Words from Brunton's Propellers about their product...

"What makes SigmaDrives different is their ability to resist propeller thrust when installed as a standalone coupling, directly from the shaft to the output flange on the engine’s transmission. It reduces and even eliminates vibrations that originate from an imperfect alignment between the propeller shaft and the engine, compensating for up to 3 degrees in all directions"

Bruntons Propellers
Sigma Drive

The Sigma Drive is something to install that replaces the coupling joining the shaft to the gearbox. Our old coupling had a rubber flange in between the shaft coupling and the gearbox flange, this is quite common amongst sail boats. The rubber flange is to allow for vibration and acts as a shock absorber between the shaft and gearbox. Aligning the shaft to the gearbox needs to be aligned with 100th of a millimetre, therefore allowing not a lot of tolerance or room to play.

By installing the Sigma drive, it counteracts any play or mistakes made in the alignment. The Sigma Drive allows up to 3 degrees in change of angle, therefore protecting the gearbox and cutlass bearing from damage or wear. 

We first completed the engine alignment with a standard coupling to ensure we were spot on (Within 100th of a millimetre) and then removed the coupling to install the Sigma Drive. 

As Nandji is a wooden boat, the alignment should generally be completed once again when back in the water. Wooden boats can change shape slightly whilst on the hardstand therefore to have an exact alignment with shaft and gearbox, it needs to completed once again in the water. Installing the Sigma Drive means I did not do this again! As we were aligned perfectly on the hard stand, the SIgma Drive protects us against any small movements that Nandji made as she went back in the water.