Don.
The clutches for the Roots blower on the GW engines have to be interlocked for several reasons, for the direct reversible engine an interlock is needed to prevent the clutch to be engaged when the maneuvering handle is in astern position, for both reversible and nonreversible engines it is an interlock preventing the clutch to be engaged when fuel handle is in a position corresponding to, at normal load, the enginerevs are below 420 rpm. At the same time the normal aspirated inletair ducts have to be shut when the Roots blower is engaged.
All these functions are obtained via linkages from the manual clutch lever up front of the mainengines as can be seen on the drawing below. There is a slight deviation between the basic arrangement for the direct reversible and non reversible engines. The maneuvering diagram shown is a direct reversible execution. The Roots blower drive is a PTO of the camshaftdrive and having a wrap spring coupling for torsional vibration reasons and a double cone friction clutch as can be seen on the drawing. In order to save the friction lining of the clutches, you would like to engage the clutch at lowest possible revs as the inertia forces caused a heavily wear on the friction lining.
On the MAN engines these clutches were not needed as the turbocharging ( Buchi system) could operate during the whole rpm range as well as astern.
Tore