From everything that I have read, a 68", three blade, square tip, Warp Drive prop exceeds the MMOI* rating on a Rotax gearbox. Tapering the tips, including the nickel edge, brings it back to the acceptable, yet still at the max, rating.
*MMOI - Mass Moment of Inertia.
http://www.aviasport.com/Rotax/Docum...les/11ul91.pdf
http://www.eaavideo.org/video.aspx?v=2010764613001