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IFocus said..RoyStuart said..
Compressing and/or constricting water flow will always create more drag, the idea that by constricting the flow one speeds the water up is a complete myth.
Not quite (from what I know of process control) the water exiting around the tail inside the runners against the surfboard tail surface will definitely have a higher velocity than the water outside on the rail.
Actually the water cannot be accelerated aft. If there is a velocity difference it's a matter of how much the water is dragged along with the board, and constricting the flow will only cause water to be dragged along in the direction of travel more than elsewhere.
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BTW you cannot compress water (any time but we are talking flow here) only increase the pressure applied to it which will increase its velocity.
Regarding compression of the water that was part of Ron's explanation of the bonzer setup, I mentioned it so that he knew that I was referring to that part of the description and amended it by saying 'constricted.
I'm aware that the common lore is that water can't be compressed but in fact every material compresses under pressure, no matter how insignificantly.
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There will be a differential pressure across the whole construct (include the fin) as you say creating drag when the board is flat in the water but once its placed on its rail while turning the applied dynamic forces change (water flow across the board surface as a whole due to pivot and arc ie angle) would be quite dramatic no idea about the out come interested in Rons feed back.
Even when the board is on the rail with only one runner being used drag will be increased as the runner will lie across the water flow more as angle of attack increases, the hook in the trailing area guarantees this.
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Note that boards using single / double concave and combinations seem to me to have a higher degree of speed (talking short boards, less control as speed increases in my experience) are I assume using differential pressure to create lift and higher speed.
Interested in your thoughts on that?
Pressure differences fore and aft will be less on concave boards ( more even pressure with less pressure drop towards the tail) thereby increasing lift and ideally overall efficiency.