We decided to share our progress on building an electrical hydrofoil. Here you will find our V1 design, part list and 3d files.
We want to make it easier for everyone to build their own, as its a lot of fun building and riding :), and that is why we are sharing our progress. We will split the instructable in two parts. Part 1 (board, hydrofoil and propulsion unit) and part 2 (battery and controller) which is still under development. We will show the battery and controller setup for the first prototype, but we do not recommend it because of fire hazard and there is no safety features implemented, do at your own risk!. We will post Part 2 with a more thorough built battery and controller unit with included safety features once developed.
The video is from the first day testing.
more pictures at. https://www.facebook.com/nikolai.hiorth/media_set?...
The main components:
Step 1: What Parts Do You Need?
Can be bought, or made custom. In this case the hydrofoil is bought from a supplier.
Standard part: Motor, Gearbox Aluminium tube, bearings, shaft seals, shaft + nuts and bolts
Custom parts: Motor mount, propeller, duct and structural components (3d printed)
Control unit --> PART 2
A more detailed instruction will come.
Battery pack --> PART 2
A more detailed instruction will come.
The part list includes the most important components. But does not include everything.
Step 2: Hydrofoil
Originally we were planning to build over own. But to save time we bought a new one (NOBILE). We think a SUP hydrofoil might be better, as they have a larger front wing which will give more lift at lower speeds.
Having a hydrofoil with detachable wings Is highly recommended, it makes it very easy to trim the angle of attack by adding washers. Especially changing the angle of the rear wing is important to get a smooth ride. It also helps to make the board more compact during transportation.
Our 3d parts are made for the Nobile foil Mast. But they can easily be changed to fit other types of mast, as long as you know the cross section of your mast.
The Nobile mast we bought is 85mm long, and the front wing is the largest type.
Step 3: Propulsion Unit (Thruster) Overview
The thruster consist of a motor, gearbox, propeller, duct and housing. The gearbox reduces the RPM of the motor by 5:1 thereby increasing the torque and allowing a larger propeller --> higher efficiency.
The majority of the structural components are 3d printed, in SLS nylon (super strong and watertight) and FDM PLA (not that strong).
The gearbox and motor assembly is glued inside an aluminium tube with epoxy, so it is not possible to disassemble. So take extra precaution and use strong locktite when putting these parts together.
Step 4: Propulsion Unit 3D Print
The Propulsion unit have structural parts made from SLS nylon and FDM pla.
SLS Nylon is thin layers of powder that are melted together with a laser layer by layer. A thin layer on the surface is porous, which makes it ideal for liquid epoxy gluing without any prepping, as the epoxy is pulled inside the outer layer of the part making a strong bond. Epoxy or grease can also be used to seal the porous surface of the part, such as in O-ring groves (We would recommend grease for this). If the parts are sanded, the porous layer will be removed, and a solid surface appear, this surface is harder to bond with epoxy.
We ordered the SLS nylon parts from 3Dprint-UK. https://www.3dprint-uk.co.uk/
Specify that you want a mesh structure inside the parts, or they will print solid sections greater than 8mm with 4mm walls. Total solid parts needs to be specified.
The duct and Prop was printed on our own FDM printer in PLA (wanhao i3 plus), to allow optimizing changes. You might think that a PLA prop would break, but is actually really rigid. There is no sign to damage on our prop or duct, which we have used on several tests already. We tested with ABS as well, but we would say that PLA is better, as the final dimensions of solid parts are more accurate than with ABS.
The propeller, duct and structural components are included. Note that the shaft, and motor mount must be custom made, based on your gearbox/motor setup.
Step 5: Propeller and Duct Design Considerations
An optimal propeller and duct may greatly increase the efficiency. There are however several factor that mus be taken into account:
- Propeller size--> bigger is better, but bigger may make it harder to ride. (as the propeller may reach the surface)
- Propeller pitch, blade area, sweep and so on.. There are many!! parameters that can be optimized
- Duct design, improves safety and may also greatly improves thrust at low speeds
- Safety, small propeller is "safer" as its harder to place your fingers/toes inside
The ducts main purpose is to protect body parts from entering the propeller, and increase thrust. We used an airfoil profile (NACA 6721, AOA 5 degrees) to increase the propeller efficiency at speeds up to about 15knots (which is proven to be efficient for large ships). Notice the twisted foil stators (“arms” holding the duct) they rotate the water stream prior to the propeller, the propeller rotates the water in the opposite direction, the result is less rotational energy loss to the water downstream of the propeller and thereby increasing its efficiency. Our next iteration of the duct will include twisted stators down-stream of the propeller, to further increase safety.
