Help me understand Prop Diameter and Pitch.

[cat toon]

Well, I guess you realize by now I am Prop stupied,I took off a 15 diameter and a 19 pitch prop.I bought a 13.5 diameter and a 15 pitch,I never ran the toon with the 19 pitch,the Motor is a 70 horse Johnson what should I look for to make the motor perform.I changed the prop because my buddy said that 19 might not work on the much heaver toon. The motor came off my Bass Cat boat.

 

[coastalrichard]

Re: Help me understand Prop Diameter and Pitch.

Determine the max rpm for your motor at WOT. You'll want a prop pitch that will allow your motor to reach max rpms' at WOT with a typical load (people, gear, etc.) Too little pitch could allow you to exceed max rpms (not good) at WOT while too much pitch will put more load on the motor, preventing it from achieving max rpm at WOT (also, not good). You will have to try different pitches until you find the one that gives you the best "hole shot" (if a pontoon has one) while still allowing you to achieve (at or near) max rpms at WOT.

 

[tpenfield]

Re: Help me understand Prop Diameter and Pitch.

yea, a 15d x 19p seems way big for a 70 hp . . . so you might be in the ballpark with the new prop. As CR mentioned, you want your engine to reach its specified RPM at full throttle.

Generally, within a horsepower range, you go with a certain diameter prop and then the pitch is matched to get you to your engine spec's.


In terms of diameter, that is pretty straight forward, and pitch is measured as to how far the prop would want to travel per revolution (as if were going through a solid). so, the lower pitch 'throws' less water per revolution, the higher pitch throws more . . .

 

[jestor68]

Re: Help me understand Prop Diameter and Pitch.

Pontoon boats require larger diameter propellers with less pitch(more blade area) in order to develop the thrust necessary to move their semi displacement hulls.

What year motor? Large or small gear case? What size pontoon boat? The 70's ability to spin up to recommended WOT rpm with a 15" pitch is in doubt.

Your motor's WOT rpm range is 5000-6000 rpm; with the hp rated at 5500 rpm.

You need a tachometer to determine if the prop is correct.

Evinrude(BRP) makes special "Hydrus" pontoon props for your motor. They are 13-7/8 diameter large blade area props available in 9,11, and 13 pitch.

What you have(13.5 X 15)is a "regular" boat prop not specifically designed for moving pontoon boats.