5-10KW Gear Reduction

In stock

Belt Drive Now Standard 

High torque drive belts and pulleys are now included in our reduction kits. These are strong, quiet solutions for sailboat conversions. If steel chain is preferred, please contact us for pricing and availability. 

What is a gear reduction?

  • Mounting assembly for the sailboat kit motors (will also work with most NemaC face motors)
  • Thrust protection to isolate the motor from axial propeller forces
  • Speed reducer/ torque multiplier to prevent cavitation
  • Includes a shaft coupler to attach to your existing prop shaft


This ThunderStruck Gear Reduction comes standard with a 2:1 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. Diagonal support arms are provided for maintaining correct alignment. A belt tensioner is also included.

Motors are sold separately from the reduction assembly. Included motor pulley/hub is for a 7/8" motor shaft, which fits motors sold by Thunderstruck. Please contact us if this is not your motor shaft size. 

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.  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 2:1, 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.

* 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.

* Replacement bearing pn: L44643


Included Components

  • Stainless Steel Mounting Plate with installed components:
    • thrust bearing
    • reduction output shaft
    • drive pulley
  • 2 stainless "angle iron" support mounts
  • 2 diagonal support arms and angle attachments
  • Motor pulley and hub 
  • Prop shaft coupler
  • Belt tensioner with hardware
  • Motor mounting bolts 
  • Machine keys (for motor pulley and coupler)
  • One drive belt

Stainless steel hardware is included for completing the assembly. Hardware required for final connections to the boat structure is not included.

All components except the prop shaft coupler, bearing housing and drive pulley are made from stainless steel.

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