After four attempts with my motor, I've finally learned how to eliminate cavitation or any change in speeds using a rock guard. The complete recipe took some design changes, until perfection was achieved. So far, I've finalized my template rock guards for the 6.5, the 12, and hopefully Sunday, the 18 will finally be covered. My rock guard is built from 1 inch thick UHMW that is grooved to fit over the skeg. The fit is so tight, that a rubber mallet is needed to get it on. 5 counter sunk screws with nylock nuts, and she's a done deal.
I don't like using metal as a rock guard because every time you hit a rock, all that force (metal to rock) is transferred to your frame. The UHMW provides a vibration deadening absorber that eases the harsh impacts that would've normally gone to the frame.
The rock guard provides an additional 1.5 inches of protection just below the skeg. This will also eliminate abrasion to the skeg. The guard terminates just shy of the furthest most tip of the blade. A deep 45 degree routered edge helps the guard slice through the water without cavitation or change of speed. Thanks Boudarc and KKalaska for showing up and helping with video and pictures. Here's a customer's armored motor tested on my Esquif Cargo(first rock guard for the 6.5):