Rocker
Introduction
While most consumers in the surfboard market do not typically consider rocker details when making a purchase, this design element is absolutely critical to a board’s performance. It is surprising that more attention is not paid to rocker subtleties, given the fact that small changes in rocker can result in dramatic changes in performance. Many shapers agree that adjusting rocker on the scale of 1/8 inch can make all the difference in the world when it comes to high performance surfboards. In the end, it is ironic that it has been said that rocker is the single most important design element of all, yet most surfers seem unconcerned with rocker specifications, even when ordering a custom board from a local shaper.
Perhaps this oversight is caused, at least in part, by the state of the surfboard industry today. Since the mid 1960s, shapers have been refining rocker curves to suit not only individual rider preferences and local wave conditions, but also what is valued in modern surfing in terms of aesthetics and athleticism. Prior to the “shortboard revolution,” the emphasis was on glide, trim, and length of ride, and longer, flatter rockered boards were perfectly designed for these goals. But when board designers like Bob McTavish, George Greenough, Nat Young, and many others started exploring new frontiers in surfing (both conceptually and physically), the demands of their equipment changed. Boards became shorter, lighter, more delicately foiled, and more highly rockered. Since then, rocker curves have changed to accommodate the ever-evolving styles of wave riding, and those changes continue to persist today.
Measuring Rocker
Bottom rocker is usually measured along the stringer, with the center of a long, leveled straight edge placed at the apex of the rocker as the board is centered on the shaping racks, bottom up. The distance between the leveled stick and the board’s bottom is measured. Measurements are typically taken at the nose and tail endpoints, but also at a number of points along the length of the board. The more measurements taken, the more accurately the rocker curve can be described. For example, typical modern performance shortboards have about 5 inches of nose rocker at the tip of the nose, and about 2 1/4 inches of rocker at the tip of the tail, with more variation found in the former than that latter.
However, how the curve changes as it moves from the rocker apex to the endpoints can vary greatly. For example, the curve may be relatively even over the entire distance from apex to nose tip, or may be very subtle and flatter through the middle of the board, then accelerate rapidly in the last 12 to 18 inches of nose, giving the two boards vastly different performance characteristics, despite the fact that they both may have “five inches of nose rocker.”
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Typical Rocker Numbers
Still, it is useful to know, in general terms, how much nose and tail rocker different types of boards typically have. Because each type of board (retro fish, high performance shortboard, modern fish, etc.) are intended for different types of waves or different riding styles, each has an almost pre-determined set of performance parameters for most riders. Many shapers and surfers alike agree that rocker is one of the most limiting factors determining those parameters. Here is a simple chart that shows some typical nose and tail rocker specifications for a few different types of boards of common length.
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Surfboard Blanks & Rocker
The rocker manufactured into the blank when you buy it is referred to as natural rocker, and may be changed through the shaping process only if the blank is thick enough to accommodate such changes. Most of today’s “close tolerance” blanks are generally too thin to make major bottom rocker adjustments. However, minor changes can be made, particularly in the tail. Because many shapers feel it is easier to add rocker to the nose and tail than to remove rocker by flattening the apex, it is most often better to buy a longer or thicker blank and add rocker, than to buy the correct length of blank and try to take rocker out. In most cases, rocker is adjusted by moving the board’s template forward or backward until the desired rocker shape “fits” into the blank, and foam from either end of the blank is removed until the endpoint rocker measurements are reached. The endpoint rockers are then blended into the middle of the blank’s natural rocker, minimizing the possibility of creating flat spots and abrupt transitions in the rocker that create turbulence and drag.
Lift
In general, increasing rocker increases lift as a board gains speed. At paddling speed, the effect of rocker in terms of lift is negligible. In order for rocker to provide a significant degree of lift, the board must be moving faster than typical paddling speed. Therefore, at paddling speed, increased rocker results in increased form drag, without much payoff in lift. Form drag is the resistance an object has as it moves through a fluid due to its shape. In general, flattening a board’s rocker reduces form drag, while adding rocker increases it. At very low speeds, a board’s volume is what determines it’s “lift,” as the displacement it affords results in a buoyant force that keeps the board and paddler afloat. Consequently, when sitting or paddling, the upward force keeping the rider afloat is not technically lift. Rather, it is a result of the board’s buoyancy, which is a direct function of its volume – more volume means more displacement, and more upward buoyant force. In this sense, the greater the board’s volume, the more buoyant it becomes, and the easier it will paddle, all other elements being equal. However, if we were to add rocker without a subsequent increase in volume, the result would be added form drag, and the board would become much more difficult to paddle.
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But once a paddler starts to catch a wave, and the board begins to approach planing speed, the rocker curves of the board’s bottom begin to create true lift. At this point, the shape and degree of rocker begins to determine where and how much lift is created, and where and how much the board’s rocker continues to create drag. One basic but essential understanding is that rocker induced lift is created mostly in the entry rocker section of the board – the first 20-30% of the board’s length. In the entry rocker section, the bottom is dramatically inclined to the plane of the water’s surface relative to its motion. The more entry rocker the board has, the greater the angle of inclination, the greater the potential lift, but also the greater the drag.
