Understanding Kitesurf board fins
What are the fins on a twin tip for?
Fins help prevent the board from feeling skatey underfoot. Without them a board feels loose and slippery on the water, if you've not ridden without fins before, the sensation is a bit like standing on a swivelling office chair. Only your foot pressure and body position keeps everything going where you want it!
What size fins do I need?
To start with, big fins about 5cm deep are helpful as they make the board feel 'grippy' and want to go in a straight line, so it will be more tolerant of mistakes in your edging. Very helpful if you ride in choppy conditions, where the engle of your edge to the water is always changing. Bigger fins are also good for carving turns as they stop the board from sliding out.
Why would I want smaller fins?
Some boards have lots of grip from channels, sidecuts etc and the just dont need such big fins to get the grip. Too much grip makes the board feel unpleasantly 'sticky' and difficult to slide out when you want to. This is particularly the case for lighter smaller riders, who exert less pressure on the board. Smaller boards also have less lift in the tips and can be used with slightly smaller fins.
Proper skim boards have no fins at all and will teach you lots about correct foot pressure and edge control!
What are fins made of?
The primary material used for kitesurf fins is a very dense grade of fiberglass called G10. It's extremely hard wearing and has good resistance to impacts. G10 is easy to repair with epoxy and can be sanded back when damaged. Other fins are made of cheaper fiberglass, expensive carbon or high density plastics.
Does the shape matter?
The surface area is what counts on a twin tip fin. Long narrow fins will grip like a shorter deeper fin. The profile is usually curved on the side of the fin closest to the edge of the board. This adds a small amount of lift and helps the fin to grip. Finer profiles are better for reducing drag, although they can be more fragile. Fins are a compromise between grip and drag, the shapes of most fins aim to optimise the amount of usable grip for the least drag.
How do they attach to the board?
The most common method is two M6 or 1/4" stainless steel bolts. The industry standard for the spacing is 38.5mm for the hole centers. In practise however the tolerances can be a bit off and you may need to tweak the holes in the board a bit, even on a brand new board. Some manufacturers will countersink the heads of the bolts and others will use flat heads and some have plates to help distribute the loads from the fin onto the top of the board.
Is anyone doing anything radical with fins?
F-One have a unique system where the fin has a head that fits into a hole in the board and a plate bolts on top to retain it. This allows a super fine profile for the blade of the fin as there are no bolts inside it.
Nobile have a cam lock quick release on their KISS fins for easy removal, although the boards are still drilled for standard fins if needed.