Propeller Changes?

[Scherbacj]

I have a 75HP Mercury 4-stroke that currently has a 4 blade Spitfire 13.4” x 15P propeller and am getting about 5,000 to 5,100RPM at WOT and only getting about 33-34MPH top speed. I would like to pickup a little more top end speed so I was thinking about trying a Turning Point 13.25” x 17P 4 blade hoping that slightly smaller diameter will make up for the increased pitch and keep me around 5,000RPM but wanted to ask the pros if I am thinking about this properly? 
 

I am new to boating but currently the boat seems to get up on plane just fine and stays on plane even at lower RPM so other than wanting a bit more top end the current prop seems good all around so I don’t think a huge change is necessary. I also did a prop slip calculation and currently only have 6% slip so I am in the proper range there. I worry that trying a 3 blade would make it harder to get on plane. 
 

I welcome any advice/input. 
 

P.S.-Do the add on/oversized cavitation plates really do anything? 
 

 

[Tim Kelly]

What sort of boat is it? Do you have vessel view, or similar to confirm that the rev counter is accurate?

[Scherbacj]

1 hour ago, Tim Kelly said:

What sort of boat is it? Do you have vessel view, or similar to confirm that the rev counter is accurate?

It’s a Lowe Stinger 178. I do not have vessel view. 

[Tim Kelly]

I don't know whether you're that far away with what you have. The Lowe website says the 178 is rated to 115hp and it looks like a very wide boat, so you might be struggling for much more speed. Probably worth getting vessel view as I wonder if your tach is a bit off. Motor height might be something to play with too, but I think you need to be sure of your RPMs before doing anything much to it as the motor should go up to 6000rpm I believe and if your tach is reading correctly you would want to go down in pitch, not up, to get the max RPM to nearer 6000. 

[gim]

I've gone down this rabbit hole more than once.  I looked into changing props and determined that it wasn't worth the headaches required to get a little more speed because of the number of variables involved.

 

My suggestion would be to stick with what you currently have, which seems to be working fine.

11 hours ago, Scherbacj said:

I worry that trying a 3 blade would make it harder to get on plane. 

 

I tried this very option and after less than desirable results, I went right back to the 4-blade.

[WRB]

2015+ Merc 75 4 stroke rpm 4,500-5500.

Your boat is rated to 115 hp.

1/4” smaller diameter prop isn’t going to help with your hull. Adding 2” pitch you need more hp to increase rpm’s.

Your flat bottom hull isn’t designed for speed, too much wet surface.

Weight distribution is the easiest method to increase speed. Dry livewell, put anything that adds weight behind the drive seat. Change the trolling motor batteries to LifePO4 to save weight.

Tom