12-24kw/ AC 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
  • 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 2.67:1 gear reduction pulley set mounted on a 1" stainless steel drive shaft and thrust bearing. The stainless steel plate is 3/8" thick, and has multiple mounting holes to make aligning your gear reduction easier. The kit also now comes with a belt tensioner. The side supports can be cut if needed for custom mount requirements. Plate mounting bosses can be used for triangulated support and alignment. Support arms are provided with the kit for this purpose.

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 drive sprockets are for a 7/8" or 1 1/8"motor shaft. Please include your existing prop shaft diameter with your order.

Do I need a gear reduction?

Due to the high RPM of 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 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 usable 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.

Thunderstruck Gear Reduction manual

Included Components

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

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

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

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