zfennell wrote:johnney and lobo,
i believe that you are both correct.
however that does not alter the point i was trying to make.
as long as there is an attraction between your butt and the earth (gravity) there will be some divergance in the flow field exiting the board.
you are certainly correct that the additional flow conditions (i.e. true velocity of the board or kite traction force, etc) should be added.
however you cannot simply toss out the ones that dont fit your model.
....unless you believe the magnitudes are negligable in this case.
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.
I'll let you provide the free-body diagram that explains why the pressure (lift) on the bottom of your board compells the flow to travel directly from front to back.
....in particular along the side rails, with no attempt to travel laterally
davesails7 wrote:Lobodomar is saying the direction of flow is never paralell to the centerline of the board when you're sailing upwind, it's on an angle (beta in the picture above, usually called the leeway angle in sailing). The board doesn't travel in the direction it's pointing, it travels at a slightly downwind angle from where the centerline is pointing.
He's also saying: Maybe the flow also diverges away from centerline, but who cares? The fins are too deep to notice.
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