In Part 1 we looked at the placement of fins on the board and how that caused each design - twin, Thruster, quad etc - to ride the way they do. In this instalment we’ll look at the shape of fins and how that affects water flow and in turn the way fins ride.

Since first being put on the underside of a board, fins have been made in a myriad of shapes - D-fins, hatchets, half-moons and many more - however we’ll only look at the most common modern shapes.

Profile Shapes

When viewed in profile, most modern fins assume a shape similar to a tuna fin: raked backwards with a wide base tapering to a narrower tip. Even keel fins, which we’ll include in the discussion, are a bloated version of the same.

George Greenough in 1970 with a classic fin of the era (John Witzig)

If you recall from Part 1, a board with a fin closer to the middle of the board will be loose though it will lack drive, while a board with a fin near the tail will be the opposite, it’ll be stiffer and have more drive.

The same spectrum from pivot to drive is traversed when considering fin shape: An upright fin will have more pivot, while a raked fin will have more drive. That’s the simple rule, and though fins come in a range of shapes the same principle always applies. However, it’s also worth considering why that’s the case, and there are two reasons.

Rake measure the degree to which a fin is swept backwards

The first is that upright fins - measured from the leading edge where the base meets the board to the trailing edge tip - sit in a narrower column of water, so there’s less resistance when it swivels through a turn. In contrast, a raked fin disturbs a larger column of water when turning; they’re hard to initiate into a turn - hence stiffer - but have more hold when moving through the turn.

The second reason is that raked fins generally have more fin area so there’s more surface to drive off when pumping a board. The opposite is also true, something the author can attest to after recently testing a batch of boutique upright fins that, while very fast and responsive, often slipped while the board was being pumped for speed.

Cheyne Horan's EZ3 fins are an extreme example of upright fins, having no rake at all

Keel fins are a wide base version of the classic tuna fin shape, and they’re predominantly used on fish outlines in a twin fin set up. Keels tick both of the above points: Being long fins they occupy a wide column of water, and they also have a large surface area. The result is a stiff fin with lots of drive, an ideal choice for shorter boards with less rail in the water - such as fish. The keels can provide the drive and control that the shorter rail line can’t.

Another fin worth analysing is the C-Drive. Designed by Troy Clutton, they’re a clever mix of outlines, having both a wide base and a narrow tip. The wide base provides drive, especially through the beginning of turns, while the narrow tip allows for release towards the end. C-Drives still have more surface area than many orthodox shapes, and they also occupy a wide column of water, so all things being equal they tend towards the drive side of the equation. For this reason some surfers choose to ride their C-Drives a size down.

C-Drive fins have both a broad base and a narrow tip

When considering the rule about fin shape - upright equals pivot, raked equals drive - it’s worth considering how this could be toyed with in multi-fin boards. For instance, though Simon Anderson put three fins of equal shape on his board and called it a Thruster, what about if the rear fin was more upright so it had less drag and was looser? What if the front fins were upright and the rear fin was raked..?

Don’t ask me, try it yourself. Report back with your findings.

Cross-Sectional Shapes

From viewing fins in profile, let’s now tilt the axis 90 degrees and look at the shape of fins in cross-section.

George Greenough is generally credited with popularising and refining the tuna fin shape on surfboard fins, and he can be similarly credited with introducing foil to the fin’s cross-section. With sandpaper in hand, Greenough reduced fin thickness, particularly along the trailing edge, and at the same time introduced flex - something we’ll touch upon later.

Cutaway diagram to show the cross-section of a fin and its streamlined shape

From ‘67 onwards, most fins had a rounded leading edge, tapering slowly to a thin trailing edge, the design mimicking the shape of pelagic fish when viewed from above, and for single fins considered the most efficient way of slicing through water.

