Prop size on 25hp Johnson

[scrit9mm]

I see through iboats that there are 4 different pitch sizes.
http://boatpropellers.iboats.com/Johnson-Outboard-Propellers/?chart=8&engine_id=30&*******=460466415
7,9,10,11

Are the only differences going to be speed. I am putting this motor on a 14' Aluminum boat that probably weighs 200 pounds or so. Would there be a cavacation issue on the more aggressive pitch?

What are the benefits or negatives?

 

[84toyota]

Re: Prop size on 25hp Johnson

I have a 14' Klamath boat which tends to be a bit heavy (with plywood platforms, center console, battery, gear, etc.) being pushed by a '71 Johnson 25hp with an 11 pitch prop. I recently picked up a 10 pitch prop off of Ebay, and there was a noticable gain in top speed, and hole shot. I'm almost wondering if a 9 pitch would be even better. I have not hooked up a tach to really see where I need to be. You really need a tach hooked up, and test things out on various trim settings and adjust the weight in your boat...

Ken

 

[steelespike]

Re: Prop size on 25hp Johnson

If you read the info titled at the top of the page it should help you understand better about props.
Feel free to ask any questions.
Very briefly imagine the prop as a single speed transmission that you need to pick a gear(prop)
that performs best within the motors rpm range and your load and activities.Now imagine the motor is like it is in your car pulling a trailer up a never ending hill. You can't successfuly select the right prop without a tach and the Wide open throttle rpm and speed preferably by gps.What year is your motor?
The prop size should appear near the base of the blades or somewhere on the hub.
Numbers are the inches forward the prop would move in one revolution assuming a solid.
In water there is slip measured by percent
Higher number props are like a higher gear in your car, lower numbers like lower gear.
The idea is to select a prop that allows the motor to run near the top of its rpm rating
with a light load. This will usually allow reasonable performance with additional weight.
If you run light a 10 or 11" should work. Speeds approaching 30 mph.A tinytach will help you get the rpm, speed by gps.
some cells have a gps app. If you have a prop check the size and get us the numbers from a test.
For best performance and the health of your motor the right prop is critical. Your rpm range is probably 5000-5500.