Most modern high performance surf kayaks are now sold with some form of fin configuration on the hull. To maximise these it's really helpful to understand how they affect the boat's performance, so you can tailor your setup to suit the type of wave you ride, or the type of rider you are.
Fins give you grip when you need it most!
It's really important to remember that fins are only a small part of the kayak's design. Quite often, where you position yourself on the wave, and how you use your body to initiate the turn, will have more effect than any adjustment of your fins! It is almost as important to be able to ride without fins to truly understand how to carve using the rail.
The following page is based around a standard tri fin set up which is 2 fins on the outside (thrusters) and one in the middle (centre fin). Although other configurations are available such as quads and 4 + 1's, it's simpler for us to concentrate on the most popular type of set up initially. If you'd like to know more about the other configurations let us know and we'll add them later.
The anatomy of a fin
Fins can come in a variety of shapes and sizes. Understanding how these changes in size affect the fin's performance is the first step to unlocking some of their potential.
This is the height from the base of the fin to the tip - effectively it is how much fin is in the water. The most common depth is around 3.5". The greater the depth the more stability through the turn, however the offset of this is that the boat will feel stiffer when you try to turn it.
This is the measurement from the front to the back of the fin. This baseline measurement is actually really important as it determines how much drive you can get from the fins. The longer the baseline the more drive the fin will produce. However the longer the fin base becomes, the more it encourages the boat to follow a drawn out arc through the water.
This is the measurement of the curvature of the fin. The greater the rake the more drawn out the turn will be. As the angle of rake lessens, the tighter the turning axis becomes, but the boat becomes less stable through the turn.
If you add all the ingredients listed above you get the outline of the fin, or its template. The way that the area of the fin is distributed will have a significant difference in the way that the fin handles. Opposite is a picture of different template 3.5" fins. The centre fin is more upright with a shorter base line, this has been compensated for by having a more upright rake - these fins would give tighter more aggressive turns. The outside fins have a longer base line and therefore much more pronounced rake. This type of fin would tend to draw out the turn slightly more.
Without making this too complicated!
The centre fin is symmetrical, so when the water arrives at it the water molecules pass around the fin at the same speed, arriving at the back of the fin at the same time.
Thruster fins start generating useful forces when the boat is leaned into a turn. The thrusters are asymmetrical, in that the outside face is curved and the inside face is flat. This means that when the molecules encounter the thruster fin, those going round the outside have to travel faster than those moving down the inside causing an imbalance in pressure. This imbalance in pressure creates a lateral force that pulls the boat into the face of the wave, generating both grip and drive.
The photo below clearly shows the change to the water as it leaves the fin having had the pressure altered.
The centre fin is always exactly on the centre line of the boat and inline with the nose and tail. The thrusters however are varied in their orientation relative to the centre line. The front of the fin boxes are often "toed in" towards the nose. The greater the toe in the more manoeuvrable the boat will become, until the angle becomes too great and just causes drag. Less toe in equals less drag, and is therefore faster.
There are two main systems which are used in surf kayaks. The most common is the traditional slot box system as pictured above. The advantage with this system is that you can move your fins to give a customised feel to your boat.
The second system is the Futures system. This is a fixed position set up so you are unable to move the fins to suit you. However because it is a system used in surfboard manufacture, there are hundreds of fin templates available so it's customisable in this way.
Some manufacturers combine these two systems with future fins on the outsides and a slot box in the centre, to try and give the best of both systems.
Irrespective of what system you use, the fins should be able to grip the boat to the wave face at near impossible angles!
Fin placement is incredibly individual, dependant on the type of waves that you surf and also the type of surfer that you are. Basic rules when it comes to fins are that the further forwards they are, the more manoeuvrable the boat becomes; the further back they are, the more locked in.
Take your boat and a screwdriver down to the water's edge, on a mellow surf day, and follow these simple steps to try and find fin placement nirvana.
Remove the centre fin completely, then slide the thruster fins as far forwards as you possibly can. Go out and catch five or six waves, surfing them as you normally would. The boat should feel lively and loose, you should be able to flat spin should you choose to. If it doesn't feel like this, then I would assume you're either very light, or you're in a larger boat than your frame requires. If you're in the right size boat then consider trying some smaller fins.
Back on the beach move the fins all the way back in the boxes, and go and surf again. This time you should feel that the boat is more locked in and stable through the turns. It should feel a lot more predictable than earlier, but the turns are more drawn out. Keep surfing into the beach and changing the fin position by moving it further forwards until you find a position that feels comfortable - loose on the top turn, but positive through the bottom.
Congratulations! You have just discovered your pefect twin fin set up. This is a perfect set up for those who are learning, as it allows the boat to move freely, helping the rider to transition the boat from rail to rail. It's also the setup most suited to smaller wave riding. This is a setup I ride regularly as I enjoy the ability to really throw the boat around without it being held up by the centre fin.
Slide your thruster fins all the way forwards again, and re-insert the centre fin. Get it as far forwards as you can without it overlapping with the thrusters. Go and surf again. This time you should find that the boat feels significantly more locked in than just riding on the twin fins. You should now be able to drive the boat against the fins through the bottom turn.
Keep moving the centre fin backwards, this will stiffen up the turn but give more drive. The greater the triangulation between the fins you can control, the more drive you'll get out of your turns. Opposite is a picture of Dave Speller (2009 Mens HP World Champion) landing a Pan-Am! You can see the triangulation of his set up. Thrusters all the way forwards, centre fin all the way back!
Enjoying fiddling with the screwdriver? Try these five and let us know what you discover!
- Surf with the thrusters inserted backwards in the fin box!
- Line all three fins up so there is no triangulation at all.
- Ride twin fin with one all the way forwards and one all the way back.
- Get your ideal thruster set up, then put tape over the unused sections of the boxes to seal them. What does it feel like?
- Decrease and increase your fin size (if possible).
When it all goes wrong!
Sometimes it doesn't matter how good you are, or how much drive you can get from your fins, you just get a good slap! Chris Hobson making a wipeout look good! Photos by Pete Copp.
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