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BWalnut said..Svendson said..
And lastly, remember wind energy scales with wind speed squared. Going from 10kn to 8kn is a ~40% drop in available energy, you need to move into the realm of extreme efficiency at this level so as said a proper dw board is with its very high length to beam ratio is a good solution.
Do you have links to any literature about this? I'd like to further educate myself.
If you just want to look at an equation, here's one for velocity pressure head. This is conservation of energy, simplified down to just potential and kinetic energy (no heat, work, or chemical changes).
www.engineeringtoolbox.com/velocity-head-d_916.htmlIf you want to get much further into the weeds, start here and then go through to the derivation of Bernoulli (section 3). This is a more complete form of conservation of energy as applied to a fluid flow.
engineeringlibrary.org/reference/continuity-equation-fluid-flow-doe-handbook#:~:text=The%20continuity%20equation%20is%20simply,the%20mass%20flow%20rate%20out.However, I can't find anywhere an explanation or derivation of why kinetic energy is proportional to velocity squared. This is a fascinating blind spot, thanks for illuminating it - it was just something I was taught in 1st year and not thought about since. I don't know if it is my Google-fu coming up short or if this is an empirical relation that was determined by experiments rather than a theoretical result.
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Gencion said..
P.P.S. 6'8 is optimal length which will fit into my car. 7' I can squeeze with some effort. So I am not able to go longer. And for safety reasons I prefer to keep equipment inside the car.
I get and share the issue of in the car vs on the car. However, a pretty middle of the road DW board is something like 8'6 by 20 at the moment. That is a L:B ratio of 5.1. Your 6'8x22 is at 3.63. 22 inches is, for the 3rd time this thread, not at all a narrow board for the design style being discussed. You are looking for extreme efficiency from a board that is miles away from an extremely efficient length to beam shape. If you are stuck at 6'8 long either learn to ride a ~17in wide board to keep the L:B in the required efficiency range, or accept you have to compensate elsewhere (big foil, big wing, big pumping skills, etc).
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Mark _australia said..
Well and also your cold air in the gorge is more dense so a 14kn applies more force to the wing than desert winds in 100F
Air density does drop about 10% going from 50 to 100F. Not at all insignificant, but compensated by less than half a knot of extra wind speed at 8kn. It becomes a bigger deal in the ~20kn range and up, where it start to be 1 or even 2knots extra wind to compensate the density difference.
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SlowlyButSurely said..BWalnut said..Svendson said..
And lastly, remember wind energy scales with wind speed squared. Going from 10kn to 8kn is a ~40% drop in available energy, you need to move into the realm of extreme efficiency at this level so as said a proper dw board is with its very high length to beam ratio is a good solution.
Do you have links to any literature about this? I'd like to further educate myself.
More to the cube than to the wind speed square..
Good reference here:
solidwize.com/wp-content/uploads/2013/07/7-Understanding-the-Wind-Power-Equation.pdf Select to expand quote
Taeyeony said..
Wind power increase to the CUBE of wind speed.
And drag force in increase by the SQUARE of water speed over foil (like airspeed for plane). And the power needed to overcome drag is the CUBE of the water speed.
In low power condition. Large but efficient foil (like thin HA foil on the larger size) will help a lot. You need less power to sustain the slower foil.
Yes, power scales with velocity cubed in fluid flows and power is indeed the more relevant parameter for the matter being discussed. However, I have found repeatedly that in a fluid dynamics context, power is much less accessible both conceptually and mathematically as I've not come across an explanation that works without understanding control volume analysis. That is why I chose to discuss energy, as my main point was that what is available to extract from the wind for propulsion is not linear, and a 1kn change in wind speed at the low end is very significant.
Definitely agree a big, but efficient, foil is a good way to gain efficiency without resorting to very extreme board dimensions.
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Wingfoil Rentals said..
Mines an 87ltr custom,
gets up with ha780 in 6 knots with some effort 8knots effortlessly
the hull is a true planing windsurf hull. Not much else to write
Yep, putting down the power to get over the resistance hump and onto plane with an appropriate board design is by no means irrelevant. When board length is limited for some reason, limiting possible length to beam ratio, it may even become the optimal approach. I would expect this to be very rider weight dependent though, the efficiency of a planing hull shape and the size of the resistance hump is very dependent on loading - it must be kept fairly low. For a heavy rider, this means a larger wetted surface area and thus a larger low speed drag penalty than a light rider. Would love to know your weight; the board, foil, and wind speeds you posted suggest lighter.
Foil size also matters a lot - the smaller the foil, the higher the speed required for it to engage and start generating a significant chunk of lift. Long narrow boards have their resistance advantages primarily at lower speeds, while planing boards are usually more efficient at higher speeds.