How does pitch affect diameter?

[496 Cubic Inches]

I have a 2004 Yamaha 50HP that is running at too many RPMs. The recommended range is from 4500-5500,and its running at 5850 RPMs at WOT.<br /><br />The current prop is a 12x11 and I understand that by going to a 13" pitched prop it should get me right about where I need to be.<br /><br />I found some Turning point props at Boaters World on clearance for about $35. They have a 13" pitched prop that will fit my motor but its only 10 1/2" in diameter. <br /><br />Do I need to find a 12x13 prop or will a 10 1/2x13 be what I want?

 

[Dhadley]

Re: How does pitch affect diameter?

Count the splines. I think thats for a different motor than yours.<br /><br />I'd bet youre perfect on the rpm for long life and best efficiency.

 

[Silvertip]

Re: How does pitch affect diameter?

Diameter and pitch are mutually exclusive. In other words you can have a 40inch diameter prop with 13 inches of pitch or you can have a 10 inch diameter prop with 13 inches of pitch. You are propped ok for light load and perfect for a heavy load. If you really want to change props, you want a 13 pitch and if the diameter for the selected prop comes in a 12-1/2 or 12 thats fine. You don't want a 15 inch diameter prop or a 11 inch diameter prop as they probably have wrong hubs and are not designed for your engine. As pitch increases, diameter may or may not decrease slightly. As pitch decreases, diameter may increase slightly. Its a design issue.

 

[JasonJ]

Re: How does pitch affect diameter?

I'd leave it. Just don't run WOT all the time. With your current setup, you can carry more load, and the engine will not have to work quite as hard, which will be better for the engine in the long run. My tach tickles 6000 at wot, but hovers around 5700 loaded down, and it does not have to struggle to get on plane. <br /><br />I had a 14x17 prop on my rig, but the prop became a little less than perfect, so I switched over to my 13x17 backup. What I immediatly noticed was a small loss of top end speed, but a little better holeshot, even though the previous prop was vented. The differences were not large enough to concern myself over...Good luck....

 

[ivar]

Re: How does pitch affect diameter?

Which version of the 50 is it you have? <br />the 50H (same block as the 40) , 50G (same as the 60) or the F50 four stroke? <br />And what kind of boat, at what speed? <br />On my Yamaha F40 on a 14' CC weighing 600#, I had a 11 1/4x13 and a 11 1/2x12.<br />For you' , I'd think the 11 1/2x12 would work, supposed you have the 50H. Lowering your WOT rpms to 5600-5700. Going down to a 10 1/2"x13, I suspect the smaller diameter will give you more prop slip. <br /><br />Usually, the diameter decreases as the pitch increases , as the prop manuf. figures that the props with more pitch will be used on smaller boats.

 

[496 Cubic Inches]

Re: How does pitch affect diameter?

I guess its the 50H,its has the same displacement as the 40. I have it on a 17' Alumaweld SuperVee and it runs about 29-30MPH.<br /><br />I put the 10 1/2x13 on yesterday and went for a test run this morning. <br /><br />It reduced the RPMS by almost 500,but other than that it seems to be a good match. My boat gets out of the water better than the old prop,and I gained almost 4MPH. I'm sure its the placebo effect,but my motor also sounds better at all RPMs, it just seems to be running smoother.

 

[shep70057]

Re: How does pitch affect diameter?

I've experienced that a larger diameter prop needed more power for my motor to turn it. Difference was approximately 400 r's. 15.5-17p Mirage Plus to a 14.25-18 Suzy... the 17 robbed me of 400 r's.... Boat's attitude was better but I was lugging the engine.