Pitch Vrs Diameter Question

[DMC]

I have a Suzuki Df140 on the back of my Seachaser 220 Bayrunner and I am turning a 13.25 X 21 pitch Power Tech Prop 6000 Rpms. The Question is could I turn a 14 X 21 Pitch prop the same number of Rpms? If I could then could I also gain in lift, thus gaining speed?<br /><br />Thanks<br /><br />DMC

 

[Bob_VT]

Re: Pitch Vrs Diameter Question

You will loose rpm's with the larger diameter. Been there done that!<br /><br />Bob

 

[Dhadley]

Re: Pitch Vrs Diameter Question

Bob is exactly right. The larger diameter will "bite" harder. That allows you to raise the motor (X dimension). That is where you gain. You may want to consider prop technology also.<br /><br />Good luck!

 

[Spidybot]

Re: Pitch Vrs Diameter Question

If you decide to look further into props, this links may help: Wisdom of Propellers

 

[Forktail]

Re: Pitch Vrs Diameter Question

Actually the DF140 operates best between 5600-6200 rpm. So at 6000 rpm you have some room either way.<br /><br />The .75" difference in diameter will drop your rpm, but probably not below 5600. And you may gain lost rpm back with the ability to now use more trim, depending on your hull, etc. The larger diameter will usually allow more trim, and additional cupping can help this even further. Increased trim usually picks up rpm because there is less hull/motor drag.<br /><br />On a F115 I can run a 13.5 X 15 cupped not only at a higher rpm than a 13.25 X 17, but 2 mph faster. This is mainly because I can trim the hull out more without cavitation (ventilation).

 

[BillP]

Re: Pitch Vrs Diameter Question

Generally...Diameter is for power and pitch is for speed. When increasing (power)diameter you want to decrease pitch to stay in the same rpm range. Then all the variables come in like forktail says.<br />Also, add a Doelfin and you can trim even higher without cavitation.

 

[DMC]

Re: Pitch Vrs Diameter Question

Thanks for the info guys. I think I may try the 14 X 20 Suzuki Prop. I do have Lenco's for additional trim needs. My motor is already in the 4th hole from the top and I believe thats as high as I can raise it. I have no complaints about the handling of this boat as far as cavitation or chine walking or porpoising.<br /><br />Thanks <br /><br />DMC