240 sport prop

[nhrapro332]

well i have it and like it alot. it has an alfa drive with a 20 prop. only turns 4300 rpms. should it spin up to at least 5000? my friend has a 27 fever and he had about 2 to 3 more mph on me. but he was at 5200 rpms. if i want to gain mph whitch way should i go on the prop. sorry 1st boat owner questions.... gps said 67 by the way.... thanks

[wfo1]

First, You should probably throw away your GPS because it doesen't work. No way your doing 67 with 20" prop turning 4300. Even with zero slip you would have to have like a 1.1 gear ratio. If you have a 454 alpha you probably have a 1.33 I think. If you have a stock merc 330 a 19 will be close as daredevil said.