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5-10KW Gear Reduction


What is a gear reduction?

  • It's a mounting assembly for the sailboat kit motors (will also work with most NemaC face motors).
  • It's thrust protection to isolate the motor from axial propeller forces.
  • It's a speed reducer/ torque multiplier to prevent cavitation.
  • It includes a shaft coupler to attach to your existing prop shaft.


The ThunderStruck Gear Reduction comes standard with a 2.67:1 gear reduction pulley set mounted on a 1" stainless steel drive shaft and thrust bearing. The stainless steel plate is 1/4" thick, and has multiple mounting holes to make aligning your gear reduction easier, and includes a belt tensioner. The side supports can be cut if needed for custom mount requirements. Top mounting bosses can be used for additional triangulated support and alignment. Our gear reduction also includes a flexible shaft coupler composed of a double roller chain sprocket set which makes alignment easier and takes stress off the reduction. One sprocket is mounted on the gear reduction shaft, and the other is mounted on your existing prop shaft. They are joined with a double roller chain as seen in the photo. Motors are sold separately. Included motor pulley/hub is for a 7/8" motor shaft. Please include your existing prop shaft diameter with your order.

Do I need a gear reduction?

Due to the high RPM of our motors, we recommend you consider a gear reduction system rather than direct-drive to improve efficiency, provide proper torque, cooling, and help prevent cavitation. Typical reduction is around 2.67:1, but this ratio will depend on the required prop RPM needed to reach hull speed. Ideal reduction will enable the motor to spin your prop at the hull speed RPM of your prop when the motor is spinning at its max RPM. This number can be calculated using the 50 RPM / Volt constant:

48 Volts * 50 RPM / Volt ~ 2400 rpm max

* If you choose not to use a gear reduction, a thrust bearing in line with the prop shaft is required, since most motors are not rated for axial/thrust loads on the shaft. 

* Before you pull your old engine, support the prop shaft for reference so you can properly align the new system.

Thunderstruck Gear Reduction manual

Included Components