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12-24kw/ AC Gear Reduction


* Now With Chain Drive *

This reduction drive kit for 12-25kW and AC kits uses components rated for more power to meet the needs of a larger vessel. Please call about custom installation requirements. Price varies. Plate is roughly 9.5" wide by 16" tall (plus tensioner) and shaft extends about 9.5" from the plate.

Thunderstruck reduction drives with chain and sprockets are now being developed in part because of supply issues with belts and pulleys needed for our higher performance kits. In order to offer a quiet and durable replacement, we are providing a #50 chain in two styles for each reduction drive.

One is a new style made from a combination of stainless steel and poly materials. This option has been tested in-house, but has not been proven by many hours of use in actual installations. We are very interested to hear from you about how this chain performs for you, but understand you may want to keep it as an emergency spare if you do not feel comfortable with using it as part of your standard system.

A steel chain, also included, is a proven option. Lubrication is required, and we recommend a spray-on motorcycle chain lubricant, which tends to stick well and last longer than grease or oil.

We believe that parts for the original belt drive will be available in the future. These components can fit on the chain drive systems without machining or modification. We will advise when belt drive parts become available again.

What is a gear reduction?

  • It's a mounting assembly for the sailboat kit 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.


This ThunderStruck Gear Reduction comes standard with a 2.67:1 gear reduction sprocket 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 chain 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.

Gear Reduction Manual (new chain drive version)

Thunderstruck Gear Reduction manual (legacy belt drive version)

Included Components