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Let Johnny Cabianca explain.

Squash? Rounded Pin? Swallow? Square? Diamond? Even good surfers with a solid grasp of shaping theory can find themselves feeling baffled by the range of tail shapes on offer today. Others might have a preference for a particular shape, but have no real idea why. So, we asked Johnny (a walking encyclopaedia of hydrodynamics) to explain the fundamentals of tail shape – arguably the most critical aspect of how a board performs on a wave.

“The simplest way to understand how a tail shape effects the way a board will handle is to think in terms of overall surface area under the fins, and the smoothness of its curve. At one end of the spectrum you have the smooth curve and low surface area of a pintail, and at the other, the sharp corners and high area of a square tail, which is the oldest tail shape. All other shapes lie somewhere between the two.

Although a board’s overall speed is a combination of many factors (especially rocker and concave), the surface area of the tail is a key element. This is what dictates the flotation and lift of the board when you push down on the tail – the less it sinks, the more speed it maintains, especially at lower speeds.

The curve dictates the looseness – a smooth curve creates less turbulence and grips the wave, while a broken curve releases more easily. So, a smooth shape like a rounded pintail equals more stability and “drive” – perfect for holding your line in big, steep, powerful waves. A broken curve, high area shape like a square tail or squash will plane more and feel looser – better for maintaining speed and throwing the board around in smaller, weak surf.

Then, like I said, every other shape is some kind of compromise between the two extremes. And, while there are lots of different shapes out there, for my shortboard designs I generally stick to four basic shapes that work best and cover all bases – the rounded pintail, round, swallow and squash. I only use classic pintails for guns, and I don’t use square tails because a squash does the same job, but with smoother turns.

SQUASH: the most popular shape, especially for small to medium waves. The extra volume under the fins creates lift which helps prevent the board from bogging at low speeds, while the corners release quickly allowing for faster looser turns and radical moves. The tradeoff is that squash tails have less control in big surf – it’s like bombing a hill on a skateboard with loose trucks.

SWALLOW: this is a really versatile design that comes in many variations, from super wide fishes for small waves to narrow ones for pumping surf. What they all have in common is how the smoother curved rail line and reduced tail area gives more stability and drive to a wide, fast board.

ROUND: good for surfers looking for an all-rounder in quality waves, this is a good midpoint where the speed and manoeuvrability of a squash meets the hold and smooth flowing turns of a pintail. By the way: Gabriel is almost only using round tails, because he is skilled enough to surf fast in any type of wave, and the round is more complete for every situation. Up until 2014, about 70% of his surfboards had swallowtails. This changed as his surfing changed – he has more power now.

ROUNDED PIN: Again, a good midpoint between a round tail and pintail, the rounded pin provides lots of hold on the wave face and smooth, flowing turns in cranking surf – perfect for a step-up board and driving down the line.

At the end of the day, the biggest single factor affecting a tail’s performance is the exit surface area. So while tail shape theory is good to know, it’s important to remember that the key to a good board isn’t one single element: it’s how all the different elements work together.