# Passive foil power usage

i am developing a passive hydrofoil withou motor.
i simulated it for 3 m/s and came that it produces 30N drag. That leads to 100Watt Power to maintain that speed.
Now compare that speed to the 7kW motors youre using.
I am trying to say that our designs are highly inefficient.

100Wâ€¦ Thereâ€™s an issue with that calculation somewhereâ€¦

Anton,
3m per second seems to be on the low sideâ€¦
6 to 9 might be more accurate.
Also, in your simulation, what kind of lift are you getting?

To be fair when I designed my first prototype, i was convinced you can foil in the couple 100 watt range after seeing a video of a guy on a foil pulled by a bicycle on the dock or by someone running pulling a rope.
I kind of got the same feeling when kite foiling by unhooking my kite and trying to feel how much pull from the kite I needed to get goingâ€¦ roughly 2 to 3 x 1.5L water bottles.

so yes, thereâ€™s some gap between What we need in theory and the power actually coming out of the batteryâ€¦

Note that even if we use motors capable of up to 7Kw, we usually donâ€™t go that highâ€¦
wet out-runner are flying in the 1600 w range and i think dry in-runner with gearbox are in the 1000w range.
The problem is usually taking off where you have to bring the board to a plane first before flyingâ€¦ thatâ€™s why the power overhead is needed.

i use Outrunners and i need 750Watt for fly with 25km/h
Greetings Frank

How much do you weigh?

65kg Driver
Greetings Frank

Thats 30Wh/km
Usually a normal setup consumes about 80Wh/km, and a good one about 60Wh/km

30 would be incredible

Indeed, our foils usually need a speed of 12-15 kph, this means around 4m/s.
But note that, until you get 100% on foil, most of the drag comes from the board.
Then, when you are on the foil, you quickly accelerate up to 20/25 kph (5.5 / 7 m/s).
At this speed, my foil lifts 72 (me) + 28kg (board) = 100kg.
It needs around 800W at 20kph and 1000 at 25kph.

Also, compared to your â€śsetupâ€ť, we have a motor and propeller (+ duct for some) that adds a lot of drag).

In your simulation, did you consider the loss around the fusalge, mast and wing tips ?

Using 1200w at 25KM/H too, this after trying many motor, propeller, foil combos,
i think it is hard to get a lot below these values, Frank do you have some records ?
750W is crazy low, be careful with Metr module, you need to average the consumption for kilometers to get something accurate, not instant consumption as GPS speed as some delayâ€¦

To cut drag down even more, the trail has been opened by the kitefoil racing scene followed by the windfoiling scene:

• a thin mast (kite: 11mm carbon - efoil 15 to 18mm)
• low thickness high AR front wing (RL, windfoils, Mantafoils, Gong Veloce to come end of June, Mosesâ€¦)
• low thickness high AR stabilizer (coming as well)

Chubanga (Italy) Kiterace hydrofoil - Thin high AR in Prepreg carbon = \$\$\$

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My complete Board ca. 16kg incl. Batteryâ€™s 6s 24Ah
My Front wing 800mm with 16mm thickness Profil
My back wing from Liquid Force 220mm x 10mm
Mast 13mm
No gear and a nice Prop construkted by Volker skale of 170mm and more Pitch.

Greetings Frank

The small black Front wing I used.
The gear engine is change too Outrunner.
The Backwing change from Caprinha to liquid force

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Your 65kg body + 16kg board = 81kg (world record ? ) are part of the explanation in your very good results.
Can your 80kg friends fly with your 800cm2 front wing setup and with your 63100 outrunner motor ? If no, you could try flipping you stab round face up. As the original -3 degree angle will become +3 degrees, you will need to make (3D print ?) a -3 degree shim to have a neutral (zero degrees) surface. Then you would have a 1000cm2 total lifting area that should be no problem for 80+kg riders. Consequences of this tandem setup : 1-the rider position will move 10-15cm to the rear of the board. 2- you and your 65kg will have an even earlier take off.

Is the high AR (long) black wing home made ? What is the (root ?) chord where you have the 16mm max thickness ?

The Front wing have I found by Bic. Is not handmade.
I have no 80kg Drivers, my family are all lightweight
But we have not - 3 and +3,
We used +2 and 0.

Thanks for the infos,
I have also out runners 80, 63 they perform very well but direct drive inrunner gave about 5-10% more efficiency

Is only comparable if the same prop was used.

But donâ€™t worry, because my own outrunner will be ready shortly.

Greetings Frank

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If this front wing is the one from the Bic Techno windfoil 230 (82cm span) , it has been measured at 31.3knots = 58kmh. Food for thought.
May I ask how much it is sold separately ?

Are you going to make several of them ?

Of course we have tested with same exact setups, wings foils etc even same vescs we tested maybe 15 motors also for 3 years
Overall they are all in the same range except 80 outrunner that waste a bit more due to the drag of rotating part and diameter.

We built our own inrunner and we love the silence and efficiency, but the con is the weight.
Outrunner are nice for cost reason and simplicity, but the con are finding right bearings for salt water and the ability to add a duct. Also we wonâ€™t achieve same efficiency with outrunner. Our motor efficiency is 92% at 2kw
Our first efoil was made with a 6374 motor and worked great

How is the 92% heard / calculated?
Test bench, eddy current brake or foil.
A value that seems too good.
I had already developed RC Heli Motoren Outrunner for Torcman (today Torqueedo) in 2005 or 2006 â€¦ With 88% Eta Iâ€™m operating range.
Greetings Frank

It called the Bic Kerfoil

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Simulated, but after 1 year real world testing benchs no outrunner could compete in efficiency (we started our first working efoil in 2017).

If you think the contrary i will be really glad and happy to see the logs, because outrunner are cheaper to build all the community could benefit from this.

Our inrunner motor has cleverly made distributed winding, high quality magnets inside the rotor made of high quality materials and other features. And was designed by a world class electric motor engineer.

Moreover dont forget the rotating can in the outrunner.
For example :
Full throttle no load test in water at 12S :
80 outrunner (100kv) approx 300 watts, an energy waster
63 outrunner (120kv) 200watts, a good match
50 outrunner (280kv) 120 watts, difficult to use, need low pitch strong prop, fragile bearings

Direct drive inrunner 120kv as low as 60 watts to 100 wattes, depend on two or one lip sealâ€¦

Applying load would reduce RPM and the waste but that is anyway a big difference.

Regarding our inrunner, we dont know if we will commerciliaze, because production is really costly and we are afraid not to be able to reach a final product less than 700â‚¬.

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