2013 Mercury 4-stroke 115hp – 115EXLPT - Prop?

[toddrun]

Hello,

I own a 2013 Mercury 4-stroke 115hp – 115EXLPT outboard, it is on a 2013 Larson FX1750DC VEC hulled boat. Currently it has a Mercury Vengeance 48-16988A46 3 Blade Stainless Steel 13" x 18 RH propeller, and it functions very well at 6000 RPM at WOT, at a speed of around 36 MPH. Being a heavy boat, I would like to try a 4 blade propeller, to get better hull lift and take off. Using the Mercury propeller selector, it suggested a Mercury Trophy Plus 4-Blade Stainless Steel Propeller, 13.75" Diameter x 17" Pitch, Right Hand Rotation - Part number: 8M0151389.

My question is - does that seem like a good propeller replacement, being it is .75" diameter larger, and 1" pitch less?

My understanding is a 4 blade propeller requires a 1" pitch drop, but what about the larger diameter added to that?

Any assistance would be greatly appreciated.

 

[alldodge]

Welcome
A 4 blade prop has more drag than a 3 blade and the rule of thumb is 2 inch pitch drop. While the the 4 blade will provide more lift it looks to me like you need more HP. Max HP rating for your boat list 150, so with 115 your at the lower end of it

 

[toddrun]

I agree on the HP suggestion, but I am not after top speed, just want the boat to get out of the hole and plane out faster. Outboard swap is cost prohibitive, and the extra weight causes a low transom water height.

You kind of answered my question though, I think the 4 blade, 1" pitch drop, but added .75" diameter, will be too much for the 115. Just not sure. Can't get that prop in a 16 pitch.

 

[jlh3rd]

trying to understand what is meant by "too much for the 115". All the engine cares about is rpm@WOT.
I've got a 2021 115 Merc. Currently running a 4 blade 14x14. I've also run 3 blade 16x13 and 16x12. All aluminum Mercs. Also a Solas 3 - 15.5x13.
This is on a pontoon.
The 16x12 gave me the rpm's I needed for tubing. The other 3 gave me practically identical performance #'s...( I can post).
But none of the props diameters were "too much" for the engine. (what I need is a 4-14x13, but no one makes one for my gearcase)

rule of thumb on paper: ....3-13x18 vs 4-13.75x17...prop material and manufacturer being the same.
adding a blade - decreases rpm
adding diameter - decreases rpm
decreasing pitch - increases rpm
exact rpm depends on whose opinion you believe.
no way to accurately tell you it will or will not give you what you want.
Propping on paper is a guess, a beginning point. That's why I went through/had 4 props, even though I knew what the rule of thumb is.

I think the 4-13.75x17 might give you what you want.