Prop Selection

[breymeyer32]

Ok so I have a 17.5 Lowe boat fish and ski with an EFI 115 Mercury Outboard. I currently have a 3.75x15 pitch 3 blade prop. It seems to run on the upper part of my rpm limit. My question is if i switch to the 4 blade should i increase the prop size to 17 or keep it at 15 the four blade prop im looking at is the spit fire by mercury. My main concern is running in my rpm range which is 5800-6400. My goal is to find better hole shot and quicker planning without losing to speed. Does anyone have any input on which pitch i should run for this 4 blade.

 

[breymeyer32]

Re: Prop Selection

I also wanted to add the sizes for the 4 blade have a diameter are 12.8 if that makes a difference. and the rpms with the 15 pitch 3 blade is between 6200 and 6500. But i do remember it being of 6000. The dealer said the stock prop for the boat is a 17 pitch if that helps.

 

[QC]

Re: Prop Selection

Typically you should go down one inch when going from 3 to 4 blades. Your goals are kind of everyone's. With the perfect match, you may be successful. If your 3 is well matched it will be hard to get what you want. Hardest thing is duplicating a good 3 blades top speed.

Edit: new info from OP.

What year Merc?

 

[breymeyer32]

Re: Prop Selection

The mercury motor is a 2009 with the 2.33.1 gear ratio.

 

[QC]

Re: Prop Selection

the rpms with the 15 pitch 3 blade is between 6200 and 6500. But i do remember it being of 6000.

Can you clarify this please? Trimmed out? Loads? Also your corresponding speed in GPS would help. Is this prop Aluminum? Going to be hard to improve hole shot and maintain top speed as is. A 17 would make your hole shot worse for sure. If you want lower planing speeds, then you should consider trim tabs.

 

[steelespike]

Re: Prop Selection

We need your wot rpm and speed.
If your setup is ideal for speed then a 4 blade will very likely cost you some speed.
It should however improve hole shot,stay on plane easier at reduced throttle and it may respond better around the dock.
You could also try Smart Tabs. Carefully applied they can improve hole shot and some times improve speed.

 

[breymeyer32]

Re: Prop Selection

My wot rpm is 6400 believe it or not and we usually have 4 people on the boat. We pull alot of skiers and tubers sometimes weven two skiers. I need a prop that gunna have the power im not so much worried about the top end. Im looking at the mercury spitfire prop i guess its the new aluminum 4 blade prop that doesnt sacrafice speed. O yeah and my max speed was 40mph.

 

[QC]

Re: Prop Selection

Well there is something wrong with the numbers as I am getting negative slip. What I am using is 6400 RPM, 40 MPH, 2.33 gear ratio, 15 inch pitch. Speed was GPS?

 

[cloumar]

Re: Prop Selection

Well there is something wrong with the numbers as I am getting negative slip. What I am using is 6400 RPM, 40 MPH, 2.33 gear ratio, 15 inch pitch. Speed was GPS?

Hi QC,

Well myself I have a 2012 Merc 115-hp 4 strokes 2.33 gear ratio on a 17.5-feet aluminum boat a little bit heavier than the Low and I am running at 6400-rpm and 40-MPH ( GPS ) with a 17'' pitch which gives me a slip of 9%.

MC

 

[cloumar]

Re: Prop Selection

Hi QC,

Well myself I have a 2012 Merc 115-hp 4 strokes 2.33 gear ratio on a 17.5-feet aluminum boat a little bit heavier than the Low and I am running at 6400-rpm and 40-MPH ( GPS ) with a 17'' pitch which gives me a slip of 9%.

MC

OUPSSS ! I just saw that Breymeyer is running now with a 15'' so you are right QC that can't be good figures.

 

[breymeyer32]

Re: Prop Selection

I dont have a gps to calc speed i was using my tachometer. So what you guys are trying to say is that i should have got a 17 instead of that 15 even though its a four blade. I know someone who has the same boat running a 16 pitch and hes a 6150 rpm at 44mph. I called the previous owner today and he said he switched the prop and the lowe dealer gave him the 15 pitch.

 

[QC]

Re: Prop Selection

You're drawing conclusions from bad data. How did you get your speed from your tachometer?? You really can't calc speed in a boat as the medium you travel in is squishy (water) as opposed to fixed (pavement).

 

[breymeyer32]

Re: Prop Selection

Ok i see what your saying i didn't record all of the data before i broke my prop because i didnt know i was going to bust it. I have almost the exact same boat as cloumar. I called the manufacturer and they come with 17 pitch 3 blades. If I want to run that spitfire 4 blade what pitch should I run and it does not come in a sixteen. I will also be pulling skier and tubers.