Being on the run there arises no drift and no lift. Several parameters are of importance.
- The leading edge works as a flow barrier (barrier point), the flow is split up into two halves at this point.
- The boundary layer is the transitional region between fin surface and laminar water flow.
- In a laminar flow pattern, the streamlines are parallel to each other. Each water particle follows its own streamline path with no mass transfer between adjacent fluid layers.
- At the point of transition, the boundary layer suddenly is growing thicker and the flow is becoming instable. The laminar flow conditions are replaced by a more confused pattern which is termed turbulent flow. The position of the point of transition is depending on speed, on profile characteristics, on roughness of the fin surface especially in the forward third of the cross section and on accidental disturbances of the flow. Damages of the leading edge and increasing speed are drawing the breaking point further forwards.
- In turbulent flow, there is an impulse exchange between the water particles in various directions.
- At the point of detachment the flow is leaving the fin.
Figure 8: Flow diagram along the fin profile
S barrier point
U point of transition
A point of detachment
The boundary layer can be understood as an area of slowed down flow. When the fin moves through the water the water particles that adhere to the surface of the fin have no velocity relative to the fin, they are part of the extremely thin laminar sub-layer. With increasing distance from the surface they gain speed and eventually they become part of the surrounding flow. Across the boundary layer the water is in shear as the water particles have different speed. As a consequence the fin experiences a resistance which is termed frictional resistance. The dimension of the frictional resistance depends on the sailing speed.
We call a fin streamlined if the flow does not break down on the fin surface. The streamlines should meet behind the fin without shedding a lot of eddies.