Performance Differences In Relation To Gear Ratio's

A 1:81-1 ratio would be for every one revolution of the prop the engine crank turns 1:81 times....correct? Having everything identical - engine, props, rpms, load, etc.... with a 1:81 gear ratio the prop would turn slower than a 1:50 (as an example) gear ratio? What is the purpose for different ratio's and the pros and cons of each if all else remains the same?

My answer would be for the torque, ie. for the same revs of the prop I am getting more engine rpms from a 1:81 than I would from a 1:50....but a 1:50 ratio the boat will go faster than a 1:81. This is my guess timate only! That is why I am throwing this out to the boating gods.....

Re: Performance Differences In Relation To Gear Ratio's

I posted this some time ago, maybe it will help out with your question.
http://forums.iboats.com/prop-questions-topics/gear-ratio-vs-prop-pitch-514326.html#post3465942

Re: Performance Differences In Relation To Gear Ratio's

To keep the props in that 17 to 23 range is done by gear ratio in respect to the torque of the engine and the hp.
As an example, with Mercruiser Alpha drives. A SBC V8 will use a 1.5:1 gear ratio, while the 3.0L 4 cylinder will take a 1.98:1 gear ratio. But both may use the same exact 21" pitch prop.

Click to expand...

Don...thanks! and as I understand, same pitch prop on different gear ratios because of the different size engines due to torque and hp...to be able to fall btwn 17 and 23 range....sound right???

I'm maybe looking for the difference of effect (pros and cons) btwn a 1.81:1 / 1.62:1 or 2.0:1 gear ratio with same 4.3l engine. Is it just based on where the torque and hp band are in relation to engine rpm's?

Re: Performance Differences In Relation To Gear Ratio's

tahoejag said:

Don...thanks! and as I understand, same pitch prop on different gear ratios because of the different size engines due to torque and hp...to be able to fall btwn 17 and 23 range....sound right???

I'm maybe looking for the difference of effect (pros and cons) btwn a 1.81:1 / 1.62:1 or 2.0:1 gear ratio with same 4.3l engine. Is it just based on where the torque and hp band are in relation to engine rpm's?

Click to expand...

Ayuh,....I too, have a 4.3l in my tinbarge,...
I run a Torque Shift prop, it starts out at 11" of pitch, 'n shifts up to where ever ya set the screws,....
Top end pitch can be from 'bout 20" up to 26"s...

Years ago, when I put the 4.3l in the hull, I had a 1.50:1 drive....
It was a Pig, couldn't find a prop small enough to make it work, 'n the Torque shift wouldn't turn down to a small enough pitch...
I bought a used 1.65:1 for it, had My guy go through it, 'n have run it ever since, with the prop set to 'bout 20"s,...
4800 rpms at Wot, 'bout 43 mph..

Wantin' to use More of the props adjustability, I've tried a 1.98:1 on it,...
It yields a Wildly over-revin' motor, 'n 'bout 38 mph, at 26"s of pitch on the prop...

Last summer, I happened into a Bud, with a 1.84:1 drive, that needed a 1.50:1,...
I Swapped a spare of mine, for his...
Soon, I hope to have My guy go through this 1 to inspect, 'n reseal it..
Hopefully, this summer, with the 1.84:1 drive, it'll run, with the prop set considerably higher than the 20"s....
Hopefully, near the top of the prop's range...

Now, of course, this All has to do with the effects yer lookin' for,... Donno what Yer lookin' for,..??
But, I'm a Fisherman 1st, 'n everything else comes after that....
My goal is to be able to troll, down low, at 1 mph or less, hence The higher my props overall top end settin', the slower it'll troll at idle, at 11"s of pitch...

Sooooo,.... You can use a 1.65:1, or a 1.84:1, but a 1.98:1 just Ain't possible, with a 4.3l, in a normal boat...

Hope that Helps,.....

Re: Performance Differences In Relation To Gear Ratio's

Bond-o said:

Years ago, when I put the 4.3l in the hull, I had a 1.50:1 drive....
It was a Pig, couldn't find a prop small enough to make it work, 'n the Torque shift wouldn't turn down to a small enough pitch...
I bought a used 1.65:1 for it, had My guy go through it, 'n have run it ever since, with the prop set to 'bout 20"s,...
4800 rpms at Wot, 'bout 43 mph..

Click to expand...

Thats about where I am running...maybe more in the 4900-5000 range, but same speed with 1:81 w 20p.

Wantin' to use More of the props adjustability, I've tried a 1.98:1 on it,...
It yields a Wildly over-revin' motor, 'n 'bout 38 mph, at 26"s of pitch on the prop...

Last summer, I happened into a Bud, with a 1.84:1 drive, that needed a 1.50:1,...
I Swapped a spare of mine, for his...
Soon, I hope to have My guy go through this 1 to inspect, 'n reseal it..
Hopefully, this summer, with the 1.84:1 drive, it'll run, with the prop set considerably higher than the 20"s....
Hopefully, near the top of the prop's range...

Now, of course, this All has to do with the effects yer lookin' for,... Donno what Yer lookin' for,..??
But, I'm a Fisherman 1st, 'n everything else comes after that....
My goal is to be able to troll, down low, at 1 mph or less, hence The higher my props overall top end settin', the slower it'll troll at idle, at 11"s of pitch...

Sooooo,.... You can use a 1.65:1, or a 1.84:1, but a 1.98:1 just Ain't possible, with a 4.3l, in a normal boat...

Hope that Helps,.....

Click to expand...

Boat has the 1:81 gen II. not changing it. I am re propping for next season, goal is for mid range/cruising speed/fuel eco. (discussed in prop forum).

When you speak of higher pitch, or going to a higher pitch setting - you are going down in number (example: 20p to 11p), and if you were to lower pitch setting, you would be going from 11p to 20p.....is this correct? If so, sounds backwards to me....jmo.

Thanks for the info!.....luv it! Learn somethin new everday....

Re: Performance Differences In Relation To Gear Ratio's

When you speak of higher pitch, or going to a higher pitch setting - you are going down in number (example: 20p to 11p), and if you were to lower pitch setting, you would be going from 11p to 20p.....is this correct? If so, sounds backwards to me....jmo.

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Nope,.... Higher in both number, 'n pitch...

The Torque Shift is Always 11" at idle...
Only the Overall top pitch is tune able...

I've been runnin' it set to 20", Top pitch..

If I could run it at a higher Top pitch, with a lower gear ratio drive, the 11" at idle, will be slower...
'n, I can dial the top pitch up to 26" or where ever it needs to be, to hold 4800 rpms/ Wot...

That's why I wanta try goin' from 1.65:1 to 1.84:1, as the 1.98:1 is Too Low, 'n 26" of prop can't hold the horsepower available...

Re: Performance Differences In Relation To Gear Ratio's

Bond-o said:

Nope,.... Higher in both number, 'n pitch...

The Torque Shift is Always 11" at idle...
Only the Overall top pitch is tune able...

I've been runnin' it set to 20", Top pitch..

If I could run it at a higher Top pitch, with a lower gear ratio drive, the 11" at idle, will be slower...
'n, I can dial the top pitch up to 26" or where ever it needs to be, to hold 4800 rpms/ Wot...

That's why I wanta try goin' from 1.65:1 to 1.84:1, as the 1.98:1 is Too Low, 'n 26" of prop can't hold the horsepower available...

Click to expand...

Bond o...thanks for clearin that up for me. The torque shift prop and the way it works is a new thing to me. I see about the 26p in relation to gear ratio. As if I were to go to a 26p...I would be way under recommended rpm range.