Fins and Board Shapes

Fins work best when they are in the water

This article is going to evolve over time as I learn more. I’m working on a general chart that will enable you to pick a condition–like “the board won’t maintain direction in the waves” then you choose your current fin setup and it suggests corrective action. Right now it’s a collection of post-it notes. I need to do a bunch of experiments, or find someone who already has, so I can give real information.

I realized some time ago that I didn’t know squat about surfboard shapes and fins. I have all kinds of theories, but they only hold water at the extremes. I lack the sensitivity and experience for nuances to matter. So I was shocked and pleased to discover the other day that I could tell that the standard fin on my Starboard which seemed just fine in flatwater was inadequate for my weight and clumsy maneuvering in the surf.

I learned in many years of racing cars that completely understanding how components work enables you to relate something you’re feeling in the car to likely corrections. The ability to fine tune comes only partly from experience, the rest comes from understanding the system.

So I browsed the web looking for information on board shapes, rails and fins. I found there’s not a lot of information in any one place, but there’s a little information in a lot of places. So I relied on the doctrine of fair use, stole freely, and rewrote it all to focus the information on Paddlesurfing issues. Quite a bit of information came from the Harbour Surboards site (http://www.harboursurfboards.com/). Whoever writes for Harbour is knowledgeable (or has access to knowledge) and an elegant, erudite writer, worth a read even though it’s all focused on laydown surfboards (or as one wag recently wrote in Standup Zone: Hand Paddling–which sounds like something that might make you blind). I’m not going to fully acknowledge all the other sites (Surfline, Wikipedia, Surfing-waves, Swaylocks, etc.), because the bibliography would be longer than the article and I didn’t steal as much from them.

Fins
Fins are probably the biggest mystery, they come in a bewildering selection of shapes, sizes and parameters like flex and foil. It you don’t like the way your board does certain things then the easiest and least expensive thing to change is the fin, and it can have a great effect. First let’s cover the basic metrics:

Base is the width of the blade at the widest part of the fin, generally where the fin meets the board.

Height (or depth) is the distance from the base of the board to top of the fin.

These two elements combined approximate the fin area. A fin with a larger area is going to have more holding potential and have stiffer turning characteristics. A fin with less area is going to turn easier but have less holding potential and will drift sideways in a wave. Those are two different thing–holding means continuing to control the tail of the board under turning pressure. When a fin loses hold it’s a sudden loss of control.

Drift is sideways movement without loss of hold, and there’s always some. It’s just like a car sliding without spinning out. All fins drift some, and the board shape has as much to do with this as the fin does. A wide tail increases drift, a narrow one decreases it. Drift is also expressed as it’s counterpart–Drive. Drive is the feeling a fin (and a rail) gives when it has low drift. Fins with more base will have more drive. With less base area, you get less resistance when pushing through a turn.

Changing fin height compensates for both tail width and the surfer’s weight. Increasing height creates more grip on turns but also makes it harder to turn. It also increases hold while nose riding.

Sweep (or rake) is the angle of the fin from the base to the tip (generally measured at the back of the fin–so it’s from the end of the base to the rear of the tip). A fin with more sweep will result in longer more drawn out turns that are easier to control. A fin with less sweep will have a shorter turning radius but it is easier to spin it out.

Flex is the amount a fin flexes from side to side. Hard to believe that a stubby piece of solid fiberglass flexes when you turn your board, but it does. A fin with little flex will be more responsive, faster, and have more drive. A softer flex fin will have less snap, and be more forgiving–think Cadillac vs. Porsche.

Tip Area (like football-shaped fins) helps the fin hold when you’re up on the nose, but resists turning, decreasing maneuverability.

Fins that are highly raked or sickle-shaped have wag. The tip flexes horizontally as well as vertically, and so bends with the turn, which generates snap when exiting the turn, driving the surfer forward.

The shape of the back of the board also works like a fin, stabilizing the board and influencing it’s carving characteristics. If the tail is flat and wide, there’s less of this effect, if it sweeps into a pintail there is more. Makes sense, eh?

The real hydrodynamics behind this effect are more complex, but fortunately leads to the same conclusion as a “common sense” view. As a general rule of thumb a board with a wider tail will require a larger set of fins. A board with a narrower pin type tail will require less fin. A larger more powerful surfer will require a larger, stiffer fin to give the needed hold. A smaller lighter surfer will need a smaller, softer fin. If your board is feeling loose and you would like more drive, consider a larger more swept fin. For a board that feels too responsive and has too much hold a smaller softer flex fin could be used.

Fin position
Both the design layout and adjustment affect the feel and ride. A tightly grouped thruster setup allows the board to do sharper turns. Fins that are further apart make the board more stable.

