works the way it does. He simplified the math so it’s not that hard to grasp except that you have to throw away your preconceived notions of how a surfboard planes. It’ll throw a new perspective on the things you were talking about. Keep up the good work! Newbs
Greg, It was posted on Wet Sand a few years ago. It was also used in portions in one of Nick Carroll’s design forums. Currently, I believe Larry said that an Australian Surf mag was about to republish it. When you discuss pitch or trim of a planing surface there is definitely a resistance factor to consider. Greg’s description of balancing the the two through larger or smaller fins describes tendency. Boards with larger, more toed in fins will tend to trim more tail heavy and induce more resistance. Rocker is another major factor that is part of the trim equation. But, I believe Greg was breaking the equation down to its elements and just was not focusing on that factor for this discussion.
Newbs… the addition of active, tuned flex to the board and fin(s) is another crutial aspect of surfcraft design to consider. In fact, a board without flex is not unlike a car without a suspension system. Achieving a functional balance between the board and wave is often more important than between rider and board… in fact, some things almost surf themselves!
“…In fact, some things almost surf themselves!” The magic somethings I’ve ridden require that they surf themselves. Rider input usually screws it all up… The problem with flex is that it adds the 4th dimension to Greg’s theory; time. Dimensional/resistance changing over time. We seem to have problems with the three easy ones here. My head hurts…its easier to ride a something than it is to think about it.
Greenough was tuned into theory and application of balance 30-40 years ago…
Lee.V introduced two very important words into this discussion that i’m not sure has been considered enough. That is; Force(mechanical interaction b/w an object and its surroundings) and Vector(dircetion of force). I’ll throw some more out there - Moment of interia(resistence to rotation), torque rotatioanl force), velocity(speed x displacement), Momentum (the quantity of motion possed by an object), and angle of incidence( described by the angle at which a foil hits the a fluid and results in a vector force) . Lift(as in a plane wing) is not really best described by Bernoulli’s equation (high-low presure gradient either side of a foil). Newton is the man that should be being discussed more. There is a critical velocity which is exceeded during most surfed waves that causes Benoulli priciple to become less pertenent than Newtonian law. As is the case in 99.9% of plane wings and also race car aerodynamics. Newton’s Surfing Laws 1)LAW OF INERTIA: A surfboard will not move or accelerate unless acted upon by a force(i.e. paddling-catching the wave and gravity-once riding) 2)LAW OF ACCELERATION: The rate of change of momentum of a surfboard is proportioanl to the force applied and takes place in the direction in which the force acts( i.e down the line speed and profile drag) 3)LAW OF ACTION-REACTION: To evey action there is and eqaul and opposite reaction (this allows us to walk on water and a fin to create stability and a change in direction). So back to the magic fin. The total effect of fin design is a combination of all the below statements. Newton’s laws suggest that- 1) the rake of fin and the side on surface area are most influential in creating change of direction. Both these variables influence the amount torque that is required to rotate the fins through the water thus creating a change in direction of the surfer. So the greater the rake the more resistance to rotation it has. This is also so for surface area. 2)Toe and camber in combination will induce an undesirable drag force when set at anything other than 0 degres to the flow of the water across the fin. We can’t get around that. But the anvantage of these variables is that they allow us to influence the vector forces that occur when set 0 degress. Vetor forces occur perpendicluar to the surface of the fin as water flow across and ramps of it. This ramping effect induce infinate vectors across the suface on the fin that over time(when trying to turn) and will also influnce turnablity of the board. 3) That leads us to the foil of a fin and front on surface area. The more hyrdodymaic we can make a fin, by varying foil, the less drag that will be created. The more we decrease front on surface area the less resistance (not drag) will be created. Drag affects our ability to accelerate -law 2 & 3, and front-on surface area is explained by law 3. Both drag and surface area are trying to slow us down. Again, undesirable. 4)Fin shape. The dynamic nature of water flow across the fin, the composition of the fin and the above previous ideas on toe, camber, torque, drag and all the other many variable this design concept must incoporate, will ultimately bring about the overall shape of the fin. This is just the fin being considered, albeit briefly, but Newton applies to all aspect of surfboard design. What about this idea. Consider that the dolphin fin and varients of it(for example) are widely used on many surboards. Consider also, that here we are ideally trying to get the same thrust ,turn, and stabilty out of 1, 2, or however many fins, all on the same plane(ie on the bottom), that a dolphin gets using using 4 foils all on different planes. Perhaps the surfboard being a non-natural mechanism needs a non-natural fin design approach. Pehaps we need a unique design of fin that is unique to the surfboard. Perhaps we need to copy nature more exactly, with fins on the tail rail for example, or maybe incorporate technology used from the multi million $ race yachts? I’m not sure how or what will be done but we can’t let little problems like this prevent use from creating the next great idea. It hasn’t prevent the Bob McTavish’s and George Greenough’s of this sport fom creating. Good luck! PS; The point I was trying to make was that maybe Newton makes more sense and more accurately describes the physical priciple of surboards design and riding.
