Gear ratios

[keefallan]

Gentlemen, I have a question of gearing(ratios) just for my education. And it really just is that........ for my knowledge. Since I am an OMC guy, I don't get the choices of gears like you Merc guys do. If we have our choices of props(pitch), why does it really matter about gearing? Cant you just adjust pitch to work around gearing?

Gearing is so much more important on the street on pavement..............I would think...........than on the water, where you can adjust you prop pitch..............or is my thinking skewed? Like I said, I just want to gain more knowledge on the matter. I'm always learning about many things. The moment I stop learning.............I'm dead.

[Captaincj]

I'm not an expert, but from what I've read before:

Yes, you can largely work around gear ratios by changing propeller pitch, up to a point. The problem is propeller efficiency drops off as propeller pitch gets too high (above 34" maybe?). So if your boat is light/efficient enough or you have enough horsepower to go fast enough, you run out of propeller pitch and are better off getting taller gears to stay within the efficient pitch range.

Possibly other factors as well.

Also, and now I'm really just talking out my butt here, but with motors like mine that don't have a lot of torque down low, but plenty of power at high RPM, I think I would have trouble spinning up a high performance prop if I had tall gears, so I'm better with standard gears to help my motor spool up off the bottom (idle).

[Motv18]

Running a sporty on top pitch over 28” creates a lot of steering torque. So a lot has to do with comfort.

Sometimes the pitch if to much just doesn’t run good.

Theirs a lot of math if you wanna follow it. But sometimes it comes down to that a 1:50 will only turn 25 at 6000 and a 1:60 will turn 28 at 6000. It’s rare that happens, but 100ft lb through 1:50 just wasn’t enough.

We talk gears a lot but most do like you stated and work the pitch around it

[keefallan]

Thank you men. Its much appreciated.

[davemvegas]

Thank you men. Its much appreciated.

A few tried to simplify the gear ratio. Many do not take torque multiplication effect that happens in real boating vs on paper boating.

10 spd bike in 1st with a 50 inch tall tire will be much easier to push the pedals from a dead stop, vs same bike in 10th gear with a 10 inch tall tire.

A car with a 4.88 ratio and 32 inch tire vs 3.73 ratio and a 26 inch tire is about the same final ratio. The 4.88 gear is going to accelerate noticeably faster even though the final ratio is about the same as the higher 3.73 with short 26 inch tire. Not to noticeable from a roll. But from a dead stop torque multiplication is a big factor.

And yes it is a big difference on a boat with a 300xs. 1.62 with a 24p will be a slug out of the hole compared to 1.75 with a 27 or 28p prop. So extreme on my heavy boat, before i converted it to twins. 1.75 and 26p prop would shoot out of the hole. About 3 or 4 different times i bolted on a 1.62 lower and tried the same 26p along with a few 24 and 23p props. My boat never got on a plane with the 1.62 lower. With the 1.75 it felt like it could pull a couple skiers without a problem. So in my mind you can not prop the wrong gear ratio to be efficient.

[keefallan]

Before I had posted my question, I had wondered if holeshot was an issue with gear ratios. So now, it brings me to two more questions: With omc lowers(particularly the 80's and 90's v4 and v6) what were the gear ratios of the 4cyl and 6cyl lowers?.........and How did OMC get away with it (not offering different ratios)?

[HStream1]

This question has come up many times over the years. Im not a OMC Guy even though Ive owned a few. They are solid motors for the lake and fishing sports. But I responded to a post a couple years back. it doesnt cover OMC but it should answer all your questions.

There are 4 lower merc gear ratios.

2:1, 1.87:1, 1.75:1, 1.62:1.

It works like this:
2:1 is two engine rotations to one Prop rotation

1.87:1 is 1 7/8 engine rotation to one prop rotation

1.75:1 is 1 3/4 engine rotation to one prop rotation

1.62:1 is 1 9/16 engine rotation to one prop rotation

In theory heres the difference, and Ill use 5000 RPM and a 26 pitch prop as a base line for the example.

The formula to calculate Theoretical speed is:
RPM X Pitch =
Ratio X 1056 =

2:1
5000 x 26 = 130000 = 61.5 MPH @ 0% slip
2 x 1056 = 2112

1.87:1
5000 x 26 = 130000 = 65.83 MPH @ 0% slip
1.87 x 1056 = 1974.72

1.75:1
5000 x 26 = 130000 = 70.34 MPH @ 0% slip
1.75 x 1056 = 1848

1.62:1
5000 x 26 = 130000 = 75.99 MPH @ 0% slip
1.62 x 1056 = 1710.72

So as you can see in theory by using 5000 RPM and 26 Pitch as a Base line the different ratios show around a 5 MPH difference. Now many variables come into play here, Boats weight, Can the motor carry the same pitch prop all the way through its maximum RPM range?

Now lets look at it a different way changing only prop pitch by 2 (ex 24 P vs 26 P) for this example ill use 1.87:1 as the base because as shown in the theory above the different ratios equate to around 5 MPH.

5000 x 26 = 130000 =65.83 MPH @ 0% slip
1.87 x 1056 = 1974.72

5000 x 24 = 120000 =60.76 MPH @ 0% slip
1.87 x 1056 = 1974.72

As you can see by merely changing the prop by 2 pitch sizes the MPH difference is almost the same @ 5 MPH difference.

Again many variables come into play and the above is based on Theory not real world results.

Also the tighter gearratios while providing a faster top end speed will also show a more sluggish hole shot.

[keefallan]

Bud, what a fantastic way of explaining to me on paper. I am grateful for everyones input. But they way that you used the different ratios and ran scenarios............I just had school.

Many thanks for that. I love learning new things.




I just wish I had gotten into building outboards earlier in life than 2010. Benn running them since the very early 80's with dad, but never had the drive to make 'em faster.