I’m a thermomechanical engineer specialized in Computational Fluid Dynamics.
What you cousin sais is wrong. The dimples are to force a turbulent boundary layer that postpones flow separation due to the higher momentum of the flow in a turbulent boundary layer and therefore reduces the low pressure turbulent wake behind the ball. This results in lower drag. As an aeronautical engineer, your cousin should know this.
Dimples on the bottom of a surfboard are useless since a surfboard is a planing surface. There is no reason to postpone flow separation.
Dimples on the bottom of a surfboard should have no useful effects on the boundary layer, because of the negative pressure gradient there which causes flow acceleration over the planing surface. Boundary layer separation would not normally occur with such a favourable pressure gradient.
However, if you wanted to promote water wrapping around the rails and licking the board into the water, then small dimples near the rail line may be another idea worth pursuing.
Dimples under the board may have another effect. If the bottom of the board is layered with an elastically compliant material, like smooth rubber foam, then dimpling caused by the hydrodynamic pressure of the water may actually increase lift on the board. Think of the relative thrust caused by water jetting out of a tap or hose and striking a flat plate versus a cupped spoon. The beauty of a rubber surface is that it can dynamically adjust to the prevailing planing conditions to maximise the lift on the board, whilst hopefully minimising the drag caused by cupping of the flow.
The maximum hydrodynamic pressure is located at about the quarter chord point of the planing waterline. So the compliant rubber surface is best embedded in the middle of the surfboard in front of the fins so as to maximise its areal effectiveness.
Interesting concept vincius, do you have any diagrams you could post about placement of the semi-fluid skin on the bottom? I am having trouble visualizing “the quarter chord point of the planing waterline … embedded in the middle of the surfboard in front of the fins.”
I am assuming something like a 3/8" to 1/2" thick eva yoga/exercise mat could work. Thinner? How much would thickness and softness/elasticity factor into bottom skin performance?
Firstly, its all just an idea I have had for a while, and I am putting it out there because I am not going to be working on it. :)
The pressure distribution under a planing board is similar to the pressure distribution on the lower side of a wing at positive angle of attack.
Very simply, there are two main forces influencing the forward-aft balance of a surfboard. The lift force forward of the board from the planing dynamic pressure, and the bouyancy force rearward to the board from the volumetric displacement of the board itself.
The chord-line of the planing surface can be described as the line from the leading edge where the water surface strikes the board to the trailing edge where the water leaves the board at the rail underwater. Of course, the leading edge and trailing edge as described are somewhat ill-defined because of the bifurcation of the water forward and aft at the leading edge, and the attached flow around the rail at the trailing edge preventing a clean separation.
The quarter chord point is a quarter way along the wetted chord line under the board. Ideally the maximum dynamic pressure occurs at this location. Its a rule of thumb.
If a compliant rubber material were placed under the board, its thickness and extent would be at the makers discretion. I have no idea what the practical realisation of this concept would look like - if it indeed it could be practically realised.
For further information, there are some 1940's era Nasa Technical Reports online that describe the theory and practice of simple planing boards in water.
There are other options for this compliant planing surface idea. If the forward jet of water at the leading edge (the spray) could be retained or captured under the board then there would be added thrust to the surfboard. A sawtooth like profile at the leading edge may catch the forward jet while permitting the rearward jet to flow under the board. Of course, the next question is whether the handling of the surfboard would be negatively affected. These are open questions.
For any of you guys over in Hawaii - Have you met Jerry Grantham? He used to shape boards here on the Central Coast of California through the mid-nineties before moving to Hawaii. Through that time, he shaped quite a few windsurf boards with “phasers” - dimples and thought pretty highly of them. He seemed to have “his system” down pretty good. He would probably be a good source for info on this.
I think this is relevant and should be posted in this thread as well:
Explain the underlying science in the following articles to clear up any confusion. Please distinguish between skin morphology, the dynamics of skin/fluid interface, as well as body morphology and movement effects.