It’s easy to complicate a simple idea. Resistance means: The resistance of the rail and the outboard fins to the wave face. That’s all. A thick rail resists more than a thin rail. A large fin resists more than a small fin. If you were to have a very thick rail in the tail and a thin rail in the nose you would tend to bury the nose of the board through turns more than if you had a thin rail in the tail and a thick rail in the front. The larger the outboard fins are the more they resist. So the balance of resistance is shifted forward with a larger fin. Smaller outboard fins shift the balance aft. That’s it. Take a batch of fins with you to the beach and try this out. You may disagree but I’m right. After 25 years of being aware of this, it works. There are a number of other important issues but these two are the two that make the most difference. And it’s just physics, with or without Sir Newton. And with all due respect to George and Mac, I have never heard a word from them on this subject.
You are very correct Greg. I prepare elite athletes for competition, which in a round about way is analogous to surfboard design. The more time that goes, and the more other people catch up to the ideas of modern training, as in modern board design concepts, the athlete becomes more refined, more highly tuned to perform, to the point where large steps in progress are not made anymore and and it is an acclumulative effect of of all the small incremental andvances in training, which are made by a more indepth understanding of the concepts that constrain perfomance. Any small improvement in training technique or explotation of these constraints, or surfboard design in this case, are what will make the boards of the better and more refined. All I’m suggesting is that as it gets harder to make that next improvment, perhaps we should understand more of what is really potentially holding surfboard design back. Who knows, in 20 yrs perhaps the best boards in the world will be designed by those who have the greatest understanding of these concepts and shaped by those how have the best ability to build to plan. A design team and a shaping team. The multi million dollar New Zealand Americas Cup yachting teams designing resources and Bob McT shaping!!The surf business of the future???
Oh Brother … have fun
Greg, There is one point you made that I must disagree with. You said “Toe and camber in combination will induce an undesirable drag”. A certain amount of drag is a good thing when you intend direction change. That is why the side fins are not set parallel to the stringer and they are predominantly single sided foil on a Thruster. A certain amount of instability is always induced which allows for quick direction changes. A Fish with a keel fin setup or a single fin which are both set parallel to the stringer get there instability by acting as you stated in a neutral balance. Just as much water flows around either side of the fin. There is no natural lift vector until you reorient the fin askew to the flow of water. Then the lift vector is induced by the flow seperating from the down stream side of the fin’s orientation to the flow of water. Fins have been making gains influenced by yacht design. The trend in fin templates has been getting less rakey, shorter cord lengths and the chamfer at the base is gone or going away. Up until Australia II won the Americas Cup. Keels were very rakey and had very gradual transitions from the keel to the hull. Since that time due to incredible sums spent on tank testing Keels have become shorter on cord length, vertical and ninety degree transitions at the keel hull joint are the norm. Yacht designers have also had to engineer stronger methods of attachment to deal with greater and greater load bearing on smaller and smaller attachment points. We at Red X had that in mind when we came up with are attachment method. The smaller we make our boxes the easier it will be to contour them to mate to the bottom contour. But, the smaller you make the attachment point the more load you place upon it. Consequently, we tied into the deck to gain an I-beam structure rather than relying upon a single flange. We also noted that the traditional point loading method of fastening a fin to a box via a fastener had a tendency to become loose over repeated load cycles. So, rather than rely upon the fastener pressing against a single tab point on the fin base. We made the fin root a matching taper to the cavity of the box. Our fastener draws the fin root into the taper. But, the intimate contact of the walls of the system are what transfer the load. This keeps the fin base from loosening and causing excess turbulence when transitioning through foil orientations to the flow.
I never wrote, “Toe and camber in combination will induce an undesirable drag”. Where did you get that? I don’t even beleive that.
Now,now,Greg. Settle down. This kind of thing happened to me all of the time around here. I believe you! Nobody can take that away … have fun
My bad it was in Naki’s post on Newton’s Theory Application. I gotta read more carefully…Sorry
OK guys. It happens. Apology accepted.