Prop diameter

[rinker115]

Looking to get a new prop or 2 for my 115 h.p.<br />Johnson..My question is this..I have a 13x17 for<br />tubing,kneeboarding ect..And a 13x19 for cruising...What will happen to boat speed and engine rpm's if i change to a smaller diameter prop...Say 11x17 or 12x17 I know what pitch does but how does Diameter affect performance and engine rpm's???????

 

[Ross J]

Re: Prop diameter

Imagine you're driving a 4x4 with 16" wheels and then you change down to 13" wheels. Two things happen.<br />1 - The motor has to turn faster to make up to the desired speed.<br />2 - A lot more fuel is required to achieve the same result.<br />Generally unless you have a specific reason to change down, like doing slow barge work without idling all day, then don't!<br />If you have to go down in size then you'll need to go up in pitch. Might be best to seek pro advice and try before you buy.<br />Ross