A sailboard is the picture of elegance. It has only two moving parts, is easy to transport and set up, and is capable of delivering more fun per dollar than just about any grown-up toy one can imagine. Until the invention of the sailboard, a whitecap-filled lake, river, or ocean was not even considered as a place to play! After two decades of evolution the sailboard, including the rig, has greatly improved in versatility, performance, handling, and durability. However, it still requires considerable skill development to master. It is the author’s opinion that a solid understanding of how the sailboard works - the mechanics of its operation- enables the sailor an easier and faster progression in learning the skills required for maximum excitement and fun. That’s the purpose of this article.
A sailboard has three distinct speeds, or modes of operation. For each of these modes the board controls differently, so it is important to know which mode you are sailing in. The techniques specific to each mode must be learned, then reinforced by experience and committed to reflex action so sailing can be handled automatically, without thinking. The cognitive mind isn’t fast enough, but the body is. For purposes of explaining, these three sailing modes will be termed "displacement sailing", "planing", and, for lack of a better term, "barely planing". The miraculous thing is how all of this works together on a sailing machine this simple!
Displacement Sailing This mode of sailing is analogous to a walk. The board is moving slowly through the water, and most of the lift which holds you and the rig up out of the water comes from the board’s natural buoyancy, as it would if you were standing on it in no wind. The sail force is small and easily controlled, and you are not required to lean or hike out against the pull of the sail to any significant degree. A major part of your lateral balancing is achieved by tipping the hull side-to-side with your feet. A big, wide board which has lots of volume, say 160 liters or more, is fairly easy to sail in this mode. If the board has a centerboard or daggerboard, this also helps to make the board less tippy, and helps the board go upwind at a steeper angle.
The lateral, or sideways resistance of the board - that keeps the wind in the sail from just moving you sideways and downwind - is provided by a combination of the centerboard (if there is one), the board’s lateral edges, or rails, and the fin. These three elements work together, flying through the water like the wing, fuselage, and tail of an airplane. Because water is about 800 times denser or heavier than air, the wing-like centerboard and fin are much smaller than their counterpart in the air, the sail. If a board has a centerboard and rounded rails, the centerboard and fin produce most of the lateral resistance, so there is a fairly fixed location for what is called the "center of lateral resistance"(or CLR), an imaginary point somewhere along the board’s centerline where the sideways "lift" of the centerboard, fin, and rails could theoretically be concentrated. The sail, in generating force from the wind, also has a "center of lateral effort"(or CLE), but its location along the length of the board can be varied by tilting the mast forward or aft. In fact this is how the board is steered at low speed, in the displacement sailing mode. What you’re doing in effect is moving the sail’s CLE ahead of the board’s CLR to turn downwind, and moving the sail’s CLE aft of the board’s CLR to turn upwind. It goes "sail back, head up, sail forward, bear off".
But don’t underestimate the rails! 19th century square riggers were capable of tacking and making their way upwind with nothing more than a squared-off wooden keel running along the bottom of the hull! For improved planing performance, many modern boards, and virtually all shortboards have angular, or "hard" rails, and no centerboard at all. The rails usually make a well-defined angle with the bottom of the board. These boards, in displacement sailing, must get all of their lateral resistance or side force from the rails and fin. Interestingly, this makes the shape and size of the fin more critical. Since a large part of the lateral resistance actually comes from the rails, it is important to understand the way the rails work, and how this affects the way the board behaves, and how to control it.
The key idea is that the fore-and-aft distribution of weight on the board determines where along the board’s length the rails are the most submerged, and hence the location of the board’s center of lateral resistance, or CLR. If your weight is forward on the board, the forward part of the rails will be dug in further, so the CLR of the board (rails and fin combination) will be also, and vice-versa. In this case, unless the sail is tilted well forward, its center of lateral effort (CLE) will be behind the board’s CLR, and the board will really try to point up into the wind. However, to get the board to turn more downwind requires you to tilt the sail further forward, causing you to shift your weight even further forward, making the problem worse! This is, in fact, typical of slalom boards, particularly the earlier "volume-forward" designs with thin tails and hard (sharp) rails all the way along the length. If the fin is too small, the rails can easily overpower it, making it hard to turn the board downwind. If the fin stalls and loses lift, it may seem nearly impossible to make the board turn downwind. A large fin mounted well aft goes a long way towards curing this problem, and the one thing the sailor can do is to tilt the sail as far forward as possible while simultaneously shifting as much weight as far aft as possible, and onto the back foot. It helps to bend your legs to bring your body down closer to the deck of the board, and stick your butt aft as much as possible, while extending your arms straight to allow the boom to tilt forward as much as possible. This is where powerful hindquarters can be an asset. Don’t worry about how you look. You look mahvellous dahling! And you’re turning downwind.
(Sidenote: The new board designs with increased volume aft go a long way towards curing this problem. Interestingly, however, board designers seem to have clung to the myth that hard rails in the front half of the board help with early planing. I personally don’t buy it! On the other hand, nicely rounded rails forward of the mast track help even more to limit the CLR shift and make a board much easier to control when it’s not planing. The new Seatrends and certain dedicated waveboards show this design trend.)
Planing This is what we live for; where the need for speed gets satisfied. In planing, several things happen differently that change how the board behaves, including how you steer it. The wonderful thing is that it is so comfortable. You clip into the harness lines, put your feet in the footstraps, and you could stay there all day, blasting along, passing unsuspecting powerboats and catamarans, leaping over chop, the wind in your hair...what a rush!
