Three vs four blades

[Desertsky]

This is a hypothetical question so please don't ask about the particular boat, engine, speed, etc.

Say I have a 15" pitch three blade on my boat right now. Would going to a four blade 15" pitch prop mean I would have to change the diameter to maintain the same loading? The prop charts i have seen seem to indicate that the same diameter and pitch would be used regardless of how many blades the prop has.

 

[emilsr]

Re: Three vs four blades

General rule of thumb is that adding a blade will reduce rpm by 200-300. Reducing pitch will gain this rpm back. In your case, if your 15" pitch 3-blade is running in the recommended rpm range you'd want to go down to a 14p or even a 13p in a 4-blade.

I'm not sure you could make such a "rule of thumb" for diameter as there are so many different variations on different props that it would be virtually impossible to make a direct comparison between one type of prop and another. Blade shape, blade area, rake, cup, prop depth.....there are just too many factors for me to get my head (and my calculator) around. I suppose someone smarter than me COULD calculate it all but they'd need a very sharp pencil.

 

[steelespike]

Re: Three vs four blades

Generally diameter doesn't enter directly in prop selection.Usually only in extreme diameter changes
3/4" to an 1". Usually as pitch goes down diameter goes up.I've seen applications where the same pitch is suggested and others suggesting a 1" drop in pitch.
If I was selecting a prop for a hard to push boat like a pontoon I would lean toward a larger diameter.

 

[dingbat]

Re: Three vs four blades

I run a 14.5 x 17, 4 blade prop and use a 14.25 x 17, 3-blade aluminum as back up. The aluminum prop is on the high end of the acceptable RPM range and the SS prop is right in the middle. I originally tired a 14.5 x 15? SS prop but it had no bite on the lower end and the RPM was a bit too high up top.

 

[PROPer]

Re: Three vs four blades

emilsr has about the same feeling I do regarding the swith from 3-blade to 4-blade. There is usually a little diameter change going from a 3-blade to 4-blade, especially if the prop is produced by the same manufacturer. The best "rule of thumb" is typically just drop 2 pitch when going from 3 to 4 blades.

 

[Desertsky]

Re: Three vs four blades

Thanks for all of your replies. The reason I asked was that all the online prop selection charts seemed to show the same diameter and pitch for the three and four blade props for my boat. There is no substitute for real world experience!