How does “changing gears” with the Gori propellers work? Is it done with RPM changes?

What is the benefit of having the Gori props in overdrive?

Liken it to being in 5th gear on the highway. Lower torque, but good fuel efficiency.

In brief, this "gear change" happens by performing a specific motion that open the folding propellor in reverse, making what is normally the leading edge the training edge, and vice-versa.

To do this, the vessel needs to have water passing over the prop moving stern-to, allowing the force to open the prop in the overdrive mode. 

The below video shows this motion:

Are we looking for increased speed or better fuel efficiency when using the overdrive function?

Whilst there may be a marginal improvement on top speed, the overdrive function is really about extending cruising range and efficiency. When motor sailing on a cat, it is very common to run on one motor and our experience of running the Gori 3 blade folding props in overdrive is around a 15-50% fuel saving. 

The below are some real figures from a Seawind 1260, running on a single Yanmar 29hp motor at 5 Kts boat speed. 

As you will see, the RPM Is far lower in the overdrive function, reducing fuel burn. 

Using the standard Yanmar fuel burn charts for the Yanmar 3YM30AE, we can then see some assumed range figures: 

The above figures are based on single engine motoring at 5 Kts boat speed in flat water.