A lot of people have asked us why some paddleboards have one main fin and others have more than one. The simple answer is that some paddleboards are designed for surfing. You can still surf a single fin paddleboard, but those extra fins near the rails of the board will give you some clear advantages when riding the surf. These additional fins are called side fins, or side bites.
When looking at a side fin you will notice that one side is rounded and the other is flat. The rounded side faces the outside and the flat side faces the inside. It’s this rounded vs flat aspect of the fin that allows it to function much like the wing of an airplane, but in water.
Water molecules travelling over the rounded side of the fin have a longer distance to go before reaching the back of the fin than the water molecules travelling over the flat side. This, coupled with the inward facing angle at which they are positioned helps generate forward speed.
When turning onto the wave, the surfer applies pressure to the inside rail that results in a certain amount of drag slowing the board down on that side. At the same time, the outside rail maintains the faster speed through the turn. This is how side fins help turn the paddleboard onto a wave by generating extra speed on that outside rail while the wave side of the paddleboard slows down.
Surf style paddleboards come primarily in two fin configurations– quad and 2+1. The 2+1consists of the larger main fin in the center on the tail and two smaller side fins farther up and near the sides of the paddleboard. The quad configuration has four smaller side fins two on each side of the paddleboard and normally a larger center fin back on the tail.
Generally speaking, 2+1 fin set-ups tend to be more maneuverable, while quad set ups can generate more speed and work well for big, sweeping power turns. Of course, you can change out the size of the fins on either set up to change how the board turns. Smaller fins will give you a looser feel and larger fins give you more hold on the wave.
Side fins aren’t necessary to surf as plenty of people use the one fin system with great results, but they give you more options and are standard on most surf style paddleboards.