3D Printed RC Airboat

Helllo Hello! I think one small or big fear that most people have is getting their clothes wet. Whether it be when it is raining or carrying an open cup with water at full capacity, no one wants to be the wet guy! What is even worse than wet clothes however? Wet plushies of course! (Seriously if my plushies ever got soaked in dirty water I would be livid). Thus, I decided to make an RC Airboat. I took a lot of inspiration from a video from Hyperspace Pirate who also made an RC Airboat so I suggest checking it out here. Additionally, this boat is going to be 3D-printed. I have never done a project that requires waterproofing before, so I guess this is another fear this project deals with haha.

Airboats differ from regular boats in that instead of typical boat propellers, they use aircraft propellers to move around. This has the distinct advantage of being able to move in shallow waters, which is really useful for me as I do not live around large bodies of water that I can use and my main test setup is a small container.

Finally, I want to have music playing in it. This will be done via a bluetooth module that is connected to an old car speaker I found in my garage. This large speaker is definitely overkill, but I did not have any other speakers on hand I could use. This has the upside however in that I can maybe get away with not using an audio amplifier and that it can balance the weight of the motors and rudders. Furthermore, I wanted to be able to turn this off and on from my transmitter, but was unable to do so for reasons I will explain later.

For this project, this is what I used:

1. A combination of M3 and M4 screws
2. A pair of Brushless Motors
3. A pair of Brushless Motor ESCs
4. Transmitter and Receiver
5. Bluetooth Module
6. Car Speakers
7. Servo
8. 3s Lipo Battery
9. Spar Urethane, you could also use epoxy if you have any
10. Silicone Caulk
11. Superglue
12. Wires, Solder, Heatshrink for Splicing
13. Plushie (The most important part)

For designing the hull, I first rearranged my plushie and speaker in different orientations to get a sense of the size of the ship. In the end, I figured the best way to do this is to have my plushie sit halfway on top of the speaker and have the speaker slightly tilted against the front of the hull. For enough room for the plushie and electronics, this would require a space of at least 165 x 254 x 55 mm inside the hull. Though, I went over this inner volume for more buoyancy in the final design.

After that, I weighed all of my electrical components and plushie which gave about 2 lbs. Now this does not include the weight of the boat, so that has to be considered as well in the design. To limit the amount of guesswork, I designed a quick mockup of the hull to get a volume and approximate weight. This resulted in the boat probably being about 2.5 lbs with consideration from the weight added from waterproofing, thus putting my target buoyancy at around 4.5 lbs.

However, just because the boat is buoyant does not mean it is good. If the entire boat's buoyancy was 4.5 lbs that would mean the waterline is right at the top of the boat in a static water, any sort of tipping or incoming waves would sink it! Therefore, you want enough buoyancy so that the waterline is around half from the bottom of the hull.

To do this, I used to a Google Sheet to plot the buoyancy force vs. the height of the boat from the bottom. For those unaware of Archimedes Principle, the buoyancy force is equivalent to the weight the water being displaced, or in math notation, F = ρgV. To get the volume per chunk of the hull, I actually had to make a separate CAD and cut it into sections manually. I think there is a way in Fusion360 to just use cross section feature, but I might be wrong. Aside from that, after some tinkering with the dimensions, I came up with a final design that had a waterline at about 55% from the bottom of the hull with a weight of 6 lbs. Going over 4.5 lbs by a bit, gives me a factor of safety of about 1.33. The part of the graph where the buoyancy has a sharp decrease in rate of change is where the hull transitions from the hull. Originally, I had the hull be made up of 3 pieces, but after some printer failures I switched it to 5 pieces to decrease the probability of failure occurring.

For motion, I used two 850kV brushless motors to run the propellers. Two motors are used with opposite rotations so that any tipping caused by one motor rotation is cancelled out by the other. To ensure the propellers never pop out, threadlocker is used. Moreover, these motors are probably in the lower-end when it comes to power, so if I could I would probably swap them with higher rated motors. For the propellers, I did not have any on hand, which gave me an idea. Could I 3D-print my propellers? To do this, I just sort of guessed the propeller profile and indeed was able to create thrust with a motor. In retrospect, I probably should have switched them with actual propellers or made the propellers longer as the boat ended up really slow.

For turning, I just have two rudders with a 4-bar linkage controlled by a servo. In initial tests, the turning was terrible, especially if the water was not static. My solution to this was to increase the area of the rudders, however I was not able to test this in a large body of water as of right now. (Hopefully in the weekend though haha).

The electronics are quite simple for the most part. Power comes from a 3s lipo battery and goes into the two ESCs. The ESCs have a 5V voltage regulator on them which are used to power the receiver, servo, and bluetooth module. For the motors, make sure that at least one of the pair of wires from the motor to ESC is switched from the other motor. This ensures that the motors have opposite rotations. The entire schematic can be seen above.

Additionally, the motors and servo on the outside of the boat have their wires go inside via a small hole on the cover which can be sealed by silicone caulk. Inside the hull, all of the electronics are put into a disposable trash bag to act as a final shield against water and just an easy way to store all of the electronics in a single convenient area.

If you look at the schematic I have in Step 3, I currently have the bluetooth module just connected to the speakers directly. However, I originally had it so that it can be turned off and on via the switches on the transmitter. This was done using a lowside mosfet switch that was switched via an Arduino Nano connected to the receiver. The 100k resistor is simply a pull-down resistor that pulls down any residual current to ground to prevent any unwanted turn-ons. An Arduino is necessary as a mosfet cannot understand the PWM signal from the receiver. The schematic can be seen above.

This was working, however in my final test I accidentally fried my Arduino board (I think I had polarity for the power switched D:) and thus had to removing the switching feature. This does give me an opportunity however, to make my own RC switches. RC switches unlike typical switches are able to understand the receiver PWM signal to turn off and on. This is better than using an Arduino as using a microcontroller is way overkill for a simple action like this, so expect my next Instructables to be on that.

After 3D-printing everything, I superglued all the parts together. If you are going to make your own RC boat, I suggest to first screw in the cover first before gluing to make sure it is entirely aligned as misalignment is common in large assemblies.

Afterwards, I went on to waterproofing. This is done via two coats of spar urethane on the hull and one coat on the cover which took a few days. To make sure there are truly no leaks, all of the crevices where two parts meet were covered in silicone caulk. Furthermore, I noticed there was a small opening as the back hull and middle hull pieces were uneven. To fix this, I just put a layer of silicone caulk that evens everything out. There will still be some water leaks possible most likely, but the opening is at the middle back of the hull where water basically should never reach. Initially I was worried that the boat would look pretty ugly at the end, but I actually really enjoy the glossy look with the black and white filament. (Though yeah the wood filament does not look the best lol).

To test for water leaks, I just put the hull in a small container with weights on them for about 40-50 minutes and was able to see that no water leaking occurred. I did this test again with the full boat assembly afterwards where no leaks occurred.

And finished! Float tests went perfectly and the boat was able to move forward, though turning was not the greatest. There are definitely a lot of areas of improvement that I intend to work on soon, but for now I am quite happy for a first boat project. As said before, some changes I would like to make are switching the motors for more powerful ones, increasing the rudder area, and making RC switches for the speakers. Furthermore, just for general aesthetics, painting the boat would make it look a lot better. Even with these problems, I do believe that I was able to face my fear of getting my plushies wet with this project and approached the problem with an appropriate amount of precaution.