What size prop should I use

[nyykk1]

What size prop should I use for max performance?

I have a 3.0L Alpha 1 Gen 2 Mercruiser I/O, 135Hp in a 18ft Bayliner

 

[95yj]

Re: What size prop should I use

You should ask this in the prop forum. You also want to let us know what prop you have now, aluminum/stainless, brand, # blades, size and pitch along with what speed / RPMs you are getting at full throttle. Speed should be measured with GPS if possible and RPMs with a shop tach, not the tach on your dash. Without this information, it's just guessing

 

[Don S]

Re: What size prop should I use

Moving to the Prop forum.

 

[nyykk1]

Re: What size prop should I use

Currently I have a 15" with a 15" pitch aluminum 3 blade prop. My top speed is about 32 MPH, not sure about RPM's.

 

[jpg1130]

Re: What size prop should I use

Currently I have a 15" with a 15" pitch aluminum 3 blade prop. My top speed is about 32 MPH, not sure about RPM's.

You need to know what your RPM's are before you make any adjustments.

You stated in your first post that you wanted the bes "performance". Well, what are you doing with this boat. Are you looking for top speed performance or are you looking for hole shot performance?

That being said, I have an 18' Maxum with the same motor and use a 4 blade 14x20p prop. I get a 4700RPM WOT @ about 41 MPH, with 3 adults and a cooler of drinks. Hole shot is pretty decent but I'm adding smart tabs as well.

IMO, the key here is knowing what your RPM's are. The recommended WOT RPM's for MY motor is 4400-4800 RPM's. I had a 14.25x21p prop that sucked out of the hole but had a top speed of 43MPH's @ 4600 RPM's.
I would imagine that your RPM's @ WOT with a 15x15p prop is overrevving the engine but you're probably out of the hole like a bat out of hell.

LONG POST SHORT-know your RPM's first. Make a decision afterwards.

 

[nyykk1]

Re: What size prop should I use

Thanks for the input, I will try to hookup a tach to get some readings. My in-dash tach is broken. The reason I was asking was I just dropped in a brand new Mercruiser Alpha I 135hp engine with my original 1992 outdrive. My old engine kept bogging down so my mechanic decided to go with my current 15" 15" pitch prop. After installing the new engine, I'm thinking I should replace the prop.

I mostly use my boat tubing, fishing and cruising both the ocean and fresh water.