This assumes you need to turn a 6 foot diameter propeller, which seems
pretty
generous for the power. I calculate nearly 80 HP at about 3200 rpm, which
is
similar to the crankshaft speed of the Jabiru engines, so the 75 HP figure
should be acheivable.

Hopefully, any resonance problems have been dealt with by the engine or
crankshaft manufacturer, and you are using the same propeller as they
recommend.

Peter

Peter, the VW I'll be flying is on a Dragonfly, which is pretty low to the
ground, and will only be swinging a 52" prop (one recommended by GPAS). Max
RPM on the engine is 3600 (takeoff power), but you're correct that cruise
RPM is 3200.

Bob, I'll accept both your luck and your warning :-)

Eric