prop diameter

[ultimbow]

Hi i just got an evinrude 9.9 1991. I want to get a spare prop in case of a brake while out fishing. In the manual they say 9 1/2 x 10. From what i read the 9 1/2 is the diameter 10 the pitch. Now for my question. I can get a 9 1/4 x 10 for 50$ (a lot easy to find too) and the cheapest in 9 1/5 x 10 i found is 90$. Can i put a 9 1/4 on it? Will it just resault in slowing me down a bit? What the major difference? Should i stick with 9 1/2?

Thanks

 

[steelespike]

Re: prop diameter

Don't worry about the diameter if its intended for you motor the diameter is right.
However the pitch is critical.If its a light reasonably fast boat with a light load the pitch should be higher.
If the boat is heavy or load always heavy the pitch should be lower. test the boat with your present prop .Would be best to have the prop size and wide open throttle rpm and speed. with the 9.9 a good detailed description of the performance and a description of the boat may be enough.
A good accurate gps speed is a must.

 

[ultimbow]

Re: prop diameter

its a small 14' aluminum row boat capacity max 760lbs normal loaded is about 400Lbs( equipment and 2 person) The Outboard is an evinrude 9.9 1991 manual says full speed 5000 to 6000rpm. Check top speed 18mph tested with handheld gps. Prop is an 9 1/2 X 10. Seem to run good but this is my first boat so i have no experience to compare.

 

[steelespike]

Re: prop diameter

Is the 18mph lightly loaded? When figuring weight don't forget motor,gas batteries,oars,anchor etc.
400 does seem you have it covered. what is the weight of the 2 people?

 

[ultimbow]

Re: prop diameter

with 2 adult would probably be around 500 total but i usual go with my 10 year old kid

 

[steelespike]

Re: prop diameter

A 9.9 should be able to run around 20 mph on a light reasonably fast boat. Your 18 seems ok if that is your normal load.
The question is the calculator says with 10% slip (a guess) your rpm is about 5100.barely within your rpm rating.
A working tach would be better to figure out the setup.
If your 10" is in good shape and your looking for a spare I would drop down one size from the 10".
it will let the motor rev easier and higher into the rpm range.should improve hole shot and may add a bit of speed.