As a wakeboard goes through the water it creates a wake that can be seen by the rider. What the rider doesn't see is another wake that goes downward starting just past the point where the water first contacts the board. The actual line that is formed by this wake is called the boundary layer (refer to Figure 1). As you can imagine, the water inside the boundary layer or wake is very turbulent and unstable.

The boundary layer plays an important role on how the fin functions. The part of the fin that is below the boundary layer provides most of the stability for the board. This is why changing the height of a fin has a lot more of an effect on stability than changing the length of it. For example, some of the newer style boards with short fins must have a much longer base in order to obtain the same stability.