Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to pitch

[griffin800]

Hello,

The question is...

for a 3.0L mercruiser, if the 2.4:1 ratio drive was installed vs the 1.98:1 or 2.0:1 ratio drive... would this require a smaller pitch prop to compensate for the difference?

I ask because... I'm worried the previous owner may have swapped the lower half of the drive with one from a 2.0:1 or a 2.4:1, because I couldn't get good results with even a 19 pitch prop. (The lower end of the drive is obviously newer than the upper half... although the sticker on the upper says 1.98:1)

I'm getting all my information about this from this old thread:

http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/alpha-gen2-2-0-lower-unit-differences-471939.html

granted... this would probably cause other adverse effects... and pardon my posting this in the prop forum but i'm not sure where this question belongs.


But what i don't understand is, Does the 2.4:1 ratio drive make the rpms on the engine run higher? or lower?

(again, pardon my ignorance just can't wrap my head around why my engine seemed to be lugging after switching a few props and... i don't want to invest in another stern drive on just a suspicion... or without fully understanding the relation between the drive ratio and prop pitch)

thanks

-gavin

 

[MaPaHa]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

2:1 means the engine turns 2 times to the prop shaft of 1. Therefore, 4,000 rpm @ 2:1 or (.5) = 2,000 prop shaft rpms. For the 2.4:1 I use the inverse math and convert it to decimal format so 1 / 2.4 = .415 or (41.5%) Therefore 4,000 rpm x .415 = 1,660 prop shaft rpm. With the same prop which is now turning slower your engine is unloaded and will rev up higher and the boat won't go as fast. You would need a higher pitch prop that moves more water at less prop shaft rpm or you could over rev the motor.

Look a few days back for "prop calculations" that I posted and there is a manual method along with links to prop calculators for speed, pitch and rpm.

 

[QC]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

Here's a decent one: Prop Slip Calculator | Propellers | Mercury Racing

 

[griffin800]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

thanks for your answers.

so if i understand your post, the 2.4:1 drive would rev the engine higher, not lower... so it wouldn't lug it?

also that calculator said i have a 19% prop slip.. guess i need to read if this is bad or acceptable

 

[QC]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

Yes, 1:1 propshaft speed = engine speed, 2:1 propshaft speed half of engine speed, 3:1 propshaft speed 1/3 of engine speed. So propshaft slows down, or engine speed goes up as the number get's larger.

19% usually indicates that improvements could be made. First place to look is trim.

 

[jestor68]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

I thought the gear ratio changes took place in the upper housing; not the lower gear case. So unless the upper has been changed, you still have the same gear ratio.

Am I wrong?

Food for thought: Shaft speed determines diameter.

 

[airshot]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

Gear ratio does take place in the upper housing so any change to the lower half will not affect the gear ratio. Many other things to consider when an engine is lugging. For example...the prop you are using, is it new? Pitch can be changed when a prop is repaired so unless it is new or has been checked by a prop specialist you never know. Is there any binding in the outdrive caused by improper alignment, or excess weight in the boat or lack of a properly tuned motor. There are some stickies at the top of the mercruiser forum
that may well help give some additional suggestions for your problem.

 

[griffin800]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

from what i've read on here... the final gear ratio of the drive is determined by the number of teeth on the 4 main gears in the drive... both upper and lower halves... not just the upper.

there are two gears in the upper and two in the lower.

they're referred to as the drive gears and the driven gears... and have different numbers of teeth to get different ratios.

the lower half of the units came with TWO different sets of drive and driven gear teeth...

so if you mix and match lowers in some cases, you will change the ratio of the drive as a whole...



if you don't believe me... read it here from a post two years ago...



http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/alpha-gen2-2-0-lower-unit-differences-471939.html


I'm no expert... but if you read Don S.'s post, number 14 down a ways, which shows you a photo of how to calculate the final drive ratio by using the number of teeth on each gear to fill in the formula... you can see how i would be worried about this... seeing as how my lower is different than my upper and the previous owner "didn't know anything about it"...

anyways, turns out this may not be my problem but i figured i'd ask on here.

thanks for all your posts.

-Gavin

 

[QC]

Re: Hypothetical question about drive ratios for 3.0 Mercruiser and the relation to p

OK guys, I am with the OP on the upper vs. lower discussion in regards to gear ratio. Here is another link from the Adults only section. Let's keep Don's legacy alive by referring to these sections as much as possible. Also, another part of Don's legacy is limiting speculation and referring to facts. The Adult's Only section(s) is full of facts!!

http://forums.iboats.com/mercruiser...n-mc-1-thru-alpha-i-gen-ii-drives-586050.html