Want to verify that I have the right prop (Mercruiser MC-1)

[jimmwaller]

Hi all,
I have a mercruiser MC-1 (pre-alpha) that had a bad outdrive and got replaced with a similar but slightly different one. The consensus here is that I may need to re-prop, but I'm not sure yet and it seems worthwhile to test.
Just to verify: the goal is to go out into the water, and run things at WOT and see where the RPMs land, is that correct? And I just need to make sure that when I'm at WOT, I'm within some specified RPM range? And if I am, I'm good with my current prop, and if not, I may need to reprop?

Is there anything else I need to know about this process? Do I just need to hit WOT for even a few seconds, or do I need to run it for a while? Is there any chance I could damage anything if I do this test with the wrong prop? I'm still adjusting my shift cable, is there any chance that I might have my shift cable adjusted weird so that my throttle is all the way forward but I'm not at WOT or anything like that (just trying to think up bad scenarios, ha)?

Thanks!

 

[Chris1956]

Make sure the shift cable is adjusted properly, before you do anything else. This is not straightforward, so I would recommend a service manual.

Any idea if the gear ratio between the old and new ODs is the same? That would be the issue, I would think. You can look up the gear ratio on the old OD, by year and HP. Hopefully the new one came with that info.

A 2" increase/decrease in pitch will reduce/add about 400RPM. The RPM test should be done with a light to normal load in the boat, say 2 passengers and 1/2 tank of fuel.

Run the motor up to max throttle until revs either pass the max recommend RPMs, or fail to reach it. if the revs get more than 400RPM over max, you will need at lease 2" more pitch, and must be careful during this test.so you do not blow the motor. Ideally you want the RPMs in the center of the max RPM range.

If the revs fail to reach max, you will need to reduce the prop pitch by 2" for every 400RPM that the motor needs to run in the middle of the max range.

If you see any more than an 800 RPM difference from max RPM, you might post back, as that is an unusual issue. Obviously the tach needs to be accurate.

 

[ahicks]

Just a thought, but if you are at your max rpm at a lower throttle setting (say 3/4 throttle), that should be the end of the test. The prop is too small.

If max performance isn't necessary, as in the boat is never being used to pull anyone tubing or skiing, you don't necessarily need the prop to allow max rpm.

Tons of variables depending on you and how the boat is being used....

 

[jimmwaller]

oh, good point, I should definitely be keeping an eye on ending the test early. Thanks!

 

[Bondo]

Hi all,
I have a mercruiser MC-1 (pre-alpha) that had a bad outdrive and got replaced with a similar but slightly different one. The consensus here is that I may need to re-prop, but I'm not sure yet and it seems worthwhile to test.
Just to verify: the goal is to go out into the water, and run things at WOT and see where the RPMs land, is that correct? And I just need to make sure that when I'm at WOT, I'm within some specified RPM range? And if I am, I'm good with my current prop, and if not, I may need to reprop?

Is there anything else I need to know about this process? Do I just need to hit WOT for even a few seconds, or do I need to run it for a while? Is there any chance I could damage anything if I do this test with the wrong prop? I'm still adjusting my shift cable, is there any chance that I might have my shift cable adjusted weird so that my throttle is all the way forward but I'm not at WOT or anything like that (just trying to think up bad scenarios, ha)?

Thanks!

Ayuh,...... What motor ya got,..??

What's different about this new drive,..??

 

[jimmwaller]

@Bondo Motor is a 165hp inline 6. Old drive was a MC-1 with the marking worn off but that worked, so I think it was properly matched. New outdrive is for a 120-140hp so people here let me know that the gear ratio is different. They all suggested that I'd need to increase my pitch and reprop, which I think is true, but I wanted to verify the process of how to actually figure out how to reprop and what new pitch or diameter I'd need.

thanks!

 

[Scott06]

@Bondo Motor is a 165hp inline 6. Old drive was a MC-1 with the marking worn off but that worked, so I think it was properly matched. New outdrive is for a 120-140hp so people here let me know that the gear ratio is different. They all suggested that I'd need to increase my pitch and reprop, which I think is true, but I wanted to verify the process of how to actually figure out how to reprop and what new pitch or diameter I'd need.

thanks!

Yes gear ratio is like 1.98:1 for the 120-140 hp drive. Likely your 165 hp came with a 1.84 or so

If that is the case and the boat was properly propped before you will likely need to go up in pitch 2”. For example I swapped out a 4.3 for a 5.0 which would have come with different ratio drives , kept the 4.3 drive and went from a 19” to 21” prop.

Depending on the boat and how you use it I have typically been happier propping so I am at the top of the rpm range. Holeshot Performance and throttle response will be snappier …

 

[jimmwaller]

@Scott06 thanks. I don't do anything fancy, my entire goal is just get from point A to point B for fishing. I'm not sure I've ever even gone full throttle as long as I've owned this boat, so I mostly just want to prop for engine reliability and longevity.
I'll probably reprop 2" and see what happens. Just to confirm, I got some warnings that I may be near the top of the pitch range already, and then I'd have a hard time finding a reasonable prop. But in that case, I can increase diameter, is that correct? Is there some formula for trying to calculate how much additional diameter will correspond to RPMs for a given pitch?
Thanks!

