Timing Belt Reduction

New here and contemplating building an Efoil this summer.

All the builds that I’m seeing in this community (except for the X-jets open belt concept) rely on a planetary gearbox. It seems to me that achieving a 5:1 reduction using an enclosed timing belt system would not be difficult to implement and ultimately would offer increased drivetrain efficiency. The only downside I see would be decreased hydrodynamics of such a system due to the likelihood of diverging from the fully concentric assembly that most of the builds take on.

Has anyone actually tried a timing belt reduction in their Efoil? Is there anything I’m missing on this one?


The planetary reduction can get very high efficiency, the neugart ones are listed at 92%-98%. I suspect the one stage 5:1 reduction is at the upper end for efficiency.
I don’t think you’ll have many gains because of the added drag of having an enclosed belt assembly, however I may also not be considering something. You also have to consider that the belt center distance needs to be fairly accurate so the belt doesn’t slip.

Alternatively, you could have the belt feeding through the mast and put the reduction all up top and just have the propeller at the bottom. This would probably lead to very high efficiency and very low drag but added complexity to the design.

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Here was my thinking:

Gearboxes take 4 months to order them and they cost 220€. They can still strip or burn. They need to be waterproofed and need an added thrust bearing, shaft couplers and a waterproof seal. It is very hard to deal with i put speeds of more than 18,000 RPM

A rim drive with timing belt would be great, but cause an aweful lot of machining. The duct will probably be difficult to swap out, so it cannot easily be optimized for different prop sizes. I would like to test all kinds of diameters and pitches with matching ducts. This was my second option.

Direct drive is much cheaper and has less things that can go wrong. The only challenge there is cooling and waterproofing. They are also shorter which reduces drag. Assembly allone will be much easier. Chances are that we have to take our stuff apart all the time. So I went with this.

Motor inside the board would make everything so much more easy, but once I saw the crossection of my mast, I scrapped that idea. It is hard to stick even a finger into mine. Its tiny!

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Yes, am working on it. Seems promising. Simple and cheap. More details soon.


Looks good @pacificmeister. I have been designing something similar internal to the pod. 4:1?

2:1 and a 360kv SSS


Nice! Looking forward to hearing the results.

I like this idea the best. Only problem could be belt slip.
But those gears look like a nice wide groove and square.
I’ve used belts before on other projects and had slip with 2kw of power on hard throttle. But my belts were no where near as “beefy” as what your using.

And it doesn’t matter it gets wet. I like it. Belts are cheap to replace as they wear out and stretch too

Yes! Exactly what I was thinking. Looking forward to seeing the results.

That looks very promising. All of the red is 3D printed I assume? Is there a way to adjust the tension of the belt?
Thanks for all of your work guy!

Oh, and its a two bladed prop now? I must have missed that one. Why 2?

If I had to guess- Pacific Meister’s original design uses a 550KV motor with a 5:1 reduction, nominally producing 110KV. The timing belt design uses a 360KV motor with a 2:1 reduction meaning he ends up with 180KV. This means faster rotation and less torque than the 5:1 design, these can be compensated for by reducing the number of blades on the prop.

yep, exactly :slight_smile: Could still be a bit fast but we will find out. I plan to post a video with my findings soon.


Exactly the kind of progress i have Bern waiting for! Great.

With the idea of a belt or chain running down the mast to the driven shaft would be easier to set the tension because you could shim either end. The driven shaft idea would also make for a very thin design. Of course a Caterpillar Drive would be the best and most quiet choice!

I’ll be interested in your amp draw with the 2 blade prop vs the 3 blade.

2 blade should be less drag in the water yes? Could result in better battery life and less strain on components and heat, may have less “take off” punch though. Keep us posted with the results.

Those mast profiles are supper small. Mine is just 9 mm at the thickest point.


I am trying to figure out… Would that be similar to that ?
Go straight to 3:07 until 6:05 :

At a high level it’s the same in that it is also a timing belt drive reduction. Timing belts are used a drive reductions all over the place. Regarding the details of the design, i.e. timing belt pitch, center distance, speed etc, it’s probably a lot different but it’s a different application, so it doesn’t matter. SDP-SI is a great place to learn about timing belts.

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The propeller can not be made of plastic and has two blades. For e-foil it’s the worst solution!

If the transmission produces a small torque for the propeller, the problem can not be remedied by reducing the number of blades to 2 (as shown in the picture).

The correct procedure is to replace the engine with a stronger engine or to use a gearbox with a higher gear ratio. Another solution is to replace the propeller with another that has a smaller diameter or optimizes the shape of the blades. 3 blades are the smallest possible number, 2 blades are used only for special reasons.

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