Max Prop Diameter for Alpha One

Brentathon · Jun 21, 2012

What's the maximum prop diameter that you can run on an Alpha One?
I was thinking of buying a 16"D x 16"P prop.
I expect I'll have to use the flat trim tab anode.

Oshkosh1 · Jun 21, 2012

Re: Max Prop Diameter for Alpha One

I'm thinking this is more suited for the gurus in the prop forum?

Seems big. Don't see any advantage over using whats recommended.

steelespike · Jun 21, 2012

Re: Max Prop Diameter for Alpha One

A 16 should fit; but what is it that you want to accomplish?What size boat what year and hp?
Sounds like your dealing with something heavy?

chrisvr6 · Jun 21, 2012

Re: Max Prop Diameter for Alpha One

To achieve top speed on a 175 bowrider? That would be my scenario. Any ideas?

chrisvr6 · Jun 21, 2012

Re: Max Prop Diameter for Alpha One

Oh yeah, its got a 4.3merc in it. So 4800rpm topout is recommended.

steelespike · Jun 21, 2012

Re: Max Prop Diameter for Alpha One

Chris,You need to start your own thread.It gets very confusing.
There Is a Thread starter tab at the top of the prop page.
Also read "read this first".
with the right info we'll get you going.

Brentathon · Jun 22, 2012

Re: Max Prop Diameter for Alpha One

23' Mid-cabin... my WOT range should be 4200-4600, and with a 17P I'm at 4100, and with a 15P I easily go over 4700.......so, I thought I'd try the 16D/16P.

steelespike said:
A 16 should fit; but what is it that you want to accomplish?What size boat what year and hp?
Sounds like your dealing with something heavy?

steelespike · Jun 23, 2012

Re: Max Prop Diameter for Alpha One

We need your speed at wot rpm both props.
Better still get us the info needed in "read this first" at the top of the prop forum.
Typical rpm change is 100 to 200 rpm.per inch of change.
Are the props same brand,material and model?

Brentathon · Jun 25, 2012

Re: Max Prop Diameter for Alpha One

1) Performance issue you are trying to correct: Trying to get WOT rpm within 4200-4600 range.
2) Current prop manufacturer, model, aluminum or stainless as a minimum: Quicksilver SS QS5052X (same performance noticed as with Aluminum QA2012X)
3) Current prop diameter and pitch (required). 14.5"D x 17"P
4) Wide open throttle RPM and speed with an average load (very helpful): 4100rpm @ 35mph
5) Engine/drive make, model, year, and HP: 1991 Merc 5.7 Alpha One (Gen I)...hp is 260, but I believe it was rated at flywheel in that year.
6) Boat make model, year, length and weight: 1991 Regal Ambassador 233XL (23 foot midcabin), approx 5000lb with gear.

Basically, the 17P is at the very bottom of the range (actually 100 rpm too low), and when I went to 15P (SS p/n QS5050X), it easily over-reved to 4700rpm (I didn't hold it there too long). That's why I wanted to try a 16P. It looks like they don't make a 16P 3-blade in SS, so I was thinking of the Aluminum 16D x 16P.
Not sure why the 16P's diameter is so big, though...??

TilliamWe · Jun 25, 2012

Re: Max Prop Diameter for Alpha One

Brent, a 2 inch pitch change (from 17 to 15) should NOT have raised your rpms over 600 rpms. So, it's possible that one of the props you have tried is not exactly what it says it is. (damaged? repaired? mislabelled?) Or your tachometer is not reading correctly at all times.

I *believe* the MAX diameter for a and Alpha is 17 or 17.5 inches. And yes, you have to use the flat anode. A four blade prop would almost certainly be better suited for your application of a cruiser. And the more blade area, probably the better. So you are on the right path with a 4 blade in a big diameter. If you could borrow some props to try out, you'd be better off. And if you could confirm your tachometer reading somehow, that would be real helpful.

