Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

[h20mobius]

Boat: 1995 Maxum, 20ft, Open Bow
Engine: Mercruiser 5.0LX
Outdrive: Alpha 1
Current Prop: started with a 21p, went to a 19p and now on a 17p, which has worked the best so far; all are 3-blade
Use: 50% wakeboarding, 50% cruising
Goal: to plane fast, and hold speed better. Not as concerned with WOT.
# of people: 2-6 on boat
Material: Need to stick with aluminum
WOT: not sure, but I think its around 4200 RPMs with the 17p.

I currently like my 17p, but wouldn't be opposed to more ability to maintain a constant speed. When I cut hard wakeboarding I can slow the boat a few hundred rpms, so more torque would be preferred. W
Questions:
What's the 4blade equivalent to the 17p 3-blade I'm running?
If I wanted a little more hole shot, in a 4blade, would I go down to a 16p 4 blade?
Solas Amita vs. Mercury Match?
I've never owned a prop with a hub system - can someone explain the pro's/cons?

thanks!

Chris

 

[Bondo]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

WOT: not sure, but I think its around 4200 RPMs with the 17p.

Ayuh,..... Without you being Sure of that little tidbit,...
There Ain't no answers...

 

[h20mobius]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

I say not sure because I doubt if the RPM gauge is accurate - it tends to bob a bit at higher RPM's, even though the engine does not. Furthermore, my range varies by a couple hundred depending on if I'm going upstream or down.

Assuming it is accurate, what prop would you recommend?

 

[Steve91T]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

I say not sure because I doubt if the RPM gauge is accurate - it tends to bob a bit at higher RPM's, even though the engine does not. Furthermore, my range varies by a couple hundred depending on if I'm going upstream or down.

Assuming it is accurate, what prop would you recommend?

You mean UPWIND or DOWNWIND. Water current makes no difference to the boat. It only changes your ground speed, per a GPS.

Wind will make a difference though.

Steve

 

[h20mobius]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

You mean UPWIND or DOWNWIND. Water current makes no difference to the boat. It only changes your ground speed, per a GPS.

Wind will make a difference though.

Steve

Correct - perhaps better stated, to maintain the same speed upstream, I am at higher RPM, assuming wind is constant.

Any reco's on prop?

 

[1980Coronado]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

As Bond-o says....to have any chance of hitting the target you have to know what you're starting with. You don't give a diameter for any of the props you've tried....you've reduced the pitch from 21 to 17, but what did you do with diameter...were they all the same diameter?

Regarding the 4-blade vs 3 blade question....typically if you buy the same pitch 4-blade the diameter is reduce a little to compensate for the extra blade area...this gives you the same rpm at wot in theory. The advantage of the 4 blade is better holeshot. Without data....it's a crap shoot.

Why the need to stay with aluminum? Some guys are using the High Five props to do exactly what you want...but you still need to know what your current performance is in order to select the right one.

 

[h20mobius]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

As Bond-o says....to have any chance of hitting the target you have to know what you're starting with. You don't give a diameter for any of the props you've tried....you've reduced the pitch from 21 to 17, but what did you do with diameter...were they all the same diameter?

Regarding the 4-blade vs 3 blade question....typically if you buy the same pitch 4-blade the diameter is reduce a little to compensate for the extra blade area...this gives you the same rpm at wot in theory. The advantage of the 4 blade is better holeshot. Without data....it's a crap shoot.

Why the need to stay with aluminum? Some guys are using the High Five props to do exactly what you want...but you still need to know what your current performance is in order to select the right one.

Prefer to stick with aluminum because the river we run in has quite a bit of debris... and people will think you're crazy to run SS in this particular area. Just dont want to lose an entire OD for the season - would rather shred a prop.

I don't have any experience with the spin away hubs on SS, so I've just stuck with what I know.

 

[1980Coronado]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

Prefer to stick with aluminum because the river we run in has quite a bit of debris... and people will think you're crazy to run SS in this particular area. Just dont want to lose an entire OD for the season - would rather shred a prop.

I don't have any experience with the spin away hubs on SS, so I've just stuck with what I know.