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Therefore, boards that are heavily rockered (very curvy), create a relatively large amount of both lift and drag, and therefore tend to take off later than boards that have what is referred to as relaxed rocker (less curve). Boards with relaxed rocker create little lift, but also little drag, and therefore take off earlier than more curvy boards.
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As the board’s speed continues to increase until it is finally up and planing, the entry rocker section of the board comes almost completely out of the water, and only the gentle curves of the middle third continue to provide lift. At this point, little lift is required, and the small amount of lift that is generated is ideal, as it also produces only a small amount of drag. As the rider makes subtle shifts in weight fore and aft, the planing surface that provides lift shifts fore and aft in response. In this way, the rider can plane of different parts of the bottom curve, adjusting lift and drag for speed and control. One pro surfer known for his mastery of riding different parts of a board’s rocker thought having a board’s bottom curves dialed in gives the competitor such an advantage that, “it’s like cheating.”
In the back third or so of the board, well behind the rocker apex, the rocker curve begins to accelerate upward. Here, particularly in the tail, under and behind the rider’s back foot where the upward curve is greatest, water must actually travel upward as the board’s length slides over it in order to stay attached to the bottom of the board. Just before the water is released out the tail of the board, water is actually pulled by the board. This is due to what is known as the Coanda Effect. The coanda effect describes the tendency of flowing water to hug a curved surface - in this case, the curved surface is the upturned rocker along the board’s tail. Newton’s third law of motion states that any water being pulled upward along the board’s bottom will pull back on the board with an equal but opposite force, creating an area of slightly lower pressure under the tail, resulting in turbulence and drag.
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One obvious question then becomes… How much rocker does a board need to be functional in terms of creating the optimal balance between lift and drag? The answer is subjective and has a great deal to do with rider preference, fitness level, wave conditions, etc. But there are some basic principles that can help guide our decision making. To sum it up, the more rocker a board has, the faster the board must go to get up and planing on the water’s surface. But once it is up on plane, the greater curve of a heavily rockered board lifts more of the board out of the water, resulting in an overall decrease in wetted surface area.
It is a topic of great debate whether this reduction in wetted surface reduces friction enough to increase a board’s speed to any significant degree, but there is a general acceptance among board designers that highly rockered boards create a much tighter turning radius than boards with flatter rocker profiles. Many shapers agree that it is more likely that highly rockered boards increase drag as the total weight of the rider is concentrated over a smaller area of wetted bottom, increasing pressure. Under these conditions, drag increases control, and the rider can turn the board very hard at speed (for the moment, not taking into account fin toe-in, which will be discussed later), as the tail penetrates the water deeply. However, as the rider begins to return the board to a flat plane, there is a general lack of projection out of the turn, as the board has stalled significantly. The rider must shift weight back to the front foot, where the rocker curve is more subtle, somewhere midway through the turn, and begin to position the board precisely on the wave face in order to regain speed quickly.
As discussed earlier, lift is relative to the direction of motion, which is not necessarily “up” in a vertical sense; lift is always opposite the direction of the force of the rider’s feet pushing down on the deck, and perpendicular to the direction of motion. Therefore, when the board is on a rail, lift under the board’s bottom is working to push the board around the turning arc. This helps explain why more highly rockered boards have tighter turning radii – they create more lift.
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The opposite can be said for boards with relaxed rocker. As the overall rocker curve is reduced, there is a general opening of the turning radius of the board, and in increase in drive and projection. In short, the board’s track through the turn is greatly (but not completely) determined by the decreased lift resulting from the decreased curve of the rocker. When the board is up on a rail, it follows the curve of the rocker. However, less board curve also means less drag, which translates into less energy wasted through the entire arc of the turn. That conservation of energy means a greater conservation of speed through the turn and an overall increase in projection.
Continuous Rocker
Boards with gently accelerating, continuous nose-to-tail curves along the bottom of the board, with no dramatic changes in rocker are generally referred to as having continuous rocker. A continuous rocker curve allows water to flow smoothly and predictably along the bottom of the board, and creates continuity of lift along the board’s entire length. The lack of relative “flat spots” along the bottom means that the rider can shift weight between the front and back foot smoothly and easily, facilitating smooth, easily controlled turns.
