Merc 5.7 prop

[Onexl85]

I'm running a 15x17p prop on a 23' ciera with a 5.7 merc alpha1 gen 2 1997 rebuilt last season
Bury the throttle and rpms only go to 4000. Top speed is about 37.

I have a 14.25x 21p availiable to me, will this get the rpms up a little? From what I understand 4000 is too low for that engine

 

[Bt Doctur]

No it will drop the rpm 200 rpm / inch of pitch
Either your dragging the trailer with you that motor should reline with a 17 inch pitch prop

 

[Rick Stephens]

Something's wrong - either an incorrect gearing or the motor is not producing. 17 inch prop should be a problem keeping RPMs down. Half throttle should hit maximum RPM with a 17.

 

[shrew]

Max RPM range for the Mercruiser 5.7 is 4400 - 4800 RPMs. Ideally, you want max rpm to be as close to the upper limit as possible. Prop size and pitch can definitely affect it. So can weight of people and gear.

I would look at the Mercruiser Prop Calculator to make sure you are running the proper size and pitch for your setup to begin with.

https://www.mercurymarine.com/en/us/propellers/selector/#/step-one

 

[alldodge]

I'm running a 15x17p prop on a 23' ciera with a 5.7 merc alpha1 gen 2 1997 rebuilt last season
Bury the throttle and rpms only go to 4000. Top speed is about 37.

I have a 14.25x 21p availiable to me, will this get the rpms up a little? From what I understand 4000 is too low for that engine

Your prop slip is about 14 which isn't bad, that is if you have a 1:50 ratio drive, and if your speed is GPS and tachometer readings are accurate. If all I previously mentioned is correct, I would suggest weighting the boat to see if it has picked up some weight. Buddy has 2550 and a 5.7 OMC and he tops out about 34 mph.

You could try a 15 pitch

 

[grewvin1]

Give some thought to a four blade, cut down on the slippage, good performance lower rpm to stay on plane but still look to be as close to 4800rpm at wot.

 

[achris]

My calculator agrees with Alldodge, the stars are aligned correctly for a 1.5 drive. (see below). I would also suggest you have the weight of the boat checked. If it's still close to factory, then consider dropping to a 15" pitch, and/or checking the engine (compression, timing, fuel delivery, carb opening fully up etc)....

Chris..........

 

[Onexl85]

How do I know what the ratio is? Btw engine rebuilt last September and the river was a bit rough when I took her out for the test run

 

[alldodge]

Other then counting teeth on the gears, easiest method is remove the drive, put it in gear and rotate the input shaft and count how many times the prop turns

That is if it's not marked, but most probable case is it is 1.5

 

[Onexl85]

Is it possible with the serial?
0k042823

 

[Fun Times]

Is it possible with the serial?
0k042823

As long as the gearing hasn't been changed due to higher elevation then yes it is... STANDARD ROTATION ALPHA 1.47:1

 

[alldodge]

That shows you have even less slip so your golden
I'm seeing 15.5 percent

I would try a 15 pitch

 

[Onexl85]

So a 15x15?

 

[alldodge]

So a 15x15?

Yes or 15x a diameter no less the 14.25

 

[Onexl85]

Ok, if I went smaller diameter (14.25) will that slow me down

 

[alldodge]

Ok, if I went smaller diameter (14.25) will that slow me down

Not necessarily, the diameter does have an impact, the point I was trying to make is if you were unable to find a 15 diameter, but could find a 14.75, 14.5 or 14.25 you could still use it.

The rule of thumb is also for every inch of pitch decrease your rpm will go up by 200 revs. So going to a 15 will put your WOT around 4400, which still gives you some more room. Going down in diameter will increase the rpms some more. These are rules of thumb and not exact. Just trying to give you some options

 

[Onexl85]

Ok, so let me give you the full scoop, you guys are very helpful btw.
Sent my prop to propmd and they shipped it to someone else. To make up for there error the sent me a brand new 15x17p quicksilver.
Originally I had a 14.5x17p. I believe that prop took me to 4300 rpm, honestly I didn't know enough then to check wot. These props were both aluminum
I'm thinking of going stainless with a 14.25x15
Thoughts?

 

[Onexl85]

Today I got to run the boat on glass like water, engine reached 4800 with the 15x17p aluminum prop. When I ran the boat a few days ago the river was very rough. I have a stainless steel 14.5 x17 availiable to me. Is this a good option?

 

[alldodge]

Today I got to run the boat on glass like water, engine reached 4800 with the 15x17p aluminum prop. When I ran the boat a few days ago the river was very rough. I have a stainless steel 14.5 x17 availiable to me. Is this a good option?

You could try going from a 15x17 SS prop, but not a 14.5x17 prop. Going to a smaller diameter would only increase the WOT rpm and your at the max at 4800

 

[Onexl85]

I've heard that ss react differently that alum.
So given that two props, on stainless one alum. Both same size......which will give higher rpm