Despite Greenough’s refinement, many 70’s single fins still had fins up to half an inch thick. In some situations this was desirable as the thickness created drag, and hence control. Cheyne Horan’s attempt to surf large Waimea Bay on a 5’8” single fin is a classic example. The thick fin shaved yards off Horan’s speed but it gave control to a board measuring half the length of a standard Waimea gun. It's worth noting that, not only do modern tow boards measure a similar length to Horan's Waimea board, but specialised tow fins are also thicker than normal to induce drag and provide control.

Cheyne Horan's 5'8" Waimea 'gun' - note the thickness of the single fin

By the time Mark Richards popularised the twin fin a more potent feature was built into the cross-section of fins: asymmetric foil for side fins.

It’s not known who first came up with the idea, however it’s based on the same principle that gives flight to airplanes. That being, a shape that’s flat on one side and bulbous on the other, so fluid flows slower across the bulbous side generating a force towards that side. On an aircraft wing it causes lift, while on a surfboard fin it creates a force perpendicular to the water flow - which results in acceleration towards the outside of the board when turning.

Importantly, not only does a foiled fin accelerate a surfboard into a turn, but it can also generate lift. If the perpendicular force that’s generated is greater than the combined weight of rider and board, the board will want to rise out of the water, thereby reducing drag from surface area and creating yet more speed.

Used on multi-fin boards - which have their fins off the centreline, hence increasing response time - foiled fins turbocharged surfing. Once concaves came along in the early-90s, which also generate lift and drive, surfers had enough speed to take their boards anywhere on a wave, including above it.

On both twins and Thrusters, there was agreement about how fins were shaped. Two asymmetrical foils on twin fins, same again on Thrusters but with a symmetrical fin in the back. But what about quads? What shape works best in quad rear fins..?

Having asymmetric foiled fins in both the front and rear fins of a quad amplifies its accelerating and lift qualities, though sometimes to an excessive level. If you have four asymmetric foiled fins in a quad and struggle to contain it through big turns, it may be harnessed through symmetric fins in the rear which will provide forward drive without an overabundance of lift.

However, with so many factors at play, fins, rocker, volume and everything else, the choice of shape in rear quads comes down to personal preference. Find your favourite set and experiment with flat foiled and double foiled, and keep in mind there’s also 80/20 foils and 70/30 foils that both sit somewhere between flat foils and symmetrical foils.

Before we drop over the clifftop of esoterica, it’s worth mentioning one more design built into fin foils: the inside foil.

When any fin cuts through water it creates drag, which as was noted in Part 1 can be a good thing. However, sometimes it’s not. Sometimes, a surfer will believe their board already offers enough control so will seek to eliminate as much drag as possible.

Whereas foiled fins create a force perpendicular to the water flow, the drag from the same fin runs parallel with the water flow. Inside foils create a pocket for disturbed water to flow, hence minimising the overall drag quotient.

Of all the popular fin configurations, quad fins create the most drag so it’s also the design where you’ll most often see fins with an inside foil used.

Flex

When a fin flexes it causes a distortion of its shape. Many shapers have experimented with flex though the original idea and inspiration flows back, once again, to George Greenough who was seeking to mimic the movements of tuna fins through the water. Unlike tuna fish, however, surfboard fins can, and indeed should, only flex through one plane. Fin-making specialists lay the fibreglass to control this movement.

There's a lot of misinformation around flex, and I hope to avoid it here, however it generally works around the idea of stored energy. When water cuts at an angle onto a fin, it can overcome the stiffness of the material and distort its shape away from the water flow. When the turning weight is released it snaps back into shape. As this generally happens around the tail and the tip of the fin - the thinnest areas - it helps propel the board forward. The effect is often very subtle.

Fin flex is best harnessed by front foot surfers who like to unweight three-quarters of the way through turns. It's also better served in small to medium-size waves. Bigger boards need stiffer fins.

Too much flex, or even uncontrolled flexing, can result in indecisve turns, such as a double pump off the bottom, or a board that's unwilling to drive through a turn on its rail.