Such a joy to read such well understood concepts!BWD wrote: ↑Fri Oct 27, 2017 6:02 pmRiding on the surface, the curve for drag:velocity is steep until you are planing.
Once on plane, you can add speed while drag increases slower.
For foiling with most foils I think you should start to feel lift from the wing soon after planing (if no before), for a quick, smooth transition to the foil.
A big or efficient wing will start to give you major lift from 6-7 knots.
So you want a board that will stop "plowing" and start semi-planing (passing its wake) around 6 knots, so it will "let go" and not stick to the water.
This is determined by aspect ratio and surface area.
Typically the overall length:width ratio ends up being 3:1. Dynamically, with the board accelerating and nose up, the immersed board area has a shape more around 3:2.
This works out to boards shaped roughly like revhed's, which is not unlike the shape of a shortened paipo, a ketos skim, or any pocket board, really. To keep some floatation for starts and a scoop to help in touch-downs, the shape is usually 3:1-4:1.
How big should the area be?
For transition to planing on surface, roughly 70-80cm^2/kg rider weight. Less for more proficient, or highly powered riders maybe.
The surface loading is the same as found in surfing, regular directional kiting, etc.
This surface loading facilitates easy planing with low power, at a board size small enough to allow good "shortboard" handling and light weight.
This is why converting a good, compact outline surfboard for foiling can work, even if not ideal.
With pocket boards, you can reduce length as much as your stance allows, but unless relying ONLY of the foil for initial lift and acceleration, you want to preserve some width, because most of the lift in surface riding is developed at the front edge of the wake. So it's good to have width for acceleration and efficiency at low power. Also somewhere to put your feet!
What about volume? 10-12L seems enough to float almost any foil so it won't sink, but gives challenges starting in light wind. Double that and the sinking when starting will be slower. If the board has a wide shape, it will also sink more slowly, even if volume is not great. To me volume seems even more dependent on taste and specific use than shape, and definitely more than surface area, which seems to follow some basic hydrodynamic principles.
Still think as Bra cru has been testing for a while now is that board shape to get the desired lift off speed in ultralite conditions such as this, might help a lot?
And the combination of what Ram air kite, on what line length, on how wide a bar, using what board with ALL its possible parameters AND not to mention probably the MOST important element the F wing to get lower and lower wind speed KBHF flying...
interesting, If I was fortunate to have had SUPER rich friends in college that gave me BIG money to do R and D, I, we would hope to tell you all!