Which foil for very low speed

I would love to build a foilboard for my kids. All three are under 25 kg and I kinda hope it would be possible for them to ride behind a small electric boat that does 7km/h
I've seen a video of a guy foiling in flat water that had easily 65-70kg. It was a GoFoil waveriding foil.
We don't have an ocean and the only foils available are kite and windsurfing foils.
I would love to do something like that

Seems easy enough.
My understanding is that low aspect foils are better for very low speeds. Which profile would I use that gets the most lift at low speed?
A longer fuselage should make the ride more stable as so winglets on the front foil?
I have read some but naturally most people (kiters especially) are aiming for high top speed. I really need the exact opposite.
Would a bigger foil help here or would it just introduce too much drag?
Please help, I really don't know where to start.

 

Instead of aspect ratio, you should think in terms of span and area. The slower you go, the more area you need. The drag due to lift goes down with increased span, so the slower you go, the longer your foils need to be.

If the boundary layer on your foil is nearly all turbulent, the foil shape is not all that important. You should size the chord so the foil is operating at the lift coefficient for best lift/drag ratio for the 2D section.

The static stability (the tendency to return toward equilibrium when disturbed) depends on the tail "volume" - the area of the tail times the distance to the wing. A small tail on a long fuselage can have the same tail volume as a large tail on a short fuselage. For the same tail volume, a small tail on a long fuselage has more damping than a large tail on a short fuselage. So a long fuselage is probably the way to go if you want a gentle foil with low maneuverability.

There are two principal sources of drag. The parasite drag depends on the wetted area, and it increases with the square of the speed. The lift-induced drag is inversely proportional to the square of the span, regardless of the area, and it actually decreases with the square of the speed. The two sources of drag are about equal at the speed for best lift/drag ratio. As you lower the design speed, the parasite drag drops, so you can stand to add area to keep the board from stalling. But it will be draggy if you don't increase the span. You might consider scaling an existing foil design, keeping the same aspect ratio. So if you want to go half as fast, make the foil twice as large in span and chord.

 

Thank you Tom.

Is there possibly some ressource where I can learn the basics or can I just keep on asking questions?

Where do I place the mast? If the wing is curved in the Z axis, what does it do? I see a lot front wings curved down. Does it add lift when tilted sideways?

What is my wing area I am aiming for?

I found a 3D model of some foil online. It's curved downward and the aft foil is curved upward.
Somewhat scaled the aft foil up and drew a new forward foil based on a shape I found somewhere else.
Again, what area am I aiming for? My forward foil is now 70cm wide.

 

If there was a single "best" answer, then you wouldn't see the variety of foil shapes that exist - they would have converged to the best design long ago. You'll need to either start estimating some numbers to evaluate potential designs, or you will need to start building them and see which is best experimentally. Experiment will eventually be required for a definitive answer, but you can use theory to make your experiments more effective.

 

That's what I thought. So many shapes around.
I will build it modular so I can test different wingshapes.
I am also looking into model airplanes, there's a lot information on slow flyers.

Thank you.

 

I have made many SUP foils of all sizes and shapes. You could get by with a normal planshape, like the middle one in the photo above. I weigh 70 kg and use a foil of around 1250-1850 sq cm, projected area(depends on wave size and speed attained). Your kids could get by with maybe 400-600 sq cm , projected area. Adjusting lift by adjusting speed is the way to get whatever size foil to function effectively. If you go to the Standupzone forum under SUP foils, and search for DIY Foils you will find a complete explanation of several types of builds. I also use a NACA 4415- NACA 4417 wing cross section. My group name is SUPeter and would be willing to share any and all information I have learned. Page 44 or 45 in the DIY foil section will bring you to my technique which I have found to be quite inexpensive, light, and stronger than current construction techniques by utilizing both an internal and an external skeleton. If you are familiar with composite construction techniques like vacuum bagging, this could be accomplished quite easily. Good Luck

 

WOW....just great.
I've used carbon fiber in a crisscross pattern over model airplane wings and on a bamboo bike frame. Using them as internal structure is really nice. On the geodesic aerolite boats Kevlar twine is used in the outside to stiffen the built.
Will take some time to read through that 47 pages.

I might start with some hollowed pine cores or something since I got a CNC to do the carving for me.

Maybe corrugated cardboard would make a nice core with carbon thread stiffening.

 

If you use cardboard, better hope the front doesn't fall off.