Outboard Engine GEAR RATIOS!

[smokin'joe]

I don't have any experience with different ratios on 3.0's but I can say for typical style props in general, that at some point on any setup you can reach a point in adding pitch that the prop will start acting more like a paddle wheel than a prop!

Yes. That reflects a limit on how low you can go in gear ratio. The lower the gear ratio the more the rpop walk and consequently the greater the gearcase drag. There are limits on low and high gear ratios that can't be precited mathematically.

[smokin'joe]

I hope it has Pixs...... some don't read well......I don't use aspirin.... hic.... My favorite OMC case 14/23 twin pinion....15 spline p/s....

It has a lot of pictures.

[smokin'joe]

SIMPLE: 1:1 is a Lower gear "RATIO" but in generic/layman terms is called "HIGH" gear being it's direct drive, 12:29 in reality is a Higher gear "RATIO" than 1:1, just as 1:50....etc., but people refer to it as "Low gear."

Many people do not consider the fact that when talking about "Gearing" and "Ratios," they are 2 different animals. Started from day 1, when all there was was Standard shift vehicles. People would pull'er down into Low gear to take off and High gear to cruise. In actuality their Low gear was the High Ratio, not High gear!!!

Your definition is not standard in the industry and in practice. You want to call greater reduction higher gear. That's not even what people meant by high gear in a car in 1920.

[DangerNewb]

Yes. That reflects a limit on how low you can go in gear ratio. The lower the gear ratio the more the rpop walk and consequently the greater the gearcase drag. There are limits on low and high gear ratios that can't be precited mathematically.

That's why I'm thinking a 28 may be more efficient than 30 or 32. But I'm sort of new to this stuff as you can deduce from my screen name. Seems like 34 or 35 is the max pitch prop I ever see, so thinking that's the top end of the range for being effective at pushing forward more than sideways. As stated, paddle wheel.

Anybody with direct experience care to chime in?

[smokin'joe]

So if you had a choice between 1.62 and 1.75 on a 3 liter Sporty on a 300 promax mounted to a 1000lb modvp hull, which would you prefer? It seems the difference is equal to about 2 inches in pitch.

30 on 1.62 is about the same top end as 32 on 1.75 at 6k rpm. within 1 mph from my calcs
28 on 1.62 is about the same top end as 30 on 1.75 at 6k.





I'm more interested in good mid range acceleration with decent top end.

Leaning towards the 1.62 so I can run a lower pitched prop. 28 or maybe 30, rather than 30 or 32.

But open to advice.

As is mentioned in one post you can't increase pitch without limit, you get into bad prop walk at some point. I would not have wanted to run a 30 or 32 when I drove a Mod VP boat. 26-28 was enough. At some point the drag increases due to prop walk and there are problems with no advantage.

[FMP]

Wait til my book is published, aspirin won't help: Hydrodynamics of Speed on the Water, Fast Boats and Surface Piercing Propellers. Filled with equations, including a complete mathematical prescription for racing prop design.

When?

[WillySteve]

Your definition is not standard in the industry and in practice. You want to call greater reduction higher gear. That's not even what people meant by high gear in a car in 1920.

Where are you coming from Joe, I'm NOT calling a greater reduction higher gear??
The problem I see is now we have 3 terms to define.
Ratio, Low and high gear and Greater reduction:
A 12:29 ratio is a Greater reduction than a 1:1. It's also a Higher gear Ratio!
In automobile's you Pull'er down to Low gear and shove'er up to High gear, but in actuality you were going from a High Ratio to a Lower Ratio.....SIMPLE!!!!

[smokin'joe]

Where are you coming from Joe, I'm NOT calling a greater reduction higher gear??
The problem I see is now we have 3 terms to define.
Ratio, Low and high gear and Greater reduction:
A 12:29 ratio is a Greater reduction than a 1:1. It's also a Higher gear Ratio!
In automobile's you Pull'er down to Low gear and shove'er up to High gear, but in actuality you were going from a High Ratio to a Lower Ratio.....SIMPLE!!!!

Sorry, then I misunderstood. Confused by your description.

[smokin'joe]

When?

Book ms is ready. Hans took the math, converted it to CAD and printed out a 3D model of an SST45 cleaver. Plan is to cut one on a CNC machine once we're sure the bugs are out of his digitalization. I'll probably have to self-publish the book. Falls into a crack between physics and marine engineering that no published so far is interested in (won't be a money-maker).

[DangerNewb]

As is mentioned in one post you can't increase pitch without limit, you get into bad prop walk at some point. I would not have wanted to run a 30 or 32 when I drove a Mod VP boat. 26-28 was enough. At some point the drag increases due to prop walk and there are problems with no advantage.

Thanks you. Sorry to hijack the thread but it seems you guys have some knowledge and experience in this area. Guess what I'm asking is, if I'm more concerned with acceleration, is it more beneficial to get the the lower overall gearing (like 4th instead of 5th in car) I want more from the gear itself or from the prop? I'm thinking a prop is pretty inefficient compared to the gear itself, so a easier to turn lower pitch prop and harder to turn gear is better than a high pitch prop and easier gear. Assuming the prop range I'm talking about is 28 to 32.

Or is basically the same either way or so close to not matter? Not just hypothetical for me, I have a choice in assembling a lower and need to decide. Thanks for any input.

[Dave S]

Tall pitch break prop.... Low pitch push boat forward......

[Dave S]

Dave S. not spelling bee...... damm were is my Ring....Fell out of cracker Jack box......Yukon Jack...

[smokin'joe]

Thanks you. Sorry to hijack the thread but it seems you guys have some knowledge and experience in this area. Guess what I'm asking is, if I'm more concerned with acceleration, is it more beneficial to get the the lower overall gearing (like 4th instead of 5th in car) I want more from the gear itself or from the prop? I'm thinking a prop is pretty inefficient compared to the gear itself, so a easier to turn lower pitch prop and harder to turn gear is better than a high pitch prop and easier gear. Assuming the prop range I'm talking about is 28 to 32.

Or is basically the same either way or so close to not matter? Not just hypothetical for me, I have a choice in assembling a lower and need to decide. Thanks for any input.

Run the gearing for 26-28" pitch. That way, no prop walk disaster. Acceleration is benefited by the right camber in the right place.

[smokin'joe]

We've designed a cleaver for this SST45 rig (Hans driving) and have printed a 3D model. As it is the boat can run 2nd or 3rd nationally in either circuit, NGK or Powerboat Nationals. But we're stuck with 1:1 gearing.

[Motv18]

Such a interesting read

btw torque converters have a ratio 2:1-3:1

to answer one question! Here it is torque x time = distance is the most basic way to express accelerating. The real formula is more complicated. The more torque can be applied over a given time the more acceleration occurs. Hp is a made up number to express torque and rpm

gear ratio never effects the engines power curve but it does change the rpm required to achieve a specific action. If prop theory holds archiving 25mph with 1:1 say at 2000rpm with the same prop it would take 4000rpm at 2:1. Mechanical advantage theory has you apply twice the torque and accelerate faster.

Of corse adding pitch slows rpm gain and reduces average applied torque. So you never really see that gain. Basically once you repitch to the operating range so your crank bearings last and you don’t bend a rod switching between 1.87 and 1.62 the gains will be negligible unless your in a drag race.

Because less pitch is easier to spool up (increase rpm) if you ran applied torque numbers with proper pitch you almost come out even and real world testing is needed.

And even vent most of that in not fully accurate due to all the real world variables and just general truths.

Forinstance the 3.0 merc have the low end strength of wet toilet paper throws most of that out the window.

B.A.T. Automotive engineering