1999 Bayliner trophy 2002 35MPH @ WOT, change prop pitch or no?

[Newbie@boats]

I am wondering what you guys think, the boat is a 1999, so is the engine 1999 Mercury 150HP aluminum 3 blade prop 17 pitch.....I would really like to get at least 40MPH out of the boat....Do you think changing prop pitch would make a 5mph different or so? I feel like at the "sweet" spot trimmed and all 35MPH is all I am getting out of it....thanks for any info

 

[lmuss53]

You want to prop the boat so it runs at the recommended RPM with the throttle wide open, the speed will fall where it does. Propping the boat to squeeze 2 or 3 extra MPH out of the engine puts tremendous undue strain on the engine, and can lead to premature engine wear issues or even engine failure. Think of it as if you were going up a hill in your stick shift car and didn't downshift to keep the engine in the proper range.

Can't stress it enough, put a prop on that lets it run in the top of the recommended RPM range with an average load and let the MPH fall where it will. The recommended range for your motor is somewhere in the 5000 to 5500 range, possibly as high as 6000, depending on the 150 model you have.

At your 35 MPH what is the engine RPM? If it is in the recommended rang, let it be.

 

[Texasmark]

Let's go to www.go-fast.com. Then pull up the BAM prop slip calculator: Click on Index on left side of home page then "How to find your propeller slip". On that page, upper left, "Prop Slip Calculator".

Plug in your gear ratio. For that engine it will be around 1.78 to 1.86 if my manuals are correct.
Plug in your prop pitch: 17
Plug in your WOT best in class rpm. Let's say its 5000
Solve for max possible speed: This is a theoretical value as if the prop were a screw turning in wood, how far would it travel into the wood in one revolution.
Now add your present best WOT speed 35
Punch the solve for prop slip.

Gonna bet it's up around 25.

Was curious so I did it for you using the 186 gear rato....the 1.78 would be faster and more slip.

With the numbers above I got 43 MPH for your theoretical (screw in a block of wood) speed at 5000 rpm. I applied your present speed of 35 at 5000 rpm and got 19% slip. The 1.78 would be 45 and 22.

If you are up at 5600 where you belong, (top of recommended range where I like to run), the 1.86 gearbox would be 48 max 27% slip and the 1.78 51 max and 31% slip.

If those numbers are correct your Bayliner is a TUB. No brag, just fact sir. Something is dreadfully wrong. Check your numbers you supplied to us.