Where do I start? Help Picking Prop Pitch.

[gemcitypos]

I have a '94 Maxum 20.5' Runabout with a Alpha 1 Gen II 1.84 Ratio. I purchased this boat after it was REPO'd. It had no engine but originally camewith a 4.3 LX. And the outdrive had a 21 pitch aluminum prop on it.

I have repowered the boat with a 5.7

I put my brothers extra 23P SS prop on it for testing today with what I think are strange results. Bear in mind this is my first boat.

With only my 17 y.o. son and me in the boat it took till about 3000 rpm for the boat to plane out, then I could cruise at about 4000 rpm. I think I was only going around 25 (Speedo Busted). I could never got to WOT because the stick was only around 75% when I hit the 4800 max rpms. One time I accidentally put the stick down and I think the rpms went to around 6500 rpms but can not be sure cause I backed it off real fast.

I know the outdrive ratio is not the recommended ratio, but I have been told (Merc Dealer)I should be able to handle this with a new prop.

Any Help is appreciated. Thanks in advance for all the great Info.

 

[a70eliminator]

Re: Where do I start? Help Picking Prop Pitch.

Isn't the 4.3 a six cyl? If so your tach is way off unless you changed the switch to 8 cyl. on the back of the guage.
I can't say much about ratio.

 

[Bondo]

Re: Where do I start? Help Picking Prop Pitch.

I know the outdrive ratio is not the recommended ratio, but I have been told (Merc Dealer)I should be able to handle this with a new prop.
and I think the rpms went to around 6500 rpms but can not be sure cause I backed it off real fast.

Ayuh,...

That might be True, But, Only If you can buy a Prop, Big Enough.....

Standard props are Only available from 15" to 23", maybe 25".....

If you Regear the drive to 1.5:1,....
You'll be back into the window allowing you to run a prop from the Standard pitches...

Otherwise,...
Break-out the checkbook,...
You'll be needing a Custom Prop, in the 27" to 30" range...
Which probably Still won't give you the performance you're lookin' for...

 

[mthieme]

Re: Where do I start? Help Picking Prop Pitch.

I agree with a70... you need to get the tach squared away before dropping any cash down on a prop.
You're in the right neighborhood with pitch now.

 

[Silvertip]

Re: Where do I start? Help Picking Prop Pitch.

And quessing at your speed is not much help either. Your tach needs to be set for 8 cylinders. The tach was looking for 3 pulses/revolution with the V6 and it is getting 4 pulses/rev with the V8 which puts WOT above 6000 rpm. Set the switch on the back of the tach to the proper setting.

 

[Tail_Gunner]

Re: Where do I start? Help Picking Prop Pitch.

Ayuh,...

That might be True, But, Only If you can buy a Prop, Big Enough.....

Standard props are Only available from 15" to 23", maybe 25".....

If you Regear the drive to 1.5:1,....
You'll be back into the window allowing you to run a prop from the Standard pitches...

Otherwise,...
Break-out the checkbook,...
You'll be needing a Custom Prop, in the 27" to 30" range...
Which probably Still won't give you the performance you're lookin' for...

As above.....a 5.7 with 1.84 gears is absolutely going to need lower gearing regardless of tach setting's alought that is a key to proper operating the engine...a good tach that is..

 

[tmh]

Re: Where do I start? Help Picking Prop Pitch.

What horse power is your 5.7L? 4bbl or 2 bbl?

let's see.....my boat is roughly the weight of that Maxum and I put a 5.7L 4bbl (260 hp ?) in it and now use a 20" SS 4-blade (we are usually loaded up and doing water sports. A 21" SS worked great also. That's with a 1.47 ratio I think.

So....using those ratios, about where does that put you? At maybe a 26" pitch or so, right? Not sure you'll find many of those for an Alpha one drive. That's just rough math, mind you. As was said, get things straightened out before wasting $$$ on a prop.

It may be that you're better off replacing the gearbox/upper with a 1.5 (1.47 is what I got from Sterndrive Engineering when my 1.5 original died).

 

[gemcitypos]

Re: Where do I start? Help Picking Prop Pitch.

Well thanks for the info on the tachometer switch. I found the switch and changed it. Took the boat to the water and ....WOT 4800 rpm's Now the question is.... approx how many rps/s up or down will it change if I keep the 23P but change to a 4 blade?

Again thanks for the info.

 

[Tail_Gunner]

Re: Where do I start? Help Picking Prop Pitch.

I you used a 22p 4 blade stilletto baypro 11 or turbo equivlant your rpm should remain the same

 

[tmh]

Re: Where do I start? Help Picking Prop Pitch.

You need to provide more feedback on how the current prop performs. how is it out of the hole? Is it sluugish or does it pin you back in your seat? Get or borrow a gps and tell us the actual speed. What is the max rpm recommended for your engine?

I have a Bay pro II 4 blade SS and I think it gets about the same rpm as the 3-blade SS, maybe a tad more. So, depending on what the proper max rpm is and what you plan to use the boat for, I'd think a 24P BPII might be excellent for you, or maybe a 22" for better hole shot if water sports are your thing.

The BPII will pick your boat up higher so a bit less drag (slip). It's not cheap, but well worth the price for me.