Help with prop?

[coachkwj]

I recently bought a 2018 18' pontoon boat with 40 hp Yamaha 4 str. Boat hits 15 mph at 5000 rpm full throttle. Full throttle can be 5500-6000 rpm. Current prop is 11 3/4 X 9. Any thoughts on a different prop for better top end without over revving?

 

[alldodge]

Don't think you will get much better top end

Could go to a 10 pitch if you can find it, but don't think it will be much better

 

[jlh3rd]

you understand that you are 500 rpm below the minimum of 5500. You are lugging your motor.

 

[coachkwj]

you understand that you are 500 rpm below the minimum of 5500. You are lugging your motor.

So should I go with a 10" or 11" pitch?
I checked the prop again and the diameter is actually 12 1/4", not 11 3/4" as I earlier stated. Does diameter have a big effect?
Thanks in advance.

 

[Scott Danforth]

You are already over propped. You do not want to go up in pitch

 

[jlh3rd]

So should I go with a 10" or 11" pitch?
I checked the prop again and the diameter is actually 12 1/4", not 11 3/4" as I earlier stated. Does diameter have a big effect?
Thanks in advance.

no...you are overpropped as stated.
to increase rpm you reduce pitch, or diameter, or # of blades.
yes, diameter plays as much a role as pitch.
So, you go to an 11x8....or 11 1/4x8...maybe get 5600-5700 max rpm, maybe more...might be less...
see what happens.....but you want to be no less than 5500@max throttle...
propping a boat is not exact science. and can get expensive...

 

[Scott Danforth]

I recently bought a 2018 18' pontoon boat with 40 hp Yamaha 4 str. Boat hits 15 mph at 5000 rpm full throttle. Full throttle can be 5500-6000 rpm. Current prop is 11 3/4 X 9. Any thoughts on a different prop for better top end without over revving?

if you want a better top end, you need more motor, or your current motor is at the wrong mounted height.

I would suggest looking to see where the cavitation plate is while you are underway. you may need to raise the motor.