# Why Dimples?

As we all now know, a golf ball with dimples will fly further than a golf ball without. What most of us don’t know is why? There are two main reasons:

• The Dimpled Surface reduces drag.
• The Dimpled Surface improves lift.

First a quick lesson in hydrodynamics (aerodynamics).

There are two types of flow around an object: laminar and turbulent. Laminar flow has less drag, but it is also prone to a phenomenon called “separation.” Once separation of a laminar boundary layer occurs, drag rises dramatically because of eddies that form in the gap. Turbulent flow has more drag initially but also better adhesion, and therefore is less prone to separation. Therefore, if the shape of an object is such that separation occurs easily, it is better to turbulate the boundary layer in order to increase adhesion and reduce eddies (which means a significant reduction in drag) Dimples on golf balls turbulate the boundary layer.

So we at 3DFINSCIENCE decided to test these principles and apply them to a surfboard fin. With advanced Computer Flow Dynamic testing 3DFINSCIENCE compared two identical fins: the MOONRAKERR dimpled fin and the same fin without dimples. The tests were conducted by flow dynamics expert Darrin Stephens (formally of CSIRO) and the results were surprising. A surfboard fin with a smooth surface is much like a golf ball with a smooth surface, it has a lamina flow over the surface. This works quite well when going straight and at lower speeds, but when you start to turn and reach higher speeds the fluid starts to separate from the foil or fins surface.

A surfboard fin with dimples creates a turbulent flow. Turbulent flow has more adhesion so when you start to turn, the dimpled fin surface delays the flow separation, reducing cavitations (the separation bubble) allowing the foil to maintain performance. When the surfer is turning at high speeds, the turbulent boundary layer helps the flow overcome an adverse pressure gradient and allows the fin to remain attached to the surface longer than it would otherwise. This reduces drag, increases lift and improves overall performance of the fin design.