Prop diameters and RPM
 

Someone learn me on prop diameter and RPM. Going to #6 props there seem to be more diameters to choose from. I have 4 blade 32 pitch x 16.25 props on my boat right now and I am looking to go with 5 blade props. I know on bravo boats that as a rule of thumb if you from a 4 blade to a 5 you drop 1" in pitch staying with the same diameter, that should keep about the same RPM on top. So, what lets say I go up a 1/2" diameter, what is the RPM drop?

With other things being constant, 1/4" = about 100rpm

Hi JRider,

You're right, diameters with the #6 props are limitless. Actually diameters on all props are limitless. We modify diameters by 1/8" on OB,Bravo, NXT, Arneson, #6 and all types of drives depending on requirements. Sometimes less.
Back when I was coming up at Mercury Racing, I was taught something a little different than what has become commonplace in the industry. As always, I am willing to share this information on this forum even if it helps the competition.

When dealing with #6 props specific, we would opt to go down 1/2" of diameter when going from 4 blades to 5 blades to maintain rpm. We did this rather than maintaining the same diameter and reducing pitch by 1 inch.
The reason was.............ready.............more blades create stern lift lift. Our goal is to maintain the hulls manners while offering the features the 5 blade has to offer and getting the 4 blade rpm. Keeping the diameter the same on the 5 blade and reducing the pitch may keep the rpm levels similar but the bow my begin to drive itself in certain rough situations. Of course, every application needs to be dealt with on its own merit.

And, there is a basic rpm rule of thumb for a 1/2" diameter reduction on #6 props. 1/2" of reduction in about 150 rpm gain.

Thank you,
Brett

So, currently I have 16.25 x 32 4 blade props, DCB F34 #6 drives 1.24 ratio, those props seem to work OK but the boat never seems to quite take a "set". In smooth water it porpoises slightly, I hate to even call it a porpoise. I have talked to a lot of people about props for this hull, Tony from DCB, Teague and every other swingin dick that has had an F34. Teague did a set up an identical hull to mine (similar power/motor placement) and he said it really needed 20*/21* rake props in 16.25"/16.5" diameter, I tend to agree with him about more rake to lift the bow. Could I test this with 18* props in the same diameter to get a feel for what pitch I need and then order 21* rake? What does rake do to RPM and efficiency?

About the 3 minute mark I have my bud in my sights, accelerate and the boat finally take a set as I go by him...I want to run like that all the time.

https://www.youtube.com/watch?v=cqzDOeld5x8&t=474s

One issue about testing with 18 degree props is that the higher rake angles create a bit more drag. Because of this 20,21 rake props will pull the rpm down a couple to few hundred over the 18 degree rake props throughout the first 1/2-3/4 throttle. The idea is after 1/2-3/4 throttle the bow will carry higher and the drag will decrease and the rpm and speed will rise.
I'm not convinced the props you need are in that 20ish rake zone. A slight adjustment to your current props could fix the slow roll porpoise I see in the video.
That being said, test whatever props you can get your hands on. Always use some caution when changing to new diameters and rakes. Unfortunately we do not carry 20+ rake props in our test program.

Brett

I do have a set of 5 blade 16 x 33 x 16* that I bought and tested, they are supposedly rolla but I have a feeling they are merc (see below). Anyway, they only worked in choppy water, if it was't choppy the boat started launching out of the water at 90+, like the whole boat...violently. When it was choppy, they ran smooth and lowered my cruise RPM, went back to 4 blades before I ran it out on top. My thought on going with lower pitch (proper rake) 5 blades was that it would keep my cruise RPM about the same, take care of a lot the porpoise and also give it a little more punch. I am happy to cruise at 100, and satisfied hitting 130. Also, the lower pitch might help around the docks. My slip numbers actually look good with the 4 blades like 5.5%.

You have 16.5 x 30 x 18* Herings on your web site that I have been stalking.

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JRider,

Since we own those propellers, you can test them through our test program.
These props will help around the dock.
Curious, did Tony at DCB also suggest 20 degree rake props?

Brett

Yes he did, but I did get the feeling he was going by what Teague did to set up the boat identical to mine.