Influence of the gab / cavity between fin segments

We can look closer at the boundary layer theory to find out why the gap does not cause extra drag. It is a classic problem for both numerical and experimental flow studies. The boundary layer is a thin layer of water close to the fin surface, in which the transition of the speed difference between the fin and the water occurs. The definition in Wikipedia: “the boundary layer is that layer of fluid in the immediate vicinity of a bounding surface”. The “bounding surface” in this case is the fin.

In this layer the so called skin-friction is generated, which is the largest contribution to drag of the fin (second place is taken by so called “induced drag”, which related to the planform shape and twist of the fin).

The boundary layer starts to build up at the leading edge and is initially very thin and purely laminar or “viscous”. The friction in this laminar area is relatively low. The boundary layer thickness will grow however, and at some distance, the boundary layer suddenly becomes unstable and the turbulent vortices are generated. This is the so called transition point and from this point on to the trailing edge the friction between the fin and the water becomes much higher.

Therefore, in order to keep the total friction of the fins as low as possible, the viscous boundary layer is to be kept stable for as long as possible, so the transition point should be as far back as possible. This is always a focus issue in fin design, and the shape of the profile can be optimized to achieve this. The profile shape will determine the pressure gradient along the chord and this is the main factor which determines the location of the transition point.

A smooth fin surface will also move the transition point further backward - thus reducing the skin friction. Polish the fin if the leading edge was damaged. Even any small roughness on the surface will increase the rate of growth of the viscous boundary layer - and friction !

Important for SmartFins is that the gap of only 0.5mm is too small to change the boundary layer flow from laminar to turbulent. At high speed the transition will also occur even before it reaches the gap at 40% chord length. Therefore the gap does not play a role in the transition from laminar to turbulent boundary layer and does not affect the drag.

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