Sometimes you seek inspiration. Sometimes inspiration smacks you in the face. As I was walking down the clearance isle at Walmart, I was smacked in the face. They had a few kid’s kickboards on clearance. With my Mini Alligator Tours airboat experiences still fresh on the brain, I immediately thought that one of these kickboards could be the starting point of a scratchbuilt airboat.
There were a few features of this kickboard that I particularly liked, in addition to its clearance price. First of all, it has a very wide stance. That would serve to prevent tipovers--hopefully. Another appealing aspect was its slippery plastic shell. I thought that would help it slide the water, as well as grass and other surfaces. The other kickboards that I saw had a nylon mesh-type covering. That’s probably great if you are actually using it as a kickboard, but not so great in airboat mode.
The one thing that I did not like about the kickboard was its very pronounced curvature (as viewed from the side). Most airboats use flat-bottomed hulls. I figured I would give it a try anyway and see what happened.
Keeping It Simple
Early on, I decided that my focus with this project would be to make the simplest airboat that I possibly could. That proved to be a surprisingly elusive goal. I discarded numerous design sketches over the course of an afternoon before I felt that I had shaved my concept down to the bare essentials.
Rather than using rudder surfaces behind the motor, I opted to place the motor on a swiveling mount. This would allow me to vector the thrust from the motor to provide turning authority. The simplest swivel mount I could think of placed the motor right on top of the steering servo. The downside to that plan was that the servo would be subjected to a lot of potentially strong forces that it was never designed for. Torque and thrust from the motor would impart all sorts of side loads to the output shaft of the servo. Things would be even more severe in a crash, or if a damaged/unbalanced prop caused the motor to vibrate.
To mitigate the risk that I accepted with my motor placement, I decided to use a stronger than normal servo. I pulled a Hitec HS-80MG from my servo bin. It has plenty of torque for this application. More importantly, this servo has metal gears inside (most servos use nylon gears). The metal gears are much more resistant to damage.
For propulsion, I looked in my spare motor bin and found a cheap brushless motor and ESC that I thought would work well. I have a selection of off-brand equipment that I keep on hand for proof-of-concept projects such as this. They keep my emotional and financial investments low, so I don’t feel that I have to be conservative during testing…if it blows up, it blows up. Projects that show promise get the higher quality, name brand stuff when I build v2.0.
Two motors of the exact same size might spin at vastly different speeds with the same input voltage.
Quality aside, you should understand that all motors are not created equal. Two motors of the exact same size might spin at vastly different speeds with the same input voltage. It all has to do with the type of motor and the copper windings inside of it. It is too broad a topic to explain in detail here, but I’ll cover it in a RC column before long. The bottom line is that you should know how your motor, propeller, and battery will integrate and understand the role of each component in that equation. Most vendors list a suggested input voltage and prop size for each motor. That provides a good stepping off point.
I settled on a small 2000kV brushless outrunner motor with a 4.9 x 4.3 prop and a 12 amp ESC. Power is from a Great Planes 3S-1000mAh lipo battery. This power system pulls 12 amps and generates 140 watts of power.
To control the airboat, I again turned to my older gear to reduce my emotional investment. I used a Tactic TR-210 receiver and TTX200 pistol-grip transmitter. While not as glamorous as my newer 2.4GHz radio systems, this radio still works perfectly.
I began by building the motor mount. I had an off-the-shelf mount for the motor that is intended to fasten it to a 10mm (3/8”) square tube…a common fuselage structure for RC airplanes. I took a short length of 3/8” square hardwood stick and glued it with epoxy to a plate of 1/8 lite-ply that is about 1” x 1”. I soaked all of the wood surfaces with thin CA glue to create a waterproof varnish.
I screwed the plywood base to the servo’s output wheel using two #4 screws. I then screwed the motor mount to the hardwood stick using two more #4 screws. With that, the motor mount was done. Next, I had to sort out a way to hold the servo in place.
I made a platform by stacking two layers of ¾”-thick blue foam. I cut a pocket in the top layer of foam to fit the servo and used epoxy to bond the servo in place. A strip of self-adhesive Velcro on the platform provides a handy method for mounting the receiver. Sharpened brass tubing works well to drill holes in foam. I used that technique to create a route for the servo lead to reach the receiver. Lastly, I stuck a short piece of bamboo skewer vertically into the foam. This provided a rigid base for me to ziptie the antenna tube in place.
I epoxied the completed platform to the top rear of the kickboard. As that dried, I applied a strip of self-adhesive Velcro forward of the platform. I sized the strip to provide a wide range of possible battery locations. This would let me experiment with the balance point of the boat if it became necessary (it did).
I tried out the completed airboat at a local pond. It was immediately apparent that the boat needed some trimming (okay, a lot of trimming). Even with the 1000mAh battery all the way forward, the boat bounced around the lake like a hopping car. I tried a much larger (heavier) 3300 mAh battery to balance things out, but it still wasn’t enough. I also tried bending the motor mount to alter the thrust angle of the motor. That still didn’t cure the hopping bow. I’m not ready to give up just yet, but I’m starting to think that the curved hull is the source of my troubles. Sorting out these types of issues (or at least understanding them) is half the fun for me.
While my airboat’s first outing was pretty much an aquatic disaster, I discovered an unintended talent of my design. It does well across the grass…I mean it does REALLY well across the grass! Maybe that’s an advantage of the curved hull. Even if I can’t make a decent watercraft out of it, I already know it’s a fun grass scooter.
Before I invested any more time tweaking the water handling of my airboat, I decided to explore its capabilities on land a little further. The first outing suggested that it could use nose weight. The next time I took it to the park (without a pond), I had my Mobius camera attached to the bow. I also had a collection of 1000, 1300, and 2200 mAh 3-cell lipo batteries.
Having the camera in the nose seems to have cured my balance problem. It kept the front from lifting at high speed--and wow, does it ever reach high speed! I’m not sure that my Ruckus monster truck could cover this rough terrain as fast. The smooth bottom of the hull seems to glide over the grass, which is how my design finally earned a name: Worm Burner (which is also my signature golf shot).
The Worm Burner can be tough to drive since the vectored-thrust provides a lot of steering authority and I didn’t add any fins or other aerodynamic devices to help stabilize it. It can be driven with some degree of precision; it just takes a little practice and a subtle approach.
The tradeoff to all of this instability is that the Burner can do some unusual things for a land-based vehicle. Crank the wheel and punch the throttle and the WB will spin in place faster than your eyes can track it. Jab the steering as it’s scooting across the field and it will do a 360 before resuming its original path. In many ways, it performs much like a hovercraft. Yet, there’s no air cushion here; just a dearth of friction between the hull and the ground.
Flash in the Pan?
As I write this, I’ve run about a dozen batteries through the worm burner over solid ground, much of it very rough terrain. I’ve even spent some time on sidewalks with it. Surprisingly, the hull is still in great shape. The servo, however, is already developing some play at the output shaft. I may find out very soon that this design is too much for even a metal-gear servo to withstand. If so, my next version will incorporate a way to isolate the servo from the worst of the motor’s forces.
Until it falls apart, I’ll continue exploring the Worm Burner’s capabilities. I’d like to find a smoother grass field, or maybe Astroturf to really see what this guy can do. Oh, and I’ll see if its fun level on the water ever approaches that on the grass. Just as the inspiration for this project came from out of the blue, so did the realization of its true talents.