Bob:  I use an IVOPROP 3-blade that is highly-pitched (HP) to 105 inches
helical travel.  It doesn't use a hydraulic governor and instead uses slip
rings similar to those developed for electric propeller deicing.  The rings
are located on the starter ring to supply electricity to a motor located in
front of the propeller and within the nosecone.  Previously I wrongly
reported that the harmonic dampener was located on the starter ring when in
fact it is not (I often say this because it makes explanation easier,...
sorry!).  The dampener is located on the prop flange immediately behind the
prop and I use six inch long, 1/2 inch thick bolts to secure it, the prop
and electric motor to the flange.  The dampener is a 17 lb precisely
machined piece of soft steel and it adds a considerable amount of inertial
throw to the prop which is an aid in starting.

I believe too that the dampener is the reason why I have had success using
the IVOPROP when others using 4 cyl engines have experienced trouble with
vibration and prop loosening as indicated by a metal test tape (stress) that
bridges between the three props.  In fact, Ivo is now reluctant to apply the
IVOPROP to 4 cyl engines.