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Gearing it Down

A project log for E - Kayak

An electric kayak for commuting to school

dominicDominic 09/21/2016 at 15:180 Comments

The motor that I'm using is designed for rc cars, and as such runs at very high rpm. Usually a motor like this might be paired with model boat props that are an inch or two in diameter. Instead it's turning a 9 in trolling prop, so it needs to be geared down considerably. The motor is rated at 4000 KV; that is rpm/V at no load. 8.4 V batteries will make this thing spin at over 30,000 rpm!!

Kayak speed can be approximated by prop pitch and rpm.


So the kayak will travel approximately 3.6 m/s or 12 km/h when the prop is turning at 2500 rpm. In a previous post I calculated that the power draw would be approximately 350W if travelling at this speed. I don't have any load-rpm graphs, but as a rough estimate I'll assume the motor runs at 75% of it's no load speed under this load. Therefore the motor needs to be geared down to 2500rpm / 0.75 = 3330 rpm.


So I need to gear a 10:1 reduction. In the next post I will show the build process.

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