The hydraulic gear- or fluid coupling mentioned before was invented by Prof. Föttinger and is sometimes therefore also called "Föttinger transformer".
It´s not acting exactly like the Vulcan hydraulic coupling, which allowed a theoretically infinite but practically finite number of sources of power to be transformed into the shaft (say, four engines feeding one shaft) while the Förringer transformer was a hydraulic reduction gear to transform one engine into one shaft. It´s earliest application can be found in a steamer called "Föttinger Transformator" before world war one.
The reduction ratio was usually 1:5 but could be varied. The effeciency reported was 95%.
Examples of it´s basic elements and an installation are shown below:
The main purpose for a Föttinger transformer was to allow a higher propulsive efficiency compared to direct drive turbines in reducing the rpm from the turbine to the shaft and allowing wider, larger props, slower turning to be fitted on the shaft. That combination greatly improved the total propulsive efficiency of early steam turbine driven ships. It also had a couple of other advantages, allowing a lightweighted turbine of smaller diameter to be used (running at higher rpm), and some notorious space reduction of the machinery spaces as can be shown from this comparison of direct drive and Föttinger installation in a large powerplant supposed to be for Dreadnought types:
Source: Hydraulische transformatoren. Föttinger transformer, in: Noticen aus der Stettiner Maschienenbau-Aktiengesellschaft "Vulcan", No. X (Stettin/Hamburg).