In planing, nearly all of the lifting force that holds you and the rig up above the water comes from the board behaving like a water ski. It turns like a water ski, too. Just canting the board from side to side makes it carve turns beautifully. There’s no need to tilt the sail forward or aft, in fact, it’s tilted as far aft as possible, and the bottom of the sail, or foot, is almost touching the deck. With the mast raked aft, the boom is now at a comfortable height, not in your face, but more at chest level. Your feet are in the straps, way aft on the board, near the tail, but the tail is not sinking. The sail force that is causing all this excitement and speed must be huge, but you feel almost none of it. Your lateral balance is achieved by minute adjustments in the sail’s angle of attack relative to the oncoming wind - none of it comes from the board. How is this wonderful state of affairs possible?
First and foremost, there has got to be sufficient wind. Usually this means more than about 13 miles/hour, but this depends on a number of factors, including how big your sail is, how much you weigh, the size, weight and design of your board, and curiously enough, whether or not you were planing in the first place. Oddly, even relative wind you create by going fast, although it’s dead on your nose, counts for something! Basically, the amount of wind must be sufficient to allow you to tilt the sail back and hang in the harness lines. It must be enough to allow you to get your feet back into the straps without sinking the tail and coming to a grinding halt. Being able to judge this takes practice.
The balance of forces at work while planing is complex, and a little weird. The sail, if released, would flop over downwind and forward. You, hooked in and hanging from the harness lines, prevent that from happening. The harness lines support a significant fraction of your weight so you’re not sinking the tail, even though your feet are in the footstraps at the back end of the board. This essentially transfers most of that weight forward to the mast foot. In essence the only way you feel the sail’s pull is through the harness, in which you are comfortably seated. The forward part of the sail’s pull can be thought of also acting through the mast foot, although the side force is transmitted through the harness lines to your butt, then through your legs to the back of the board and eventually down to the fin. The fact that the sail is tilted aft angles its thrust force upward, partially unweighting the board, rig, and you, so the planing lift required of the board is less. This reduces the board’s planing drag in the water, allowing you to go even faster! This is why sailboards are faster than most other sailcraft. Speeds of 20 to 30 mph are common, and the speed record is above 50 mph! Tilting the sail aft also increases its efficiency by "closing the gap" between the sail and board, effectively endplating the sail. This is a well-known speed technique.
Down under the board, the fin is producing most of the side force to counter the sail. The lift of this hard-working little wing tends to lift up the windward rail, but you are holding it down with your feet. All things equal, a longer span fin will be more efficient, but will create more of this tendency to rail up, and require more downward force from your feet, or that your feet be located further outboard on the board’s tail. For a given board width and rider weight there is clearly a limit to how long the fin can be. Long fins are for big people and wide boards. Since the fin is symmetrical side-to-side, the board never goes straight through the water. It’s always yawed slightly upwind, since the fin needs some angle of attack to produce its sideforce.
Not all of the sideforce needs to be generated by the fin, however. If the fin is located behind the CLE of the sail, some of the required lateral resistance is generated by the planing hull itself in what might be termed "asymmetrical spray-throwing". In planing only the last roughly 1/3 of the board is even in the water on the average. The front part of the board is out of the water. The part of the hull that’s in the water rides nose-up over the surface. As the board passes over the surface it pushes the water down, and the water’s inertia is manifested as high pressure acting on the bottom of the board, i.e., it pushes back! This high pressure underneath the board causes some water to be squished out the sides as spray. If the board is tilted to one side or the other, the planing lift force contains a side-force component in the direction of the tilt. An indicator of this is that more spray is squished out on the opposite side of the board. Since most of the planing surface is ahead of the fin, tilting the board from side to side creates both sideforce and turning moment, and accounts for the board steering like a water ski. But the fin is more efficient at producing sideforce than the board, so tilting the sail aft and loading more sideforce onto the fin, increases speed! This is called "riding the fin", and has long been recognized as a way to go fast.
Barely Planing This is actually planing, except there is not enough wind to produce the sail force necessary for you to step back into the footstraps. The mast is more-or less vertical, and your feet are straddling the front footstraps. The fraction of board lift due to planing is smaller, and that due to the board’s buoyancy larger, so the board does not cant-steer like a waterski as well. Here it is helpful to tilt the sail fore and aft to help with steering. Some combination of rail and asymmetric spray-throwing along with the fin produce the sideforce required to counter the sail and maintain course. However the amount of sideforce the fin can generate is limited at these low speeds.
If you hook into the harness line and try to step back on the board to put your feet in the footstraps, you quickly learn that the sail is not producing enough pull to significantly unweight your feet from the tail of the board, so it sinks. The board slows down, the fin stalls, losing lift, and the board’s rails turn it upwind. This is called rounding up
In this mode of sailing It is critical to use the hull to generate most of the sideforce to offload the fin so it doesn’t stall. Standing more forward on the board helps accomplish this, and you need to have your weight forward anyway to keep the tail from sinking. Obviously, you can’t have the sail tilted too far aft. For this reason it is important when planing in marginal conditions to be ready to quickly get out of the footstraps and step forward on the board if a momentary lull hits. Then, if a gust hits, you’ve got to be ready to step back into the straps again, and tilt the sail back again. Once you understand what’s going on, and what you need to do, sailing on a gusty but marginal day can be a great way to teach your body this part of the dance.