Johnny Rotten wrote:
Since a kite/surfboard is not a displacement hull and works on plane, we create less depth of wake than other craft. So at fin depth, (particularily for race fins) it must be considered that the overwhelming majority of the cross sectional area of the fin is moving through motionless(ish) water (relative to board speed)
As for the boundary layer between the board and water I have a VERY basic knowledge of the mechanics of planing which is enough to tell me that I truly know nothing and would not be surprised at all by any direction of flow in this boundary layer. But I think this this is where board shape not fin geometry becomes the important factor.
“Since a kite/surfboard is not a displacement hull and works on plane, we create less depth of wake than other craft. “.....
This may be true, but I don’t think its relevant.
Because your kite board is not a displacement device, the forces to keep your board on the surface must come from other means. Specifically, both the kite and the flow past the bottom of the board keep you on top of the water. The specific flow path taken past the bottom of the board should be the point of this discussion.
“So at fin depth, (particularily for race fins) it must be considered that the overwhelming majority of the cross sectional area of the fin is moving through motionless(ish) water (relative to board speed) “....
A very good point, longer fins would certainly minimize the interaction between the fins and bottom surface of the board. However, what your describing is the ‘potential flow’ or the extent of the pressure field created by the board. Historically, the pressure field of a submerged body can be ‘felt’ 1-5 diameters upstream and 10-20 diameters downstream. Not sure where our case fits , but I’m with you on this one.
“As for the boundary layer between the board and water I have a VERY basic knowledge of the mechanics of planing which is enough to tell me that I truly know nothing and would not be surprised at all by any direction of flow in this boundary layer.”....
Planning forces are typically calculated independent of the boundary layer.
I’m sure the boundary layer is of marginal thickness and will not provide much impact on the flow-field.
The boundary layer certainly contributes to the skin friction (laminar/turbulent) , impacts the flow separation point and resultant wake of the board. All factors which contribute to the overall drag. But not likely to influence the streamlines (flow path)
.”But I think this is where board shape not fin geometry becomes the important factor.”
....
Yes, I was never proposing the use of fins to alter the flow.
My point was/is “where” to locate the fins to maximize alignment with the flow.
The flow past the board is not well contrained or defined as you would imagine for flow inside a pipe. Even if all of the steady-state scenarios we have discussed are appropriate, I’m sure we’re only scratching the surface here
-bill