 

[Bondo]

@Scott06 thanks. I don't do anything fancy, my entire goal is just get from point A to point B for fishing. I'm not sure I've ever even gone full throttle as long as I've owned this boat, so I mostly just want to prop for engine reliability and longevity.
I'll probably reprop 2" and see what happens. Just to confirm, I got some warnings that I may be near the top of the pitch range already, and then I'd have a hard time finding a reasonable prop. But in that case, I can increase diameter, is that correct? Is there some formula for trying to calculate how much additional diameter will correspond to RPMs for a given pitch?
Thanks!

Ayuh,..... What pitch were you runnin' on the old drive,..??

Props are sold by pitch,..... The diameter is determined by blade design,.....
Yer motor will be it's happyist when it is Able to pull 4200 to 4600 rpms at Wot,......
Regardless the rpms you choose to run, if the motor is unable to pull it's proper rpms, at Wot, it'll be luggin' at all rpms, which leads to detonation, which burns holes in pistons,.....

The standard range of aluminum prop pitches, is from 15" to 23", or possibly 25",....
Higher pitches can be had, but are usually custom ssteel go-fast boat props,....

I'm guessin' you ran a 17" or 19" with the old drive, so I believe you'll be Ok,.....
An on-water test is the only way to know though,....

 

[Scott06]

Ayuh,..... What pitch were you runnin' on the old drive,..??

Props are sold by pitch,..... The diameter is determined by blade design,.....
Yer motor will be it's happyist when it is Able to pull 4200 to 4600 rpms at Wot,......
Regardless the rpms you choose to run, if the motor is unable to pull it's proper rpms, at Wot, it'll be luggin' at all rpms, which leads to detonation, which burns holes in pistons,.....

The standard range of aluminum prop pitches, is from 15" to 23", or possibly 25",....
Higher pitches can be had, but are usually custom ssteel go-fast boat props,....

I'm guessin' you ran a 17" or 19" with the old drive, so I believe you'll be Ok,.....
An on-water test is the only way to know though,....

Well said and spot on

 

[QBhoy]

Think you need to just try it as she is now. Tell us how she goes. See what ratio is stamped on the new drive

 

[jimmwaller]

Ok, will test. But just some additional info: Judging by what is stamped on my two props, I think my old one was a 21p and my current one (on the 120/140hp drive) is a 17p. So... my guess is that I'll have to reprop. Is it worth just switching them out before I test, because I'll be more likely to be close to the right range with a 21p?
Thanks

 

[Bondo]

Ayuh,..... That's correct,....

 

[Scott06]

Ok, will test. But just some additional info: Judging by what is stamped on my two props, I think my old one was a 21p and my current one (on the 120/140hp drive) is a 17p. So... my guess is that I'll have to reprop. Is it worth just switching them out before I test, because I'll be more likely to be close to the right range with a 21p?
Thanks

Yes try the 21 and see where the rpm land.

 

[jimmwaller]

Just a thought—if there's a formula about what pitch does to RPMs, is there also a similar formula about diameter? My current prop is a 17p but is 15" diameter, how would that stack up against a 21p but a 14" diameter? The consensus is that when going to this new lower unit, I will probably need a greater pitch or greater diameter, implying that I will need to reduce the RPMs on my motor. Which one of the 15"/17p or the 14"/21p will result in lower RPMs?
Thanks

 

[QBhoy]

Just a thought—if there's a formula about what pitch does to RPMs, is there also a similar formula about diameter? My current prop is a 17p but is 15" diameter, how would that stack up against a 21p but a 14" diameter? The consensus is that when going to this new lower unit, I will probably need a greater pitch or greater diameter, implying that I will need to reduce the RPMs on my motor. Which one of the 15"/17p or the 14"/21p will result in lower RPMs?
Thanks

The 21 for sure. And by a lot.

 

[Scott06]

Just a thought—if there's a formula about what pitch does to RPMs, is there also a similar formula about diameter? My current prop is a 17p but is 15" diameter, how would that stack up against a 21p but a 14" diameter? The consensus is that when going to this new lower unit, I will probably need a greater pitch or greater diameter, implying that I will need to reduce the RPMs on my motor. Which one of the 15"/17p or the 14"/21p will result in lower RPMs?
Thanks

Q has you steered straight try the 21 it should deliver about 400 less rpm than the 17

You can peruse mercurys prop calculator but you really need current wot speed and rpm for it to be accurate. But they don’t replace real world trials on the water. Best bet is if u can find a prop shop to try before u buy

Get it together try with the 21 willing to bet it will be close

 

[jimmwaller]

ok, will do. Thank you all

 

[jimmwaller]

One last question here: I am looking for a new prop, but I can't find one specifically for an MC-1. I see that there are Alpha One props that have the same number of splines, will an Alpha One prop work? Are there any other models with props that will be compatible? Thanks

 

[Bondo]

One last question here: I am looking for a new prop, but I can't find one specifically for an MC-1. I see that there are Alpha One props that have the same number of splines, will an Alpha One prop work? Are there any other models with props that will be compatible? Thanks

Ayuh,..... MC-1s, the Alphas, the Bravo 1s, 'n Merc large gear case outboards, All run the same props, as far as fitment goes,....