Matthew Warren

Airboat

I chose to carve a chunk of blue foam into a hull. A wet layup method was used to completely wrap the foam in carbon fiber. I used the same process for a small airfoil that became the pillar holding the motor mount. At the same time I also manufactured flat carbon fiber plates. A large plate became the main rudder while a smaller plate was cut into a motor mount. 

At this point, the airboat looked great but had some problems.

• The boat tipped laterally

• The steering was erratic and unstable

• The hull would pitch up massively at moderate speeds in the water

• The boat couldn't steer at all on snow

The snow problem was short lived because I hadn't taped the lid on the hull when I drove into a puddle of melted snow and fried the ESC. Clearly some improvements were necessary.

First, I mounted the ESC and receiver on small wood blocks so if some water gets in the hull, those critical electronics are raised just above the first bit of water. That was an easy way to prevent ruining another ESC and has worked fabulously.

The steering and tipping issues were mainly related to my choice of motors. The Hacker A50 shown on the right was powerful, and more importantly, already sitting in my basement. It is a 1.25 kW motor with 480 kV. This meant that even running on a 5s LiPo battery, it spun a 16 inch propeller to maximize static thrust on the boat. The motor needed to be mounted high to fit the large propeller moving the CG upward. The high motor placement also meant that the motor produced a nose down pitching moment which meant that the nose was the only point of contact at high speed. The motor produced significant torque which caused the boat to tilt sideways whenever the motor was energized.

I switched the hacker motor for a Cobra 2820/14 with 840 kV and that is rated for 670 W. The motor is much less powerful, weighs less, and spins a smaller prop at a much higher RPM while generating much less torque. I switched from a 16 inch to an 11 inch propeller and moved the motor mount lower. While I knew it was important to keep the motor as low as possible, I didn't realize I would need to select a motor that performs well with smaller propellers from the start to really get the motor as low as possible. Switching motors fixed the lateral tip-over issues and dramatically improved handling in all conditions. The downside is that I needed to attach another motor mount and needed to make the new one out of plywood, which is prone to delaminating. I needed to reinforce the new motor mount with some structural tape.

The biggest mystery was why the boat pitched up at higher speeds in the water and prevented the boat from going fast. The back of the boat would sink low into the water and the increased drag meant that the boat hardly went faster at full throttle than it did at half throttle. The boat sat nice and flat while still but the nose would rise up as it went faster despite the fact that the nose was being pushed down by the motor.

The problem was due to a flaw with the hull design. Being my first boat, I rounded the front and back of the hull. I didn't realize that the water would follow the curvature at the back of the hull creating a low pressure region that "sucked" the back of the boat into the water. After concluding that this was the problem I thought the easiest solution would be to chop the back 3 inches off the boat where it is rounded leaving a sharp back. This is why real boats have a sharp 90 degree back.

I concluded an easy solution would be to add a trip strip along the back of the hull to prevent the water from following the contour. My trip strip was a rubber band stretched across the beginning of the curve. This solution worked wonders and dramatically improved the handling on water.