Re: Volvo Penta duo prop what props do i need?
The F7's came on boats up to 22 feet in general, I know we are not sure why the other boat had F5's, but it is larger with the same size engine as yours. Using that as a baseline for estimating an F7 set should work well on your 20 footer with the HP bump you are giving it. If you boat at 3500+ feet then I would go down 1 in number for each additional 1,000 feet, all estimates on that.
My recommendation assumes you are not building a totally high-end horsepower engine. If you are sacrificing low end torque to gain more HP at higher RPM powerband then all bets are off, you may need F5 or even F4 to get out of the hole and into the higher power band.
F5 versus F7, the F5 will give better acceleration from a dead stop than F7 but not as much top speed. This is assuming the engine in the boat can turn the F7 set to the same top rpm. If it can then the F5 set may have an over-rev potential and you would have to watch the tach and throttle down accordingly.
I have attached a Volvo Pent prop guide, you can look up the F series and a similar engine to compare the differences.
Watch that 5,500 rpm top end, that will be pushing what the drive can handle.