Based on the measured drag we saw, that it is beneficial to lift as quickly as possible. Furthermore I was thinking on the main purpose of the efoil. I love kitesurfing in waves. Therefore I would like to optimize the foil for riding waves. Here is an example without motor - Video
I studied Aerodynamics, so I designed a wing shape using XFLR5. I was looking for high lift airfoils for the hydrofoils Reynolds numbers. After some tries I decided to apply the Eppler E398 for the root wing part and the E397 for the tip. These airfoils are thick, which gives a good structural stability. I hope to use thinner airfoils in the future, but we did not want to risk breaking a wing on the first prototype. The high structural demand results from the high span of 1meter! This is huge, but there is only one recipe for low induced drag at low speeds -> high wing span and aspect ratio. With a root chord of 20cm and a span of 100cm the resulting AR (aspect ratio) is 5. At these low ARs the profile drag is little compared to the induced drag, which is the reason why I did not dig into further airfoil optimization or search for better suited airfoils (for now). Another goal was a high wing area in order to generate enough lift at low speeds. Hence the wing area is triple the area of racing kite foils. (about 1800cm²)
Twist and shape were adjusted to have a high Oswald efficiency (close to elliptical at “lift off speed”)
The resulting drag for lifting 110kg is shown below:
The calculation implies that lifting off will be possible at 3m/s (11km/h / 6kt). On my last sailing trip I realized, that it will be possible to foil without motor behind a 30ft sailing boat, which runs usually at 6-7kts.
The polar implies that it might be beneficial to lift off at 15km/h depending on the characteristics of the board in order to be able to use an motor as small as possible.
The wing war 3D printed. The print included channels for carbon-UD-spar caps. The current status looks like this:
For the mast we decided to buy a training set from Slingshot, which gives us 3 different masts with length of 15, 24, 30".