Re: 115 Optimax and 20cc
the 115 Opti uses a 2.07:1 gear ratio. WOT range is 5000 to 5750 rpms. Hull+motor+fuel=2 persons is in the 2400 to 2500 lb range. I'd guess that using a SS prop, 3 blade similar to the Trophy plus, to run 36 mph @ 5400 you'd need a 17"P. If you'd like to keep the rpm's up higher, say at 5700, go with a 16"p.
I have an '06 90hp Opti on my 20' Seafari. 2.33:1 ratio. 3 blade SS 21"P gives 5350 rpms @ 33 mph. 4 blade adjustable composite, set at 20"P, w/ cleaver trailing edge gives 5400 rpms @ 5300 rpms, but gets on plane much faster. Problem is I keep snappping the correction/adjustment tabs off the blades at $25 each. I'll be trying a 20"P 4 blade SS Attwood this weekend. I'll post the results.
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Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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