The propeller is optimized for 3d printing. The pitch blade area and sweep is not calculated, but it works well with our setup. The propeller cross section is now a B-troost, which is good for forward motion. Note that the distance between the propeller tip and duct is tight, this improves efficiency!.
We will try to do some calculations and/or CFD analyses to figure out the optimal shape of propeller and duct, this can give Lots of extra speed and longer rides!. But this is something we will do once we have everything up and running.
Step 6: Propulsion Unit Assembly
The motor is mounted do the gearhead using a custom made plate. This parts need to be customized to the gearbox you are using. Optimal material for this part is aluminium but a 3d printed part will also work if designed correctly. THIS DESIGN DOES NOT ALLOW ACCESS TO THE MOTOR OR GEARBOX AFTER ASSEMBLY, SO BE A PERFECTIONIST, and use STRONG LOCKTITE.
The motor shaft output needs to fit the input of the gear-head. In our case it did not fit. So, we had to reduce the motor shaft slightly from 10mm to about 9mm.
A tight fit of the motor in the aluminum tube is important to maximize thermal conductivity. We also added a heat sinks to the gearhead. Aluminium tape or thermal past can be used to get good connection between the motor/gear box to the outer shell, giving good motor cooling.
The rear ball bearing was glued in with a 3d printed washer and epoxy mixed with fiberglass.
Very important to apply a substantial amount of grease where the O ring is located as the SLS nylon absorbs some grease due to the porous surface. Also apply grease inside/outside the shaft seals. 3D printed parts are generally not ideal for O-ring seals, due to the irregular surface, so use LOTS of grease. This is the reason we chose to have as few o-ring seals as possible.
The aluminium tube is permanently glued into the SLS nylon 3D printed parts with liquid epoxy 8-24Hours hardening (important). The epoxy was applied with a syringe, to get into the 0.5mm gap included in the 3d model. It is important to prepare the aluminium tube before gluing, to ensure proper bond. Sand it down to remove oxidation then apply acetone, do this 5 min prior to gluing(Important)!!!. Also make sure the SLS nylon is clean, without grease or other contaminations (we kept the sls parts in plastic, until we were ready to glue). The SLS porous surface is makes a good bond with epoxy!
A 4mm hole needs to be drilled to attach the propeller, we used a block we had lying around for this purpose. This may be challenging, a trick is to grind a flat surface on the shaft prior to drilling. The edges on the shaft MUST be sanded really smooth, we used 400-800 and 1200 grit paper, to make it shiny without sharp edges. Sharp edges on the shaft or a rough shaft surface may damage the shaft seals and cause leaks!!
The 12 AWG cables for the motor are glued in with epoxy and fiberglass filler (makes the liquid epoxy thick as toothpaste) from each side. A "double safety" layer of silicone was placed on the outside on top of the cured epoxy where the cables come out of the thruster unit.
The aluminium mast is hollow, so three holes (one for each cable) should be drilled in the mast where the cables are threaded through. Use the Rear Circular passage in the mast profile to get the wires through. The mast foot attachment plate also needs a hole to get the wires through, ensure smooth edges on the drilled holes, to prevent cuts in the cables!!
The rest of the parts are put together with stainless m4 bolts and nylon lock nuts.
Step 7: The Board
The Board is made from an old windsurf board, which has some volume to it (easy ride, and room for a large battery pack inside). The board was shortened, patched up, and reinforced with a carbon plate to hold the hydrofoil mast foot. Check out the photos!
We used fiberglass and epoxy and vacuum-bagging (there are several other instructables on this matter, including on of our own https://www.instructables.com/id/DIY-kiteboardwakeskatelongboard/ ). Or just check out how surfboards and similar stuff are produced :)
Will Improve and show some more steps on the board build in part 2, when we are implementing the battery-box and so on.
About 100l volume is nice and will make the board easy to ride.
Step 8: Battery and Controller Unit (Dangerous Prototype, Not Recommended!!!).
The batterypack consists of 2x 6s 5000mah battery’s. running in series. Total 12s. we also tried with a 8s battery. it works with 8s but we want more power!
2x 6s battery’s gives us about 10min ride time.
This version has no water-cooling so the ESC gets very hot. As of now we are building a bigger battery with about 1.5-2kwh capacity. It should give us 1-hour ride time. We recommend using an aluminum ( box due to easy heat transfer. The controller has no waterproofing. We had it in a plastic bag each time we used it.
Battery pack and controller will be greatly improved for next version.
Stay tuned for next PART 2.
Second Prize in the
Outside Contest 2017