Both SS and Aluminum props are mounted in rubber...both can be "spun".....with an aluminum prop you usually damage the blades before you "spin" the prop...since SS is stronger the prop bushing will sometimes be "spun" before the blades get damaged. If you're worried about hitting something substantial like a rock......that's one thing....but if it's floating junk like branches etc....I'd run the SS. That stuff will damage an aluminum prop where the SS will come through unscathed.

 

[craze1cars]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

When I dropped from 3 blade to 4 blade of same pitch, I lost over 300 rpms. So if you buy a 17P 4 blade I believe you'll struggle to hit 4,000.

Frankly from your description? If you're at 4,200 on a 17 pitch 3 blade and you want to go to a 4 blade?

I think you'll probably need a 15...or maybe even smaller.

You're already at the very bottom end of your RPM range. For your stated watersport usage, you should be shooting for 4,600-ish...or maybe a hair more depending on your exact engine model. If you can reach 4600 RPM with a 4 blade prop, you will SUBSTANTILLY improve your holeshot and mid-speed holding capabilities over what you are guessing you have now.

And I have no idea what your outdrive ratio is...but if you can only turn 4,200 on a 17 pitch prop with your 19 footer 5.0L boat? And you started with a 21, then a 19, then 17...and you can STILL only produce 4,200? That screams RED FLAG to me...sounds like you're quite a bit underpowered from what she used to be...like you have a slowly dying engine. She's clearly lost a few steps from what your machine should be capable of producing...is your boat due for a comprehensive tune-up maybe? Compression check? Heavily waterlogged?

As a budget-minded and rapidly aging wakeboarder myself, I've been in and around and pulled behind a WHOLE lotta 5.0L 19 ft runabouts of your 15 to 20 year old vintage...and almost ALL of them capably push 19 or 21 pitch props at 4,600-ish...WITH balast bags full. Which begs the question...why can't yours?

Of course you admit your 4200 is nothing but a guess. So my guestimate that you need a 15 pitch 4 blade or very likely some engine work...is also nothing but a guess...based on your guess...

My #1 priority if I were you? Get a working tach and an honest assessment of the health and state of tune of your powerplant. THEN do a top speed run and document top speed and RPM with your current prop. Any advice given thusfar (including that given by me) based on guestimated RPM/no top speed data on a possibly untuned/sickly motor is wasted advice. Props ain't cheap, and they ain't usually returnable if you buy the wrong size based on a bad tach, or discover your ignition timing was simply 5 degrees retarded, or that you've been running on 7 cylinders...

 

[captkevin]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

Hub insert is the only kind of prop to buy. Don't have to worry about spinning a hub ever again. One of the best improvements in boating.

 

[go_faster]

Re: Prop Recommendation for 20ft Maxum w/Mercruiser 5.0LX & Alpha 1

As a budget-minded and rapidly aging wakeboarder myself, I've been in and around and pulled behind a WHOLE lotta 5.0L 19 ft runabouts of your 15 to 20 year old vintage...and almost ALL of them capably push 19 or 21 pitch props at 4,600-ish...WITH balast bags full. Which begs the question...why can't yours?

Of course you admit your 4200 is nothing but a guess. So my guestimate that you need a 15 pitch 4 blade or very likely some engine work...is also nothing but a guess...based on your guess...

My #1 priority if I were you? Get a working tach and an honest assessment of the health and state of tune of your powerplant. THEN do a top speed run and document top speed and RPM with your current prop. Any advice given thusfar (including that given by me) based on guestimated RPM/no top speed data on a possibly untuned/sickly motor is wasted advice. Props ain't cheap, and they ain't usually returnable if you buy the wrong size based on a bad tach, or discover your ignition timing was simply 5 degrees retarded, or that you've been running on 7 cylinders...

I have to agree, as my 21' 5.0 Alpah 1.62:1 spinning a 14X21 4 blade Solas will do >4800 if held WOT. Of course when I got the boat the tach was set for a V6 instead of V8 so my rpm was always high for the speed the boat was running with the prop.

I have since did a tune up, which by the way, I set the base timing back to where it belonged, it obviously came from the factory off, because I cracked the paint on the adjustment nut. I also set the carb and I now have the tach reading the right rpm. The boat comes out the hole well, holds well in turns, and will go ~ 50 mph. I need to GPS verify. $35 for the service manual and a little work and my boat is working as it was intended.