Brentathon · Jun 25, 2012

Re: Max Prop Diameter for Alpha One

For years, I ran a 3-blade AL 14.5D x 17P (QA2012X), and was always barely at the 4200rpm min of range. So, I tried the SS version, because I thought I would pick up 100rpm from thinner blades; the QS5052X (14.5D x 17P SS) performed identical. Then, I tried going down to 15"P, so I tried the QS5050X (14.5P x 15P), and my rpms were at 4700rpm. Recently, I tried Michigan's version of the SS 17P (993043) with 14.25"D, and it was 100 rpm's lower than the QS5052X.
So, I thought of trying the AL 3-blade 16"D x 16"P. I'm wondering what the large diameter will do....
I haven't ventured into any 4-blades yet.

steelespike · Jun 25, 2012

Re: Max Prop Diameter for Alpha One

If you can find a good prop shop you could have the 15 tweaked a little with a touch more pitch or cup.
I would imagine the 15 gives a strong hole shot and is easier to keep on plane at reduced throttle.
According to my info; your fretting about 100 rpm thats like gauge error or a with wind or against wind difference.
Your missing the gains of the prop change. If the boat gets up quicker your saving fuel and wear and tear on the motor
If your able to cruise easier at reduced throttle your probably saving fuel and wear and tear.
Do you routinely run at wot?

Brentathon · Jun 25, 2012

Re: Max Prop Diameter for Alpha One

I didn't like the idea of someone bending the blades to some spec? or dimension? or ????....the "tweeking" just doesn't seem scientific enough for me. I like the idea of a factory-new product. I spoke to a few people, and they couldn't answer some basic questions about the process, and didn't like to offer any guarantee.

I avoid WOT as much as possible, may occasionally need it to get on plane when heavily loaded. Pretty rare now, as I avoid heavy loads.
I love the way the 17P cruises at 3600/3700 rpm @ 29/30 mph. But, getting on plane can take longer than I like sometimes; my boat seems stern-heavy, and asking people to go down into the cabin can be annoying.
The 15"P was a "dog"; it just felt like I wasn't going anywhere...I hated it..the motor was revving, but my speed was slow....I'm too used to the 17"P. I just can't figure out why a 2" pitch decrease resulted in about 600 rpm increase.

TilliamWe · Jun 25, 2012

Re: Max Prop Diameter for Alpha One

As Steelspike was getting at, that 15p should have been a virtual rocket out of the hole compared to the 17p. Something is wrong with that 15p prop.

As far as "tweaking" a prop vs factory-new. How do you think the factory gets to their final designs? They bend them.

Brentathon · Jun 25, 2012

Re: Max Prop Diameter for Alpha One

All props tried, have been factory new....not a scratch on them.
All trials were done with just me on board....low load (although I'm up to 255 lbs

)
The 15P planed real fast, but WOT rpms were definitely above the 4600 rpm high limit. Plus, my cruising speed probably would have had rpm's around 4000, which I really didn't like. It felt like I was in a low gear, on the highway.

TilliamWe said:
As Steelspike was getting at, that 15p should have been a virtual rocket out of the hole compared to the 17p. Something is wrong with that 15p prop.

As far as "tweaking" a prop vs factory-new. How do you think the factory gets to their final designs? They bend them.

TilliamWe · Jun 26, 2012

Re: Max Prop Diameter for Alpha One

Is this your "typical Load"? If not, almost all this prop testing is a waste of time, if you are going to be carrying a heavy load. That 15 might not turn 4600 with a big load on the boat. And you may need it's "low gear" feeling in order to plane and cruise.

ufm82 · Jun 26, 2012

Re: Max Prop Diameter for Alpha One

May not of much help to you but regarding your original question I ran a Mirage prop on my Alpha and there was barely 1/4" of clearance between the blade edge and the anti-vent plate. My anode was already the flat type. But if you increased the diameter the prop would hit. The Mirage is 15 1/4" I believe...

Don't think a 16 will work.

TilliamWe · Jun 27, 2012

Re: Max Prop Diameter for Alpha One

Brentathon said:
What's the maximum prop diameter that you can run on an Alpha One?...

I'm fairly certain it's at least 16 inches, because I ran a prop selector on your boat at MercuryMarine.com and one of the props it suggestd was a 16 in diameter Mercury Black Max prop. Now it didn't allow for the year of the drive to go older than 2000. So it is possible that a Gen 1 drive can take a larger diameter than your Alpha One. But I don't think so.

Max Prop Diameter for Alpha One

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