Testing an RC Hydroplane - Part 3

October 4, 2018|FliteTest

Part 3 of James Whomsley's Hydroplane project sees it all come together as the boat makes its first dip into the water.

The great thing about RC electronics is that they can be used to power all sorts of wonderful contraptions. They even don't have to be things that 'fly', as such. This hydroplane model is the perfect project to experiment with how different branches of fluid dynamics merge as the barriers between aero and hydro collapse. Here's what happened when I tested the RC hydroplane for the first time.

Previous Parts

If you haven't yet had a chance, make sure you check out the other articles in this series that go into the design and build of the craft.

Part 1: Designing a Hydroplane

Part 2: Building a Hydroplane

To summarise, the materials used included balsa wood, plywood, thin sheet birch, carbon fiber, and a small amount of polycarbonate.

The model speedboat was designed on paper and then cut and assembled by hand. It went from this -

- to this. Look at those sweet lines! The build took about four months working off and on.

Finishing the build

At present, the craft is finished and ready for action. Since the last installment in the series, waterproofed electronics have been installed, the hull completed and the whole thing sealed.

I feel like I could have finished the wood slightly better, but hey! This is my first go at building a boat from balsawood after all. The process was simple: the completed assembly was first sanded with something like 200 grit sandpaper. This removed all of the balsa 'fluff' and curved some edges. Next, I tried sealing the wood with wood stain. This was going to be the finished result, but I didn't like the color. After sanding once more and doping the wood, I painted the whole boat silver. Finally, I hit it with some gloss spray varnish to protect the paint.

Electronically, the motors are Emax 2204s. They are rigged up with 5" carbon props. These contra-rotate to provide around a kilogram of thrust without any sort of torque roll. They also sound pretty rad. For now, the motors are powered by just two 2s batteries. You may be thinking 'but James, why so little power?' I can explain.

On the initial tests, I really don't want to overdo it. A kilogram of thrust is quite satisfactory to get this boat skimming across the lake at speed. In future tests, the battery voltage is something I shall be upgrading along with some other components. I'd also quite like to3D print a motor mount where the thrust angle is adjustable.

The First Test

The first time the new hydroplane went into action was on the 3rd October 2018. It was only earlier that day that the final coat of varnish sealer had gone on and the polycarbonate waterproof hatch had been installed. With the radio on and all systems go, the boat was lowered into the open water for the first time.

Having only previously floated in the bathtub briefly, I was glad to see that the boat still stayed upright. After celebrating that the gleaming new craft didn't immediately capsize and sink, I decided to see how it moved under its own power. The initial impression was a good one as it seemed to move well without much resistance, although the steering authority was lacking somewhat.

After a few quick circles, the boat really got moving and managed to get up onto its plane without a problem.

However, soon a problem did become apparent. As the craft accelerated, with more and more of its hull lifting out of the water, it would suddenly swerve to one side.

After reviewing the video, it seems the issue is due to there not being enough stability when traveling at speed. With a short rudder, there simply isn't anything to keep the machine tracking in a straight line. Think of it like a dart without fins; the center of pressure is effectively too far forward meaning the rear center of thrust spins the boat around.

As a solution, a larger rudder is needed. With one that extends far into the water, it should act as a fin to keep the machine heading forwards when thrust is applied. Let's hope so anyway!

In all, these initial tests seem very promising.

Successes:

The boat didn't sink
It didn't capsize
It was stable
It got up onto a plane

Issues:

The boat spins around at a certain speed
The rudder hasn't enough authority

Thankfully, both of the issues are really quite minor and should be fixed with one solution - a bigger rudder. To see how fast this machine can really go, make sure you watch this video showing the next steps of this project.