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Is there a rule of thumb as to dia. to pitch. I have a 13 3/4 x 15 cupped prop.what would my rpms do if I went to a cupped prop 12 3/4 x 15 or say a cupped prop 12 x 17? I run a 70 hp Jonny at 5600 rpm wot on 16ft. Lund Renegade at 31mph max.
Basically speaking,....
The Engineers who develop props, design the diameter to work with the pitch,+ blade area to come up with the design,....
So,.....
NO,... There isn't really a Rule of Thumb.......
You pick the Pitch,... The Diameter is designed into it....
Just to add a little bit more, props within a given diameter range (for example the 12-3/4 x 15 and 12 x 17) would be typical props for Johny/Rude 40/50 HP engines. The 13-3/4 inch props would be typical for larger engines. Just because the prop fits the prop shaft does not mean its proper for the engine. The gearcase to prop interface plays into this as well. Putting a prop for a small gearcase on an intermediate gearcase will likely cause performance issues. Exhaust needs to pass through the prop, not around it. Conversely, if you installed the 13-3/4 inch prop on say a 40/50 HP motor it would likely hit the trim tab. So diameter and pitch do indeed work together from a prop design standpoint but diameter is also a physical limitation depending on the engine it's installed on. That's whey there are so many props and that's why the XHS system was developed so as to make a given prop be more "generic".
12 to 13 is a pretty big change all other things being equal I would think the 13 would bring down your rpm. Just logical thinking it is simply harder to swing
a longer blade.Key words being"all other things being equal"
You are misinterpreting the diameter issue. It takes HP to twist a prop. It takes more HP to twist a 13 inch 15P prop than it would a 12 inch 15P prop PROVIDED both props were of the same design, from the same manufacturer and of the same material. Of course RPM will change. How much depends on how much power you have and which way you go (up or down in diameter). Either prop "theoretically" would move the boat 15 inches forward with each rev. But slip factors for both props would not likely be the same. How much difference would there be? The only way you know is to try it. And then you apparently totally missed the "hub fit" issue. Because it slides onto the shaft does not mean the "fit is right". Too small a hub causes exhaust to pass around the hub of the prop rather than through it. You lose performance. Lastly, two props of the same pitch from two different manufacturers generally do not perform the same even if they diameter is the same or very nearly the same.
Thank you again for the info. I did not miss the hub diamiter issue,but I also did not pay attention to it for I would not use a prop that was not the proper sise as to fit correctly. I guess my question was not about putting on a prop not designed to fit my engine,but if diameter of prop (not hub)would help me gain some RPM if I did not go to different pitch.From the answers I am getting I guess there is not any rule of thumb and that I will need to try different props of same design and different dia. to see if there is a change in rpm. I do not want to spend 200-300 dollar plus to find out that my perfomance was not improved.I want to thank everyone again this is realy a good forum. Mike
OK, so why change diameter? Stay with the diameter specifically intended for use with that series of lower units and adjust engine rpm with pitch. And before you do that, experiment with engine height and trim adjustments. You can easily gain a couple hundred rpm by simply raising the engine. Less engine in the water = less drag. Just don't go so high that you lose water pressure.
The reason I wanted to change dia. was because the prop available for my motor go 16p 15p 13p.I need to gain about 3-4hundred rpm.I am afraid if I drop to a 13 pitch I may overwind and as I have a family that I boat with I dont want someone to over rev the motor nor do I want to run right at max. rpm.
