I am calculating battery options at the moment. If you run 7000W continously, you only get 5min of flight time with a 20Ah battery pack. Only 14Ah of those 20Ah are usable because of 70% depth of discharge. It would be nice to know how much power is needed when up in the air.
I think 3000w is more than enough. We ride on around 1000-1500w while crusing. We have a big wing tho, will need more power with smaller wing
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Are you planing on getting a smaller wing? I have seen yours on Facebook. Its not very expensive and looks quite long (from leading to trailing edge).
Have a windsurf foil on the way. But have used the same propulsion unit on ĂĄ kite foil as well, works fine wirh that also. Just harder to ride
Which one are you getting?
3000W is indeed quite a lot, it’s enough to lift a 280kg solar boat 
1000W is indeed a better value as @Hiorth mentioned.
A wing with a higher aspect ratio is more efficient, but also a lot harder to manoeuvre!
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So which foil are you recommending? I have my eyes on this one: Cabrinha Kite Foil Kit 2017 - Coronation Berlin
L – 1236 cm2 Naish Thrust is a good size foil for carving, assisted wave riding etc, for all out speed an 800-900cm2 high aspect foil should get you to 50kmph or so , the Larger thrust wont but will be super stable to ride. i would have 3000w on tap and run at what ever rpm suited my needs unless you are trying to get a calculated maximum efficiency in Kw/Km. eventually 1000w will get boring and if you have any initial issues 3000w will drive you past them most likely, like too many pies, or poor prop 
If anyone is interested in takeoff speed (unfortunately I can’t mesure anything else yet) I am using an Alpine Foil XLP of 682cm2 and the takeoff speed is 11knots (about 75% of power) and my weight is in the high 80kg with a wet drysuit.
Cheers
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Great info! What about max speed with freekite foil? Are you on SSS 500kv motor?
Yes, SSS 500kv and 5:1 Neugart and the Hiorth v1 propulsion. Max speed was about 15-16knots (I am not a good rider for now), and I hadn’t changed the angle of the rear foil. I just fund the shim and will put it for the next time, I guess that the takeoff and top speed will be higher?
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If you increase the angle of attack of the stabilizer, (make it more horizontal) the speed might increase, less drag and less tendency to “lift the nose”, but take off will be at higher speed and the ride becomes less stable. Try with very thin shin, one step at the time
Super info! And what battery pack,? S,? mAh
Yes 1000-1500W if using props >140mm. If you use jet drive about 2-3KW but safer and higher speed possible;-)
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I think a large prop 140-160mm prop (with duct) uses less energy regardless of speed, if designed correctly.
Yes, correct but at top speeds the jet gains again against the ducted prop
Could you elaborate this statement, with calculations or test results?
As far as I can see the fastest speed boats all use propellers (without ducts tho).
If your statement is true, the duct drag must bring the efficiency down to jet drive at a given speed. Do you know what kind of speed we are talking about?
We calculated about 1300W power at around 20kmh foiling, with first itterarion of propeller and (rice speed duct) that are not optimized at all.
Will test with a high speed duct to see what speed/efficiency we can get.
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Yes the duct is the problem…
Overall the propeller is more efficient. What I meant is just at very high speeds above 40km/h the jet used on a foil gets closer towards a ducted prop optimised for the foil lift off speed of around 15km/h, in terms of power efficiency…
I see, I think we will limit the speed to max 40kmh, 30kmh feels really fast 
What I really like about jets is the the safety part!!!
Yeah thats the major point why we went on a interchange system to change it from prop to jet within about 5minutes;-)
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