Prop size

[Carsten1979]

Hi.
Just bought a new Suzuki 150, with 3x16x21,5 prop stainless, max rpm is 5300, gets to 40 knots.
I want to buy a 4 blade alu prop, and get up to minimum 5600rpm. which size would you recommend? I dont have the opportunity to try different 4blades here in Greenland
I ask here because I dont have the oportunity to try different 4 blade props here in Greenland.

 

[Stinnett21]

Do you mean 21.5? You have a comma in there.

 

[Carsten1979]

Do you mean 21.5? You have a comma in there.

21.5

 

[jimmbo]

You probably need to drop to an 18, maybe even a 17. Why do you want a 4 blade? What is the WOT Range for your Engine?

 

[jlh3rd]

general rule of thumb....
Adding a blade, or increasing diameter, or increasing pitch, with everything else being equal, reduces rpm....throw in cupping, different manufacturers adds to the internet propping dilemma.
Going from a stainless to an aluminum also effects rpm...I think it increases rpm's because of flex.....
just stuff to consider..do some research,
Like stated, to get to 5600 rpm requires dropping the pitch. But throwing in a change to a 4 blade also complicates the answer. You might get lucky with your 1st choice.....
you didn't say if the boat will be loaded down, or just you in it....that affects max rpm also.....

 

[airshot]

Just tossing this out there....generally you change rpm by 200 for every inch of pitch, when you add a forth blade, you drop 2" of pitch ( compared to a 3 blade) to make up for the fourth blade. These are general adjustments, many things can create a difference like aluminum vs SS, cupping, rake etc. Make your own conclusions, but these ratio's have served me well in my 60 plus years of boating, no guarentee they will work for your situation.. Good luck......

 

[jimmbo]

"Flex"
That bit of Misinformation has sold Hundreds of Thousands, if not Millions of SS Props. Grab an Alunimium Prop Blade and try flexing it, The put you hand in Water and move it around. Which one Yields to your hand? Which one is stiffer than your hand? If there is any "flexing" of Prop Blade, it is in the regions of a couple of thousands of an Inch, which would equate to far less than 1 rpm

 

[jlh3rd]

"flex"....ok...
I don't know as experienced, and never claimed it as a fact. I said "I think"....and then I said "do research"
furthermore, any specific info or statements on "flexing " came specifically from this site and people that are on it....not from any knowledge of mine.
So here:....
and just why does this site's propping chart ask whether a person has an aluminum or stainless steel?.....

Your "scientific "experiment" is seriously flawed. You're not considering fluid dynamics. Put 150 hp behind your hand movement and you'll see just how "hard" that water is. Drag quadruples as speed doubles so that prop has to work harder to give smaller increases in speed. Translation, it has push harder....
Further analogy. Do a belly buster into a pool at 2 ft height. No problem...
Belly bust off of a 30' cliff and that water is like concrete....you probably will not live....
I don't know the answer to alum. flex vs steel......but until someone publishes the facts ....neither do you.....
The OP needs to do his own research.....like I said...

 

[Scott06]

Hi.
Just bought a new Suzuki 150, with 3x16x21,5 prop stainless, max rpm is 5300, gets to 40 knots.
I want to buy a 4 blade alu prop, and get up to minimum 5600rpm. which size would you recommend? I dont have the opportunity to try different 4blades here in Greenland
I ask here because I dont have the oportunity to try different 4 blade props here in Greenland.

Probably something like a Michigan Vortex 4 blade 18" pitch would be a good place to start.

What is the current SS prop that is on there? just trying to figure out how much grip it will have vs a less expensive AL prop.

Also what is the WOT rpm range for your engine. You mention at least 5600 , I find it best when you are propping into the upper end of the WOT range.