Diameter questions

[Bud420x20]

What effect does changing the diameter have? Approximate RPM change? Faster acceleration or top speed?

Is the manufacturers diameter accurate or is is rounded?

My used props diameter measures around 13.75 but the manufacturer says it's 14"

Thanks

 

[Scott Danforth]

the diameter has less impact on performance than the pitch and blade design.

 

[QBhoy]

As mentioned....all depends on the prop.
THe local Merc dealer to me loves to sell the 16x16 big blade prop to the guys with the standard 24-30ft bayliner or similar with a 5.7.
They mostly have great results but are certainly not performance orientated....just overall better jolesnot and decent mid range.
I'd like to think that we should carefully consider such things mathematically and specifically, but most just want to get care free and average results !
For the above stated, I'd rather go for a 14x17 for example, but each to their own !
Best thing to do is gather specific detail like rpm range, spec rpm, gps speed, normal use considerations ....and take it from there.
All boats are different and require a specific application, if fussy. There is much to be gained by the right prop....just that it can be a pain in ghe bum finding it.

 

[ahicks]

More diameter/blade area = more ability to transfer the engine's torque to the water = better hole shot. Downside of more diameter/blade area is drag. More drag = reduced top speed. Your favorite prop will be a compromise of both diameter and pitch.

Prop repair shops often return a "reconditioned" prop to you with a smaller diameter. It's easier to cut the blades down a little than it is to build then up with weld and reshape.

 

[Bondo]

What effect does changing the diameter have?

Ayuh,.... Little to None,....

 

[ahicks]

Judging by your post count, you know better than that. Maybe you could qualify your comment a little better?

 

[Faztbullet]

A propeller with smaller diameter and larger pitch might provide the same load on the boat's engines but may not have the same efficiency as the suggested propeller. Propeller diameter is chosen to be optimal for the boat's particular combination of horsepower, RPM and speed.
"Michigan Wheel"

 

[ahicks]

A drag boat will have more pitch and way less diameter as compared to a tug boat with the same power. There's no doubt diameter plays large in prop selection in some cases. In others, like we see in small outboards, the diameter vs. pitch ratio is pretty much taken out of your choice. The engine and prop manf's have taken care of that for you. In the case where you'd like some say and have multiple choices, all else being equal, smaller diameter will be about speed applications, and larger will be about better pulling power (eg pontoon and ski boat). It's a compromise. That's why finding your favorite all around prop is generally going to take some trial and error.