3-blade to 4-blade conversion

[jgoeden]

Hey can you guys help me figure out the math on this? I'm going to do some research myself but prop sizes has always baffled me.

Right now I have a 14" x 19P 3 blade aluminum prop.

I want to move to a 4 blade. I don't want to spend the money (or risk) on stainless, so I'll happily stay with aluminum but I want to move to a 4 blade. Right now my 19P maxes out my RPM's perfectly around 4400. I'm not concerned with top end but primarily pull tubes, skis, and wakeboards so I want to lose a little top end for some easier acceleration. I was considering getting a 17P this season but I really would like a 4 blade, and I know there's any extra conversion, so could you guys tell me what I need?

I'm running a Merc 170 with Alpha 1 outdrive.
Thanks!!

 

[jgoeden]

Re: 3-blade to 4-blade conversion

So.....

Right now with my 3-blade 19P, a 4-blade 18P would be equivalent, correct? So I should probably get a 17P 4-blade for just the slightest bit increase in acceleration with a 4-blade? Or if I wanted to stick with the same out of acceleration vs. top end, I should get an 18P 4-blade, and I will just notice smoother (possibly slightly quicker) holeshots?

That's mainly my goal is just to have slightly quicker, smoother hole-shots.

 

[Silvertip]

Re: 3-blade to 4-blade conversion

There is no simple answer to your question. Propeller "design" is a combination of a high number of factors such as blade shape, rake, cupping, to name just a few. What I'm getting at is that if you tested the prop you have now and then picked another 19P three blade from another manufacturer the two props would very likely NOT perform the same. So going from a 3 to a 4-blade introduces one set of variables and going from one manufacturer to another introduces another set of variables. Going from aluminum to stainless is another factor. In other words it is very possible that you may be very happy with whatever you choose, or you may actually hurt peformance with what you choose which in a nut-shell is why prop selection is such a crap shoot. The best way to make this change is to try and find a prop shop that will work with you by letting you test various props to see what works. Then be a sport and buy from them when you find what works. Failing that, there is a school of thought that says stay with the same pitch, and another that says drop one inch of pitch which is probably a better choice based on the fact that many four blade props are pitched in even numbers rather than odd which is true in most three blade designs.

 

[PROPer]

Re: 3-blade to 4-blade conversion

Move down to a 4-blade 17 pitch and you will be happy!

 

[jgoeden]

Re: 3-blade to 4-blade conversion

Silvertip, thanks for the awesome advice. Makes perfect sense. It'd be like buy a motorcycle I guess, a 600cc crotch rocket is very different among brands.

Also thanks to PROPer for the reassurement. I think that's what I'll do is either get the 18P to remain the same (just smoother performance) (HOPEFULLY) or try the 17.

I may try to find a place who will let me try a few out, I never thought of that!

 

[Silvertip]

Re: 3-blade to 4-blade conversion

You might even try your local boat dealers. They generally have a pile of used props and would probably let you try anything in the pile provided you left a deposit.

 

[dingbat]

Re: 3-blade to 4-blade conversion

I run the same same (14 x 17) in both 4 blade stainless and 3 blade aluminum. But as Silvertip said, each prop is different.

I used Overtons for my experimentation. They allow you to return props