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Mercruiser Alpha 1 OEM prop selection

C

Cliffs

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New boat to me. It has a Mercruiser 5.7L Alpha 1 Gen 1 Engine serial number OC759123. The 3 blade prop on the outdrive now is number QA1918X21P. This prop looks small for the outdrive so I'm looking to verify if it is the correct OEM prop and purchase a spare prop. Any help would be appreciated.
 
The only way to determine correct prop size is a water test.

You alone, 1/2 tank of gas +/-
Some normal gear for normal weight +/-

With drive down throttle up and get boat on plane. Once on plane go to full throttle and trim drive up to reach maximum rpm (not allowing over revving caused by cavitation).

Once you have achived max RPM with drive trimmed up for best performance see what RPM the tachometer is reading.

99% of 5.7 Alpha 1 Gen 1 the max rpm range is 4400-~5200 depending on Efi or carbed.
So if you are around 4800-5000 rpm your prop pitch (21) is correct.

If RPM is over ~5200 rpm then a 23 pitch prop should be used.

If RPM is around 4400 or lower then a 19 pitch prop should be used
 
Thank Jack, that does sound like the most accurate way to determine the needed prop. I was just wanting to know what prop was on the boat when new. Is there a chart or something that might tell me?
 
The only one who would know is the boat manufacturer or the original boat seller who rigged the boat when new aa they would have lake tested before delivery.

99% of 16-24 ft pleasure boats are all designed around a 19 pitch prop, and typically with +/- one pitch.
So 17 pitch to 21 pitch is extremely normal.

My 21 ft 3700 lb 5.7 carbed boat originally had a 19 pitch when rubber hubbed props were used. When the went to the current, plastic hub props, I had to go to a 21 pitch.
I run about 4900 rpm at wide open throttle and trimmed all the way possible
 
kghost said:
The only way to determine correct prop size is a water test.

You alone, 1/2 tank of gas +/-
Some normal gear for normal weight +/-

With drive down throttle up and get boat on plane. Once on plane go to full throttle and trim drive up to reach maximum rpm (not allowing over revving caused by cavitation).

Once you have achived max RPM with drive trimmed up for best performance see what RPM the tachometer is reading.

99% of 5.7 Alpha 1 Gen 1 the max rpm range is 4400-~5200 depending on Efi or carbed.
So if you are around 4800-5000 rpm your prop pitch (21) is correct.

If RPM is over ~5200 rpm then a 23 pitch prop should be used.

If RPM is around 4400 or lower then a 19 pitch prop should be used
Click to expand...
That's the factory stock test... If you are in the habit of going out using the boat with a bunch of folks on board and lots of "gear", I would suggest that you test under those conditions. Everything else in the post applies.
 
What about the diameter of the prop? My prop looks small and putting a tape measure to it comes up with about 13" but it's hard to tell. Is there any good trick to measure the diameter of the prop?
 
The diameter is around 14 inches most standard alpha 1 drive, 3 Bladed 17--21 pitch are about 14 inches
I believe as pitch changes so does the diameter to some proportional degree.

Stop over thinking or measuring.

If the part number of the prop is the ONLY one then it is what it is. If it has a second set of number that look like they were stamped in (usually bigger than oem) then that would say its a rebuilt prop. Then the diameter and pitch could be off.
 
That number I gave is the only one on the prop. If I go by that number it looks to me to be a 21 pitch prop. Looking online I see 14", 14 1/4" and 15" prop diameters with that pitch which is why I was asking about diameter. Do they all have the cupping on the edge of the prop? I don't see any mention of cupping when shopping online.
 
From my experience most of not all OEM merc props we are discussing are cupped. Most rebuilt are not.

The cupping adds greatly to performance.

In 35 years I have never even noticed that there were different diameters for the same pitch prop, (OEM).

I need a new one and i get the same diameter every time without asking or a part number.
 
kghost said:
From my experience most of not all OEM merc props we are discussing are cupped. Most rebuilt are not.

The cupping adds greatly to performance.

In 35 years I have never even noticed that there were different diameters for the same pitch prop, (OEM).

I need a new one and i get the same diameter every time without asking or a part number.
Click to expand...
Whether it's cupped depends on who rebuilds them. Moe's in Oshkosh, WI does cupped props, AFAIK.

I think the similar props with smaller diameter may have been dinged or worn down and repaired without returning them to spec.
 
Please post the part numbers and if possible the links to the sites that offer the different diameters for the same pitch prop.

I see 15 inch for 17 pitch, Black diamond Mercury props. I see 14 3/4 dia for 19 and 21 pitch mercury props.
 
kghost said:
From my experience most of not all OEM merc props we are discussing are cupped. Most rebuilt are not.

The cupping adds greatly to performance.

In 35 years I have never even noticed that there were different diameters for the same pitch prop, (OEM).

I need a new one and i get the same diameter every time without asking or a part number.
Click to expand...
I have yet to see the same prop in same prop family with different diameters for same pitch.if you do it is probably advertised wrong.

Generally as pitch goes down diameter goes up and vice versa
 
Searching again I found:
Amazon prop: Meruzue14-1/4Dia X 21Pitch Boat Outboard Propeller for Mercruiser Alpha One Prop/Mercury Engine 135 150 175 200 220 225 250 300HP,3-Blade Aluminum 14.25dia...
Amazon prop: Mercury Marine Blackmax 3 Blade Aluminum Prop Propeller 12-3/4 x 21P 48-77348A45
I did not find the 14 3/4 diam for a 21 pitch prop that you mentioned.




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I would suggest you go the the Mercury/Qicksilver web site to research props vs Amazon.

Find what you are looking for then do a search with the Quicksilver part number for best price.

Not all specifications listed on Amazon should be taken as fact...... After all its Amazon....what do they know about boat parts.

I have purchased parts but I do my research first then see if Amazon sells for a good price. Lots of items say they are the same part but they may be aftermarket or Chinese knock off...

If it doesnt say Qicksilver then its not OEM.
 
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