Daniel Riley spends his days turning ordinary hardware into machines that do more than expected. His latest project takes a small four-stroke engine of the type found on string trimmers and connects it to an electric motor normally used in skateboards. The result is a compact generator that produces up to 780 watts of electrical power. Installed in a boat, the assembly becomes the core of a hybrid drivetrain built around one clear goal: keep the gasoline engine running where it works best while the electric side manages everything else.
Small boats using a gasoline engine to directly drive a propeller face a fundamental issue. They require the engine constantly speeding up, slowing down, or changing load as they turn, go over waves, or adjust speed, but most small engines only perform well under very specific conditions, such as RPM and torque, and outside of that range, they simply burn more fuel for the power they produce while shaking violently. Riley has discovered a remedy for this problem by fully removing the engine. The engine runs at a constant speed and load, while electricity transmits power to where it is needed.
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The setup is designed as a serial hybrid, with the gasoline engine rotating the skateboard motor, which converts it to electricity. That electricity is then routed to batteries or a separate electric motor, which turns the propeller. The electric motor can adjust the thrust on the fly and even regenerate energy as the boat slows down. The engine never has to cope with any of that since it is locked on the single operating position that testing determined would utilize the least amount of gasoline for the power it delivers.
Riley had already experimented with a smaller version of this idea, building a little autonomous boat powered by a much lighter hobby engine. That one confirmed that the general idea worked, but it was unable to generate enough electricity to power anything larger. So for this project, he used a large 6354-size brushless motor rated at 200KV. These motors are often used in electric skateboards because they can withstand high torques and long periods of sustained load, and with the correct controller, they can also function as generators.
The engine is a 25cc four-stroke clone of the Honda GX25, the type of engine typically found in garden tools. Riley removed the centrifugal clutch shoes and joined the crankshaft directly to the motor via a special coupler, which he developed using CAD software and then iterated on until it functioned properly. He printed a first iteration in clear resin to test the fit, then upgraded to a stronger resin for a subsequent print. To be honest, a laser-cut aluminum plate from a source did an excellent job of maintaining the engine and motor on the same axis, and spacers maintained everything aligned, ensuring that the shafts remained in immaculate condition.
Several minor details needed to be worked out before the system could function for any period of time. The engine’s flywheel fan assists with cooling, but even so, high-torque operation for lengthy periods of time raises temperatures. Riley devised a clever solution: a little electric-powered fan fed straight from the generator output, which pulls in extra air and assists the cylinder cooling fins. Plus, a servo was installed to control the throttle position via a little metal rod that presses into the carburetor, which is a nice piece of machinery. An ESP32 board with a tiny LCD captures the RPM, voltage, and power information in real time and saves them to a file for later inspection.
Out on the water, the generator was plugged into a small catamaran and began charging a 12S LiPo battery pack, which powered the boat’s electric drive motor. He had a high speed of approximately 14 kph (8.7 mph). Throughout the run, the gas engine remained in the sweet place where it works most efficiently, known as the ‘sweet spot,’ while the electric motor handled all of the hard work of propulsion as well as any unexpected fluctuations in demand. With a full fuel tank, the entire generator setup weighed around 4.75 kg. The only thing that will stop it from running is how much juice you can put in it, because the battery does not even enter.
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