Gas Engine Gearbox

by Martin Plumer
(Sydney, N.S.W. Australia)

I am in the process of building a model of our local ferry boat. It will be powered by a 26cc petrol/gas engine, but I can't find any gearboxes for that type of engine. Can you get forward and reverse boxes for model boats.

I'm hoping this is a big boat, because that is a big power plant in the realm of scale models.

The simple answer is: No. To my knowledge, there is no commercial gearbox specifically for model boats that provides forward, idle and reverse for combustion engines. However, there are a few workarounds.

  • Water Jet Drive

    A water jet drive system is a full-size solution to the problem you are asking about. It has also been duplicated in scale models and there is at least one commercial unit by Graupner. They call it the JET Booster-5 (SKU: 2347) for electric motors or gas engine up to 30cc. Depending on your skill level and tools, you could potentially make your own.

    The principle is to have the motor generate a stream of water shooting out the rear. By the articulation of a steering nozzle, the water stream is manipulated left to right as desired, for steering.

    A reverse flap can be brought across the stream to partially block it. In doing so, the flap is forcing the stream to do a u-turn and the resulting force is pulling the boat backwards. If the reverse flap is partially closed, and the resulting force is split even forwards and backwards, the boat will be in a state of idle.

  • Kitchen Rudder

    A Kitchen rudder is a century-old solution to the same problem, mostly used for slower speeds, so it may not apply. There is little sense in me explaining, Wikipedia is doing a fine job.

  • Gas Electric Drive

    Build a system, not unlike the electric system of a car or motorcycle. Have the engine turn a generator or alternator that charges a battery. From the battery you draw current through a regular ESC to electric motors that propel the model boat. It may sound complicated, and it is. The benefit over strictly electric is if the boat is big (and it better be to take a 26cc gas engine), the batteries can be relatively small and weight will be saved. At least that’s the theory. The alternator could come out of a car or motorcycle.

  • Electric Drive

    At the end of the day, a gas powered option has to make sense as compared to an all electric solution. The one advantage, that can’t be stressed enough, is its absence of noise. An electric powered model boat doesn’t disturb wildlife and fellow human beings the way combustion engine powered models do. Motors and ESCs for big boats can be found wherever Robot Wars components are sold.

Personally, I'd go with a Graupner water jet system for the gas engine. Otherwise an all electric drive is awfully convenient and therefore always my first choice.

Best of luck with your project!

Comments for Gas Engine Gearbox

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by: Fred

You may find something by researching steam links. It was the same problem for single cylinder non-reversing steamers. A "in and out" gearbox doubled and/with the addition of an idler for reversing and a shift fork controlled by an electric solenoid or servo. No synchro involved, just stop/shift/go. If the clutch is intact on your engine it takes care of stopping gear rotation to shift. Just an idea that I had for an old tugboat but moved on to other things. Good luck.

Reverse Gearbox for IC Engine
by: Richard Hallett

I have the same dilemma and I know all the alternatives BUT sometimes One wants to do...what one wants to do..and to this end I am working on such a project.

WHEN it will come to fruition who knows, but one thing is driving me forward.. I've built the hull (5'6" long) and installed the engine so it can't be that far away!

Unfortunately, other "projects" get in the way but I'm rather determined to crack this one. Sadly commercially available components are elusive, if at all, which delays even more, but watch this space!.

Richard, Wolverhampton, UK

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