[Rick Shoop]

Could someone please tell me how to measure a prop for size and pitch. One book tells me I need a 9.1/2 in X 10 pitch. When I called to order the prop I was told I needed an 8 X 7. When I measured from the center of the hub to the end of the blade I got 4 1/4 in. The prop has been rounded on the edges and filed afew times leading me to think I need the bigger one. I saw a used 9 1/2 X 10 that came off a 15 horse johnson 1974. The ad said it fit 74 and ^. I need a couple of opinions.
HELP Thanks
Rick Shoop[/list]

 

[BobMac]

You measure the prop by diameter and pitch and shaft size. measure from the center of the shaft hole to the tip of the blade and double it. It is expressed as; D X P X S XH where D is diameter, P is pitch, S is shaft size and X is either right or left hand rotation.

The pitch is hard to measure, you need a gauge to determine it correctly but it is usually in whole inches, not fractions of an inch. In the case of outboard props it probably does go by fractions of an inch.

If you measured 4 1/4 inches then you had a 9 inch diameter prop, probably about a 1/4 inch was filed off each blade over time.

Dave Gerr has written a book about props. Get it at your library and read what he has to say.

 

[BobMac]

You measure the prop by diameter and pitch and shaft size. measure from the center of the shaft hole to the tip of the blade and double it. It is expressed as; D X P X S XH where D is diameter, P is pitch, S is shaft size and X is either right or left hand rotation.

The pitch is hard to measure, you need a gauge to determine it correctly but it is usually in whole inches, not fractions of an inch. In the case of outboard props it probably does go by fractions of an inch.

If you measured 4 1/4 inches then you had a 9 inch diameter prop, probably about a 1/4 inch was filed off each blade over time.

Dave Gerr has written a book about props. Get it at your library and read what he has to say.

 

[BushRat]

Rick,
No need for complicated formulas. Unless I read your post wrong, it looks like you've got a 9.9 or 15hp Evinrude or Johnson.
Your OEM will list a number of props that can be used on your motor.
If you were using a 9 1/2x10 that is a common diameter and pitch for most small boats. To correctly prop a larger motor where tachometers are common accessories you would need to follow a set of guidelines, but as your motor doesn't have a tach, just go by "ear":
The perfect prop for your PARTICULAR boat and load (all boats are different) is one that gets you up "on step" planing quickly without plowing water for a hundred yards. However, once on step you should still have accelleration and speed left in your throttle. If you don't and it feels like you are immediately out of throttle (like pulling out onto the freeway in 2nd or 3rd gear but forgetting to up-shift) then your pitch is too small---try going to an 11 or 12 pitch prop (don't worry about diameter as props made for your motor won't be too big). Now, if you are having problems getting up on step with that 10-pitch prop, try going DOWN in pitch to a 9 or an 8 pitch.
What you want is the best COMBINATION of getting up on step plus having throttle left to accelerate once you are planed.
Whoever told you to go to an 8x7 without first asking how your motor runs with the 9 1/2 x10 is not displaying any knowledge of propping. There isn't a guide published that can take into account your particular hull (flat or vee; hard chine or soft chine; ect) so what is needed is to base what you need by comparison to what you have. If that 9 1/2x10 has given you good service and does what its supposed to (get on step nicely but still have speed and accelleration left) then stick with it. If you need further ideas you can contact me. Good luck!
--BushRat--