Need to choose a prop for 9.5 and 18 hp motors

[BusyJ]

I have a '69 Evinrude 9.5 and a '67 Evinrude 18 that I am thinking about changing the props on. With no tachs or speedometers I have no clue which prop would be a good fit. Both these motors are used to push a late '50's 16' aluminum v-bottom boat with 2 people. Does anyone have experience with picking a prop for these motors on a similar boat?

 

[jim j geezer]

Re: Need to choose a prop for 9.5 and 18 hp motors

Assuming you want to optimize speed, correct?
1. Absent test equipment, you have the seat of your pants and experience regarding your boat's performance.
2. You probably have a wristwatch with a sweep second hand, so you can pick two landmarks and time yourself.

 

[steelespike]

Re: Need to choose a prop for 9.5 and 18 hp motors

Assuming that is a bare bones boat;no floors,casting platforms or live wells the 9.5 with a 8 1/4 X 8 1/2" and the
18 a 9 1/4 X 11. A Tinytach will work on both motors as a hand held or installed.Cost about $47.
You can get your speed with a gps and some cells have a gps app.
If you guys are 250 pounders you might consider a size smaller.
Don't worry about the diameter it follows the pitch. If you look up the props here you will see the available sizes.Size is written diameter X pitch.