3 blade alum to 4 blade stainless

[kjdunne]

Please check my thinking... I have a 20' Sunbird Neptune cuddy with a 2000 Johnson 150 hp V6, the max rated for the boat.
I currently have a 15" x 17P 3 blade alum prop which runs up to 5300 rpm, 40 mph (gps) with the boat full of fuel, 2 or 3 adults, and fishing gear. The WOT range for this engine is 4500 - 5500, so I think the prop is matched well. I would like to go to a 4 blade stainless primarily to keep on plane at lower speeds and better hole shot when needed, top end speed is not a concern if i lose a couple of mph, I usually run mid - upper 20s. I'm thinking a 14.25'" x 17P may be a even swap due to the smaller diameter. I know the rule of thumb is to drop an inch of pitch to go to 4 blades and another inch for stainless. Will the smaller diameter be enough to make up the difference without dropping pitch?

Thanks in advance!

 

[jimmbo]

Well if going slower is you goal, that will do it. Too many people have a Fixation about Diameter, Leave that to the Prop Engineers. A smaller Dia. can be countered by a Larger Blade area. As for going to a 4 blade, you are going to have to drop 2 inches in pitch to make up for the extra blade. As for the switch from AL to SS, a lot of Variables are there, If the SS has less slip, the rpms may drop, if the thinner blades of the SS help ,the boat may go faster, differences in the Cup or Rake may affect it, so there is no hard and fast guide about pitch changes there. Going from a Vanilla AL to a Vanilla SS can yeaild different results/requirements than going to a Higher Performance SS, but then the engine mounting height might need changing to gain the extra performance

 

[kjdunne]

Thanks jimmbo. I'll go with a 15 pitch if I decide to make the change. Will post results if I do.

 

[QBhoy]

Are you sure your current 3 blade is 15” diameter ? Seems rather large. If it is then it would be comparable to a 4 blade of same pitch for rpm...give or take. Usually the 4 blade of same pitch would have a bigger diameter than the 3 blade. That’s where the thinking is needed but your diameters are similar. You could go down in pitch but you might lose your intended low speed planing idea...or at least the rpm would be higher at such speeds compared to currently.

 

[dingbat]

I have to agree with Jimmbo, it’s a crap shoot at best. Different prop designs behave differently.

I ran a 14.25 x 17 Offshore Renegade prop for years. Very happy with performance (feel) but people running my same boat and motor combos where reporting much better top end performance.

Ran the numbers and sure enough my slip figures where good until mid range then took a nose dive. No idea I was sacrificing that much top end for what I perceived as a better hole shot.

Switched over to a 15 x 15 performance 3 blade prop which raised me almost 300 rpm, putting me close to the max. rated rpm.

Hole shoot is as good if not better. Increased cruise rpm a couple hundred rpm but gained 0.1 - 0.2 mpg in the process and gained 2-3 mph in my top end.


A couple of comments on planing speed.

Planing is any speed above the speed to length ratio, V/Lwl^0.5, of 1.34.

V is boat speed in knots, and Lwl is the length on the waterline in feet, and it is raised to the 0.5 power, that is, the square root.

I would very skeptical of any claims of decreased planing speeds made by a prop manufacturer. On paper maybe, but I seriously doubt you would notice a difference.

If the gains where substantial or verifiable, manufactures would have the quantified improvements plastered all over their advertising.

 

[kjdunne]

Qbouy - the 3 blade is definately 15"x 17 pitch, embossed on the hub. Part # 391200, which comes up on iboats prop search as "OEM, manufacturer unknown, 15x17, 3 balde stainless". The stainless part is wrong, it's aluminum. Still pondering if I'll make the change, will report back if I do. Won't be for a few months at least.

 

[Scott06]

I

I would very skeptical of any claims of decreased planing speeds made by a prop manufacturer. On paper maybe, but I seriously doubt you would notice a difference.

If the gains where substantial or verifiable, manufactures would have the quantified improvements plastered all over their advertising.

saw just that on two different boats when switch from 3 to 4 blade. Gave up top end but saw very noticeable improvement in hole shot and slower planing speeds. While maybe not an ideal trade off for every boat owner suited my loaded cruising and water sport uses perfectly

 

[jimmbo]

To me, it is about getting top speed at the proper RPM, all things else will fall into place