Today it’s not out of the question (though it sure looks odd) to see six fins on a board. The most typical setup for a Sup board is a 2+1 with a medium sized center fin 6 to 9.5 inches deep, and two smaller front fins called thrusters or side bites. Thrusters are normally 3 1/2 to 4 1/2 inches in depth and in a fixed position but are removable. Most thrusters have a flat foil on one side which helps them release quickly when they’re on the outer rail. Most center fins are adjustable. Moving the center fin forward makes turning easier, but if you move it too far forward it won’t hold well. Not only does it get too far from the rail, but it also starts cranking against the thrusters instead of working with them. When the center fin is increased in size it creates more drive (reduces drift) but makes the board harder to turn.

A good starting point for center fin is 8 to 8 1/2 inches from the tail. This is true both for multiple fin and single fin applications.

Single Fin or 2+1?
Multiple fin boards turn more off of the rail while single fins deliver more of a pivot type of turn. This is mainly because the side fins are close to the foot that is putting pressure on the rail being sunk into the turn, so a lot of the turning pressure is applied to them as the rail is pressed. Thrusters are usually angled towards the nose which also assists in initiating the turn and are foiled flat on the inside of the fin, so the thruster on the outside of the turn will release easily.

A single fin sits behind the pressure foot so turns are more of a pivot. The surfer lifts the nose up and rotates the board on the tail rail. Assuming you’ve optimized the size, shape and placement a single fine will feel loose but stable and will be particularly good at swoopy classic longboard turns where a frontside couples smoothly to a backside, and on and on. Because those kinds of turns are classic, a lot of people consider single fins to be the only true longboard arrangement.

So the question is, which turn do you want to make? Most Sup boards that offer thrusters have removable ones so you can experiment with both fin types in varied conditions. You’re likely to find that you like thrusters when you want to maneuver sharply and cut. Multiple fins are only faster and more maneuverable if they are pushed hard with an aggressive style. If you’re just trimming down the line looking smooth and soulful, more fins just slow the board down.

If you’re trying for the best of both worlds with a “2 X 1″ Sup, push the center fins forward some, which serves to close up the distance between the fins and loosen the board.

Board Stuff
If you’re not shaping your own boards or ordering a custom some of this might seem like it’s just not important. But it’s one more piece of what it will take to really understand how your stuff is working, and what you might do to take the next step in your surfing and paddling evolution.

There is no such thing as a perfect all-around Sup board. Every shaper works with a known set of limitations and blends all the factors to deliver what Blaine Chambers called “a specific flavor of ice cream”. You might love peach licorice cookie dough, but it gives me the willies just typing it. Here’s how some of the factors work for and against the tastes you want:

Make the nose wider. Now it’s more stable to stand forward on the board, but it’s heavier to swing from the tail so it’s harder to turn.

Wider tail: Better lift for stability in motion but now it’s harder to edge the board in turns

Wide mid section: Good initial stability, but it might wobble in waves and it’s harder to turn.

Keep the nose and tail narrow, widen the middle: The board gains stability but it gets difficult to get the rails to bite.

Widen the nose and tail and keep the center narrow: The rails become more parallel which is nice for noseriding, but straight rails don’t turn well, especially in big waves.

Widen the tail and narrow the nose width: You’ll have a slow board that turns great but it will be hard to stand on.

Tail Shapes influence every other element of the board.

Squaretail: Fast but abrupt. Water is guided back to the tail smoothly then BAM, it’s out the back door blinking in the sunlight. This shape enables volume to be retained in the tail, increasing flotation and stability. It also enhances drive (the acceleration provided by the fin and rail carving against the wave face). It’s no surprise that one of the most stable boards is the JL 11er. The squaretail suits a beginner/intermediate Sup surfers who want to catch mucho waves and not fall off all the time.

Squashtail: A softened squaretail, has width for flotation and stability, but the rounded curves at the end of the rails enable the board to carve better in bigger waves.

Pintail: Sliiiiides through the water and doesn’t separate as quickly, so it isn’t as fast as a square or squashtail but it has a lot more control and smoothness, but the least drive. The curve fits nicely into big waves, allowing the board to carve without levering itself out of the wave. A common tail shape for advanced Sup boards and big waves.

Swallowtail: Blends a squaretail’s quickness, flotation and stability with a Pintail’s smooth turning and big wave fit. When the points are very far apart, as in a fish, the shape delivers drive like a squaretail.

