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ThunderStruck Gear Reduction


What is a gear reduction?

  • It's a mounting assembly for the 5 KW and 10 KW 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.


Now with a Stainless Steel Plate and Shaft! The ThunderStruck Gear Reduction comes standard with a 2:1 gear reduction pulley set mounted on a 1" drive shaft. The stainless steel plate is 1/4" thick, and has multiple mounting holes to make aligning your gear reduction easier. Also now comes with belt tensioner! The side supports are easily adjusted by being cut down to the proper size for your application. Top mounting holes 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 are for a 7/8" motor shaft.

Do I need a gear reduction?

A gear reduction is not always necessary, and largely depends on the size, efficiency, prop style, and desired speed of your vessel. However, due to the high RPM of the some 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 2:1, 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, it's important to install a thrust bearing somewhere in line with the prop shaft, as most motors are not rated for axial/thrust loads on the shaft.  If you Do pull your old engine, it's good to support the prop shaft where it's at now so you can properly align a new system. 

Thunderstruck Gear Reduction manual