Fooling with my 18' bass boat /w mercury black max XR2 150 hp. owned it appx.17yr. It came with a Quicksilver 3 blade 23p prop, & there was appx. 3/16" space between outer wall of prop & inner wall of lower unit. I just figured that's how it should be. Appx. 12 yr. ago, I traded for a 4 blade 24p from a prop shop, no name on it, but it helped the holeshot, didn't fix but helped, same gap. Today a buddy loaned me 2 of his props,( I'm still trying to get a better hole shot ), to try. I put his on there & that gap closed to maybe 1/16". Have I had the wrong size all the time? What sizes do they make? where do you measure the hub dia.? inside, outside, outside of lip that goes up in lower unit? How much difference could be expected from my smaller hub & the larger hub, pitch & diameter being the same?