Surfboard fins explained. A guide to surfboard fins 4

Warp Drive surfboard fins

Fins have become essential as part of a surfer's arsenal: we have all heard about how they increase drive, maneuverability and speed. A board can be altered significantly by changing the fin setup. Here are a a few basic pointers to help with making your next fin purchase.

A Futures fin designed in Solidworks ready for #3dprinting

Terms and Measurements

The following  terms are traditionally used when speaking about surf fin design:

HEIGHT or DEPTH

The depth of a fin describes how deeply the fin 'reaches'  into the water. Measure  from the bottom of the surfboard, where the fin starts, perpendicular to the bottom,  to the highest point at the tip of the fin. The deeper the fin the more it can access the power of the wave underwater. At the surface some of the water will already be turbulent from wave action, but deep in the wave the flow will be 'clean'. a very rough rule of thumb employed by many shapers is to have an inch of fin for every foot of board (for singles). A deeper fin can offer more stability and power, but some surfers prefer the looser feel of shallower fins.

BASE LENGTH

The base of the fin sits on the surfboard bottom.  The length is measured fore and aft to give a number decribing the 'base length'. Like a rudder on boats longer lengths provide more surface area to lever against the water flow. Turns are stronger and drive and speed is increased.  However, the longer fore and aft length tends to 'track' more which will draw out turns. If sharper turning is desired a shorter length can help. Water temperature will also affect what base length one might choose. Cutaway templates are a way of reducing the length at the base and putting the maximum length higher up the fin where there is less turbulence from the junction of board and fin.

TOE

Traditionally shapers describe the toe angle as the angle the fin makes in relation to the center stringer. What is actually working however is the angle of the fin in relation to the rail convergence. Side fins usually, but not always, have toe-in. Faster speeds require less toe-in. Big wave boards will have nearly zero toe-in, and recently have started using toe-out.. Slower speeds can handle more toe-in. Angling in the fins is a way of steering the board. (Think rudders). By setting up fins with toe, one is forcing the board to a pre-determined turning path, which a lot of surfers enjoy. There are many ways up the mountain however, and toe is no longer a hard and fast rule to slavishly follow.

CANT

The cant is the angle a fin makes in relation to the bottom of the surfboard. Single fins and central trailing fins are usually set up with 'no cant' i.e. perpendicular to the bottom of the surfboard.   Cant generally slants outward. Theoretically one can slant it inwards too however, which will lead to tunnel fins. Cant is a way of getting some horizontal area into the fin setup. To explain horizontal area look down onto the fin, perpendicular to the board. One will see that a canted fin takes up more area than a zero-cant fin, which will present the minimum of cross-section.

A bonzer fin is canted and will thus present horizontal area.

                                                               

Increasing the fin cant increases lift aganst the surfboard bottom, much like a hydrofoil. It decreases pressure on the surfboard hull, and can increase speed. By reducing pressure on the hull, turning will feel more effortless and more like flying.  Beware of cant on big waves however. BTW tunnels overcome the difficulties made by canted fins in big waves.

FOIL

 Foil is the curvaceous shape which is the most mysterious part of the fin. Check out a fin's foil by looking down onto the fin from the tip. Foil shapes have been developed since the beginning of the twentieth century.  Names like Dr Eppler, Royal Airforce Engineers (RAE), NACA (who became NASA) and Clark-Y  are often mentioned. Of note also is Dr Martin Hepperle who designed the MH32 foil. Foilshapes come in 'families' which group together certain aspects of their behaviour. Surfboard fin foils fall into the lowish Reynolds number category similar to model aeroplanes.  (Not supersonic aircraft for example) Foils should have a bulbous leading edge (front) and a pointy trailing edge (back). Having a proper foil on one's fin(s) will make a huge difference.

Clark-Y vs MH32 

Some fins are flat on one side and foiled on the other (usually side fins) This is the famous Clark-Y foil last used on Aeroplanes in 1911. The problem with these fins is that they present a sharp corner to the waterflow. A tiny detail, you say, but tiny details like miniscule bumps in the foiling make all the difference.  Check out our range of MH32 foils, designed in the 1990's, which we believe are a massive improvement on the Clark-Y.


Single fins and trailing fins are foiled symmetrically i.e. the same on both sides.

RAKE

The rake angle measures how far back the fattest part of the fin (chord) curves in relation to the bottom of the board.  Rake is usually sloped backwards, but in the case of the Gullwing there is a forward rake component at the base of the fin.

The Gull Wing has forward rake at the base

 A fin which slopes back sharply is said to have a smaller rake angle. A sharper rake angle can make the board more forgiving, and will have a larger tip which will prevent spinout,  it will effectively give the fin more length, (see above re: length).  Fins with a large rake i.e more upright  give the surfboard a tighter turning radius, (check out our Spitfire range), Spitfires have a 90 degree rake angle and offer the ultimate in pivot. 

The Spitfire has no rake

FLEX

Flex is a much bandied about term in the surf industry. 'Flex' as a word in itself does not describe anything. What type of flex? Stiff flex or Soft flex?   When somebody says "It's got flex" One feels like that emoji which slaps its own forehead. Stiffer fins react predictably. Fins with a softer flex tend to spill some of the pressure of the tip which makes for a different feel when turning. Each to their own. My suggestion is to experiment and find out what you prefer. Some people like to say that beginners should surf such and advanced surfers should use the other, but I think in this day and age we are over such compartmentalised thinking.
When flexing a fin there is a bad way and a good way however. Fins should flex by rotating the tip around the chord when looking down onto the fin. Much like a fish flicking its tail. If the whole fin flops from side to side, that's bad.