Rocker
Rocker is the curve of the board from nose to tail. There are three primary areas of rocker: Nose, middle and tail. In a longboard it’s mostly important for determining turning characteristics–the more curve the easier it will turn, though drive will suffer as rocker increases. For a Sup board it also determines dynamic stability, recovery and glide. Rocker sometimes improves both dynamic stability and recovery. Picture a monkey (that would be you) standing on a curved stick free to rotate at both ends. The monkey’s weight pushes the curve to the bottom–it’s somewhat stable because any force that rotates the stick has to raise the monkey. In an extremely floaty board with a lot of rocker and a light monkey this all starts going the wrong way. The curved stick is no longer rotating at the ends, but instead is spinning somewhere closer to the middle. But with less floatation a 245 pound guy like me sometimes finds surprising stability on a 10’ 8” pintail that only has it’s nose and tail above water.

Increasing nose rocker decreases pearling but can increase resistance of water entry. Nose rocker can also pop the fin out of the water when you’re on the nose. Boards with a lot of nose rocker don’t usually glide as well as a similar board with a flatter nose. The board is sinking more for a given amount of weight and is shoving water ahead of it.

Rocker in the middle of the board gives less drive in the surf, and less coast when you’re paddling.

Tail rocker eases turning and adds stability under some conditions. But it decreases glide, and reduces decrease drive in the surf. Less tail rocker makes the board hard to turn from the tail, while drive and coast increases.

Thickness
A thicker board will be very buoyant and easy to stand on and paddle, but can also be sluggish and hard to maneuver. The rails will tend to be thick and can be pushed around a lot by wind and side chop.

Board Width
The wider the board the more stable it will be, as well as giving it extra planning area. Wide boards will hunt more, and sometimes wobble when you catch a wave. You generally have to set the rails more aggressively and get your weight on the turning rail quickly. The sweet spot for stable standing and paddling is bigger, but the board may be more difficult to trim in surf. Generally wide boards take a lot more fin to control them.

Board Outline or Plan View is the outline of the board, generally communicated in text or verbally by describing the nose (round, pointed, pulled in, etc), the tail (square, squash, pin, swallow, etc.) the width at the widest part and the width 12 inches from the nose and tail. It’s obviously a limited approach and a picture is worth a thousand words. For standup boards with their stability and glide issues the standard measurement approach is probably even less useful than for longboards.

Bottom Contour is the curve from rail to rail. It’s one of the most contentious areas of board design. Sup boards tend to have simple bottom contours, many are flat, but that will probably change as the designs evolve.

Concave: Guides water under the board and out through the tail, giving it more speed and acceleration. In wider Sup boards it can counter wobble and increase speed.

Channelled: Increases speed, nice in smooth waves, makes the board track, helps noseriding.

Vee: Enables fast rail to rail transitions. Slows the board but aids in quick maneuvers. .

Rails
Low rails are sensitive and tend to bite into the wave face. Good for bigger or hollow waves and longer, smoother turns. For Sup boards low rails give the added advantage of not knocking the board out from under you in side chop or windy conditions.

Boxy, full rails are more forgiving and suited for shorter, quick turns. Good for small-wave surfing and they also give the board more volume and add to initial stability, dynamic stability and recovery. When confronted with side chop on a board with thick rails it helps to sink the rail on the side the chop is hitting. If you’re paddling on that side sinking the rail will also keep you going straight.

Rail shapes come in two basic flavors–turned down and 50/50 (also called egg).

The apex of most 50/50 rails is cheated slightly below center. This makes for a smooth riding board that slides around more in a turn and often noserides better. Since the smooth rail allows water to flow up over the edge, you need either a bigger fin or need to move the fin back.

The apex of a down turned rails is generally 25% from the bottom, starting at the top with a soft roll that tightens quickly past the apex to blend into the bottom. They can (and often do) change to a hard edge fully down-turned rail near the tail. A hard edge in the tail area grips the water allowing for smaller fins. In general a board with turned down rails needs less fin than one with 50/50 rails.

MORE TO COME
Right about here is where that chart I spoke about needs to go. The nice thing about an interactive magazine is that articles can grow and improve over time. As I complete my experiments and pull the data I’ll put what i find here, and republish the article to bring it to everyone’s attention.

For now, here’s a couple of simple suggestions.

Start with your fin in about the middle of the box. Measure from the tip of the tail to the back of the fin. If it’s more than 8.5” than move it back. Surf it. If it feels tight, move the fin forward, too drifty, move it back.

If your 2+1 board seems “skatey” make sure your side bites have the foil sides to the rails, the flat side toward the center fin.

Moving the fin forwards on a 2+1 moves the center fin closer to the side bites and makes the board turn easier. Moving it too far forward makes the fin drift.

If the fin feels inadequate, remember to try more rake as well as more depth or area. More rake gives more directional drive, less rake permits faster rotation.

As for the rest, it will be in the next version.