Ok so I called powertech today. They recomended the SRD4 (4 blade with 11.25 diameter) in a 13p. My first response was "can my 50 4stroke turn that," and he said that it being a 13p it would have no problem. Said they put the SRD3s (3 blade,11.25 diameter) on boats/motor combos like mine all the time. Thoughts?