Setups / Placement

There are countless setup possibilities. Generally the overall surface area of the fins will give a good idea whether the board is 'overfinned' or not. Having more fin area overall will give more control and drive, but one is also making more drag. Fin tips create tip drag which is caused by a vortex spilling off the tip, So more fins is not necessarily the answer to wanting more drive. (One could just use a bigger single fin, for example)

Fin placement makes a difference in the way the board will respond and turn. Fins placed forward on the board closer to the centre  will make the board 'looser', as the turning axis ( think of the fin as the pivot on which the board is turning) is more balanced. The board is rotating around the fin and will seem easier to turn. However, while it may be easier to turn, it is harder to control. Having the fin cluster spaced widely apart is like having a longer wheelbase on a car.  If you want more balance and drive, you should try placing your fin or fin cluster further aft, and tightening the fin cluster.

Single Fin

 Single fins may well be called the universal fin setup. It is the difference between going finless and having one. Single fins are typical on longboards and midlengths, and some will have you believe that beginners should start with a singlefin. Again refer to my modernist compartmentalisation rave. Have however many fins you want and don't believe the hype about more fins = more ability. It just ain't that way. Single fins rule. They are fast and responsive, and have only one tip. They are beautifully placed to take advantage of the power in the wave and they feel great.  Single fins are foiled symmetrically.

#Gunproject spitfire fin

TWO FINS (a.k.a. Twin-Fin)

Popular on Fish boards, Simmons boards and some longboards. Some people like having two double foiled fins, while others prefer having a pair of assymetrically foiled fins with toe.  

Not all two finned boards are created equal either. Here is a million dollar board with a 3d printed spitfire surfboard fin as the leading fin and a hand foiled wooden tunnel with a wavy trailing edge as a tunnel plus one setup. This board surfs like a single fin (i.e. one tip) but has the added lift and directional versatility of the tunnel. A Jet boost.

Million dollar surfboard with 3d printed front fin and hand carved back fin

Twin fins for Warp Drive afficionadoes

THREE FINS (a.k.a. Thruster)

The thruster is very common.  They are set up with two side fins and a trailing fin in a triangular way. The side fins are flatter on the inside (refer to Clark-Y vs MH32 above). They are set up with toe and cant. The center or trailing fin is foiled symmetrically, and set with no toe and no cant.

FOUR FINS (Quad)

As the name suggets quads have four fins. The fins on quads are smaller than thruster fins, just like the thruster fins are smaller than twins and so on. Normally the front pair is single foiled, smaller and set up with more toe and cant than the rear pair. The double pair is set closely together so that the fin cluster is tight. Some surfers swear by the speed and manueverability they can derive from their Quads. 

FIVE FINS

This system is often seen on today's surfboards because shapers believe that it offers ultimate versatility. One can now choose whether to set the board up as any of the above setups with five fin positions  laid out on the board. Of course one doesn't have to fill all the boxes, but one certainly could. If using five fins, however,  be aware of the overall surface area and reduce the fin size even further.

Fin Systems

FIN CONTROL SYSTEMS (FCS)

Refers to fins using the square prongs which are screwed into the box with a grub screw. This fin system is currently the most popular fin system because the plugs are easy to install and the  fins are easy to make. Fins are made upright or slightly canted. Plugs come in zero, 5 degree and 9 degree cant.

FCS tabbed spitfire whale bump trailing fin from Warp Drive

FUTURES SYSTEMS 

Futures use a base which hooks into the box at one end and is screwed down at the other. They are generally stronger as a fin attachment because they are a fin tab which is longer than FCS. There are two sizes: 1/2" which is often used for the trailing fin at a thinner part of the board. and 3/4" which is more generally (but not always) used at the side fin position.

Futures based Knubster trailing fin with BLEF

Fins Unlimited (FU)/ Standard/ Chinook

This fin setting is universal for  single fins and 2 + 1 setups. The box is longer than the fin tab. The base of the fin will fill only part of its fin box. This allows  adjustability and versatility to the fin setup. The fin tab comes with a horizontal pin, which slides into a groove inside the box, and which holds down one end of the tab. The  other end is held by a bolt and 'plate'. The plate is a square nut which fits into the same groove as the pin. The bolt screws into the plate through a hole which has been drilled vertically through a protruding part of the tab.

Standard fin box tabbed fin with Bumpy leading edge.

An extra long fin box means that the fin can sit closer to the back of the board (for more control) or further forward for maneuverability or 'looseness', check the fin placement paragraph above.

TUNNEL FINS

Tunnel fins are also known as 'Annular wings' The tunnel fin gives a board more speed and maneuverability than a standard planar fin. The horizontal component works like a hydrofoil or similar to cant in ordinary fins. What makes the tunnel special, however, is the fact that they eliminate fin tips. The hydrofoil works by increasing lift and decreasing pressure on the bottom of the surfboard. Speed is increased significantly because they improve the lift/drag ratio.
It is designed for use as a single center fin or as the center fin in a Thruster setup. Tunnels are turned by rotating around a theoretical centre of rotation. By sinking the tunnel into the board (effectively making a half-circle) the centre of rotation aligns with the stringer and when one turns the surfboard the tunnel slips inside its own footprint as it were. This makes turning effortless. Having a tunnel by itself will feel like having no fins at all, however the board will fly (like a hydrofoil, by only touching the fin in the water) and the board will still have directional control. Adding a tunnel behind a single will maintain the single-fin feel of a board. Adding it as a rear fin on a thruster fin setup will make the board feel more like a Twinfin.
The size of the tunnel used is determined by the size of the tail it is placed on. Generally they should be placed as far back as possible.

A tunnel placed behind a single fin on Roy Stuart's Hotkurl

Whale bumps

A video on how they work is over here.

Million dollar hand carved hardwood surfboard fin with biomimetic leading edge tubercles a.k.a. whale bumps