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Staged Rocker
Boards that have combinations of curved and relatively flat sections of bottom where there is minimal rocker, are said to have staged rocker. Typically, the rocker is flattened in the middle section of the board in an effort to increase down-the-line speed and projection out of turns. Dramatically accelerated rocker curves in the first foot or so of nose is sometimes referred to as flip, while a similar acceleration of rocker in the last several inches of tail is referred to as kick. Increasing flip increases the board’s tendency to plough water when paddling or at low speeds. Too little and the board will have a tendency to pearl. Increasing kick in the tail increases turbulence and drag. Too little kick makes the board stall dramatically during hard, tight turns, but is faster down the line. In addition, minimizing kick in the tail helps catch waves easier as the tail engages the wave’s energy sooner, and provides greater response to the lifting force of the wave than a dramatically upturned tail.
Flattened rocker through the middle of a board lengthens wetted surface area of the bottom, allowing the board to plane higher and flatter on the water’s surface. With the board up on plane quickly, and the rider’s weight distributed over a longer distance of bottom, boards with flatter bottom sections tend to go faster, all other conditions being equal. But mentioned earlier, this increase in speed has a tradeoff – less rocker lengthens the turning radius of the board making staged rockers more suitable for waves where the emphasis is on down-the-line speed, rather than tight pocket turning. Many riders feel staged rockers facilitate staged turns, particularly on bottom turns - the transition in rocker from the flat middle to the curvy tail can be felt, and pressure must be applied deliberately with the feet to get the board to move from one curve to the other. The challenge for shapers is to blend all of the curves together smoothly to minimize disruption in water flow along the bottom, and to allow the rider to make a smooth transition from one set of curves to the next.
It cannot be overemphasized that the transitions between rocker curves must be blended in such a way that water flows smoothly along the bottom surface of the board if the purpose of the design is to maximize speed and minimize drag. No matter how fast or how high a board is planing across the water’s surface, and no matter how flat or curvy the board’s bottom may be, all surfboards disturb and displace some amount of water. The displacement of water means that water is pushed (and to some degree, pulled) and the result is a change in pressure both below and slightly behind the board. The greater or more abrupt the changes in the curves of the board’s bottom, the greater the pressure changes in the fluid, and the greater the drag created.
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It should be made clear that the leading curves of a board’s entry rocker push water, increasing pressure below and slightly ahead of the board, particularly at low speeds, creating drag. The trailing upward curves of the board’s aft section pull water, lowering pressure, also creating drag through what some refer to as “suction.” A well-foiled and smoothly rockered board helps minimize those pressure differentials, reducing drag and increasing speed. Whether the goal of the rocker design is to increase speed or increase drag is up to the board designer. For example, the hard-edged concaves under the noses of many noserider designs is put there intentionally – to deliberately create lift and increased drag in order to facilitate nose riding and keep the board in the pocket of the wave. Similarly, the increased kick in the tail of the same board is put there to help create suction, and hold the tail of the board more firmly in the face of the wave. It is therefore important to keep the goal of the design in mind when creating and blending rocker curves and transitions.
Rail Rocker and Deck Rocker
It is also important to understand that the nose-to-tail curves of the rail and deck of a board may not be the same as the curves of the board’s bottom. Rail rocker, the curve of the rail apex (the rail’s furthest point from the stringer, which makes up the outline of the board when viewed from the deck or bottom), can be changed from the bottom rocker by bottom contours. For example, adding vee in the tail removes material from the bottom of the rail, essentially lifting the rail apex deckward and increasing rail rocker, but in the tail only. By contrast, adding concave through the middle of the board removes material along the stringer, essentially flattening the rocker along the stringer, but maintaining a relatively curvy rail rocker. While the depth, length, and location of concave or vee is primarily a matter of preference, it should be understood that the changes made by these contours is not limited to the bottom of the board. They also manipulate rocker, and their transition into the natural rocker of the design must be smooth and appear natural. (More on bottom contours later!)
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Deck rocker, the nose-to-tail curve of the deck, may also vary from the bottom rocker, rail rocker, or both. Shapers use a variety of curves when designing deck rocker, and decks may be flat, concave, convex, or any combination of these, depending on the desired performance characteristics. Boards with concaved decks bring the rider’s feet closer to the bottom of the board, giving the rider a greater sense of control and stability. Boards are said to have parallel rocker when the bottom and deck rocker are the same, and only the nose and tail sections are foil. This design gives more uniform thickness to the board, and generally adds volume throughout. This profile is useful for shorter, thicker boards, like retro single fins and fish, where a reduction is length is made up for by adding width and thickness.
Flat deck rockers increase flex, while convex deck rockers thicken the middle of the board when compared to the nose and tail sections, stiffening it considerably. When combined with thinned rails, convex decks offer increased sensitivity as the deck is domed, and volume is concentrated along the stringer. Concave and convex deck rockers are combined in s-decks, where the nose section is concaved, the middle is convex, and the deck in the tail is turned down and thinned greatly. Used in hull designs, along with bellied bottoms and pinched rails, this design creates a unique combination of dynamic thickness flow and flex that allows these boards to “come alive” in fast, reeling waves. While hulls have an intense, dedicated following, this design requires exacting precision in shaping, and few shapers have become masters of the form.
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