SS Prop question

[bear_69cuda]

Hello,

I have a 1999 Bayliner 1950 3.0 L MerCruiser that gets to ~42 mph @ 4800 RPM with 21 pitch aluminum stock prop.

I'm prop shopping, so I can use my current aluminum stock prop for a spare.. The two times I've driven the boat, the current 21 pitch prop seems to have a good hole-shot, and nice top end. I now want to up-grade to SS, and have been reading about Mercury Stiletto and Apollo props. I understand these are progressive props. So that on WOT the prop has a bigger pitch correct? And that if I choose a SS 21 pitch for my boat, it would be 22-23 at WOT, and therefore, my boat may be over propped? Then my guess is to check out a 19 pitch version of the Stiletto or Apollo. That should be comparable to my stock prop correct?


Thanks in advance!

 

[Silvertip]

Re: SS Prop question

You will find that when transitioning from AL to SS the pitch range is in even numbers rather than odd like AL. Since you are altering two parameters, material and design, it is very difficult to predict the outcome. I would not drop back to a 19P. Since you seem to be happy with the 21 I would select a 20P SS. SS is generally more efficient than stainless.

 

[bear_69cuda]

Re: SS Prop question

Thank for the advice.

I'm just trying to make sure I understand things...

Pitch = The theoretical (due to water not being a solid) distance in inches a prop travels in one revolution correct? Then lower pitch props are usually for bigger slower boats that need more torque, and higher pitch are used to go fast? At WOT @ 4800 with my aluminum 21"P are the blades flexing making it act more like a 20"P Stainless? The stainless is more efficient because the leading edge is thinner correct? What about the diameter? Should I stay the same as my stock prop?

Sorry about the lame questions. I want to understand is all...

 

[Silvertip]

Re: SS Prop question

Actual pitch selection is not based totally on boat weight but rather a combination of things. The amount of power (engine size), type of hull, weight, etc., determine what is needed in a prop. In other words a light boat with a small engine may require a 19P prop and a heavy boat with a much bigger engine may also be able to use a 19P prop. The idea in prop selection is to make wide open throttle runs to determine maximum engine rpm. You want the engine to run at or near the top end of the manufacturers recommended wide open throttle rpm band. 4600 - 4800 rpm is about where your engine should be. If you dropped back to a 19P you would likely increase engine rpm by about 400 rpm which would make it rev to 5200 and therefore be outside the recommended band and obviously over reving. If you increase pitch (say 23P instead of 21) the opposite is true so you would pull 400 rpm off the 4800 and thus be lugging the engine. Since SS is generally more efficient than AL, a 20P SS would likely perform about the same or slightly better than the AL yet keep the rpm where it is now. As for prop diameter, you need to buy a prop designed for your engine. Diameter is calculated into the design of a prop and is not as much of a factor in performance as is pitch, rake, blade shape, etc. Your drive is designed to accept a fairly small range of prop diameters so the first step is to select a prop specifically for your drive, then pick the pitch you need. You should notice that as pitch numbers increase, diameter tends to decrease slightly provided you compare the same propeller manufacturers model. Decreasing pitch tends to increase diameter a little. And lastly, when comparing props, a 19P prop from Mercury may not perform exactly the same as a 19P prop from Apollo, Turning Point, or any other manufacturer. That's what makes prop selection such a crap shoot.

 

[bear_69cuda]

Re: SS Prop question

Thanks!

You have been very helpful with my understanding of props. This is my first boat so I tend to have a lot of questions. I think I'm gonna call a Bayliner dealer and see what they recommend for SS just for the hell of it. And then most likely roll the dice with either a 20"P Stiletto or Apollo.

Best regards!

 

[Frank Acampora]

Re: SS Prop question

PROGRESSIVE pitch means that while the prop may have an effective pitch of 21, it may start at the blade root (hub) as a 20 pitch (for simplified example) and the tips may be 22 pitch. It does not mean that the pitch changes between low and high speed. There can be performance advantages to progressive pitch--one of which is that the blade is shaped more like an airfoil than a flat helix segment.

 

[Tail_Gunner]

Re: SS Prop question

Thanks!

You have been very helpful with my understanding of props. This is my first boat so I tend to have a lot of questions. I think I'm gonna call a Bayliner dealer and see what they recommend for SS just for the hell of it. And then most likely roll the dice with either a 20"P Stiletto or Apollo.

Best regards!

4 bladed props more like 18...3 blades....19.....