The fin, or fins, underneath your surfboard are there to help with the steering of your board. The fins will also assist with stopping the surfboard from sliding around on the wave. The further the fin goes down in the water, the more control you will have over the surfboard on the wave. The shorter the fin, the less resistance there will be on the water. The longer the distance of the fin where it is attached to the base of your surfboard, the faster you will be able to go.
Another important factor for your surfboard is how close together the fins are. The surfboard will be looser, or easier to turn, when the fins are placed closer together. The surfboard will also become easier to turn the further towards the front (the nose of the surfboard) that the fins are positioned.
There are 2 types of fins. Fins that are able to be removed from the surfboard, and those that are permanently fixed. Fins are generally referred to by their number. For example, one fin is called a single fin, two fins are twin fins, and three fins are usually known as thrusters, or tri fins.
Single Fins - The original and classic fin setup. Single fins are most commonly found on a longboards. Single fins tend to be much larger to give your surfboard more drive and the ability to lock into a wave. Single fins are generally positioned further towards the back of your surfboard to give you a touch more control. If you want to ride the nose (or hang 10 off the front of your surfboard) you will need a fin that is deeper in the water and wider at the base where it meets the bottom of the board.
Twin Fins - Twin fins are another retro design. Usually you'll only find the twin fin setup on a retro surfboard and fish or swallow tail surfboard. Due to the lack of a center fin, you won't be able to ride bigger waves with this configuration. Twin fins make for a very loose, drifting ride---so don't be taken aback if your retro fish feels a little wild at first. Once you get a good feel for it, twin fins are a blast to ride.
Thruster - Three fins is probably the most universally popular combination, especially for the modern shortboard. Typically, in this set up the 3 fins are the same size. The 2 side fins will allow the board to be more responsive and able to perform more radical turns. The center fin makes the surfboard a bit more stable. This fin setup is common on shortboards, some hybrid surfboards, and funboards.
2+1 Fins - Some modern longboards are now available with a larger middle fin and 2 smaller outer fins. This is known as a 2+1 fin system. The two outer fins give the board a little more maneuverability over the traditional single fin approach, giving this the same all-rounder appeal as the thruster.
Quad or Four Fins - Quad fins are most commonly found on fish surfboards due to the even number. The extra set of fins adds to the speed. The lack of a center fin means there is less resistance in the water. More maneuverable and adaptable than the twin fin setup, the recent years have seen a lot of pros and big wave surfers experimenting with this design.
What Sort of Fin is Right For Me? There are many possible fin combinations. You have to consider the size of the wave you will normally be surfing. Do you want to perform radical maneuvers or glide through the water? Do you want to surf faster or slower? Some surfboards like the fish or swallow tail need a symmetrical fin set up to enhance the V-shape at the tail of the surfboard. A great option for your surfboard can be to have a removable fin box set up. This allows you to play around a bit with the placement and size of the fins. This makes your surfboard more flexible to varying surfing conditions. You can change the performance of your surfboard immensely with the fin set up.
In Summary. Radical Maneuvers. Can have two or three fins. Fast Surfing. Two fins will give you the sensation of less water resistance. Four fins can really add to the speed and drive of the wave. Hanging Ten. You need a single fin that is deep into the water and wide at the base where it joins the board. Longboards. Tend to have a big single fin. Longboards are generally more for gliding through the waves than performing radical maneuvers. Fish or Swallow tail boards. Due to the V-shaped tail (or back) of the board, these surfboards come with symmetrical fin systems, usually a twin.