Gear ratio question

[Dstyles75]

Most simply put, which would be the "faster" gear ratio 1 to 1.25 or 1 to 1.50? I'm guessing its a reduction in the speed from the engine so the lower one of 1 to 1.25 should go faster. Am I wrong? Can anyone explain. Thanks

 

[groundloop]

1:1.25 would spin the prop shaft faster, but that won't translate into a higher top speed because there would be less torque available at the prop shaft and you'd need a lower pitched prop. Assuming you had the optimum prop for each setup you'd come out real close to the same performance with each.

 

[wca_tim]

it'll depend a lot on what kind of speeds we're talking about and what the set up is. For most set-ups, the higher numerical gear ratio will be faster because you can turn a higher pitch prop - this applies when we're talking about speeds say 60 mph and up for top end. in that sort of a speed range, props in the 28" ish pitch range are going to be the most efficient at turning rotational torque into forward thrust.

sooo... the gear ratio that gets you into the right pitch range prop for optimum performance is the right one...

hope that's helpful. there's (alot) more...

 

[Big Red]

1.5 is always the best way to go. People will change the gear in the drive when their speeds exceeded a prop selection thats offered. Most common change is going from a 1.5 to a 1.36. The speed for a I/O would be over 90 before you would consider a gear change

 

[mkhammer]

1.5 is always the best way to go. People will change the gear in the drive when their speeds exceeded a prop selection thats offered. Most common change is going from a 1.5 to a 1.36. The speed for a I/O would be over 90 before you would consider a gear change

This is the real answer,


MKHammer

 

[wca_tim]

Not necessarily argung, but isn't the change from 1.5 - 1.35 done when speed get's high enough that you run out of available props that won't blow out (somewhere around 32" or so).

Everything I've read (some of it from the engineering community), what I've been told on several occassions by a very good local performance shop that I've dealt with, and my personal experience would say that when propped right turning a higher pitch prop slower will yield a higher top speed all other things being equal.

It's a combination of the angle of attack / pressure differential across the blades and the fact that the higher pitch prop has to move less water to go the same distance and so is more efficient at translating torque into forward motion.

based on that, a 1.6 gear would probably be faster up to around 90 and then a 1.5. I think the choice of drives on some of the newer performance boats may reflect this as well...

to the point, the 1.5 will be faster of the two...

 

[groundloop]

...when propped right turning a higher pitch prop slower will yield a higher top speed all other things being equal.

It's a combination of the angle of attack / pressure differential across the blades and the fact that the higher pitch prop has to move less water to go the same distance and so is more efficient at translating torque into forward motion....

I believe the increase in efficiency for a higher pitched prop is mostly that there's less parasitic drag because it's turning slower (for a given speed). Also, to go a certain speed you need to move the same amount of water no matter the pitch (up to the point that the prop is "blowing out", as you said). It's that Newton's Law thing - for every action there's an opposite and equal reaction blah blah blah.

Good discussion.

 

[Dstyles75]

I believe the increase in efficiency for a higher pitched prop is mostly that there's less parasitic drag because it's turning slower (for a given speed). Also, to go a certain speed you need to move the same amount of water no matter the pitch (up to the point that the prop is "blowing out", as you said). It's that Newton's Law thing - for every action there's an opposite and equal reaction blah blah blah.

Good discussion.

I agree. Between this one and my other posts I'm learning a ton! Very informative stuff!

 

[wca_tim]

I believe the increase in efficiency for a higher pitched prop is mostly that there's less parasitic drag because it's turning slower (for a given speed). Also, to go a certain speed you need to move the same amount of water no matter the pitch (up to the point that the prop is "blowing out", as you said). It's that Newton's Law thing - for every action there's an opposite and equal reaction blah blah blah.

Good discussion.

should have said move through less water... and as you pointed out would result in less drag... There are a few other arguents that support the same conclusion.

The bottom line is that the sweet spot (for 60+ mph) for a well designed prop is approximately 2 times diameter with diameter as small as reaosnable to keep the prop from blowing out or ventilating. for all practical purposes on boats like we're talking aobut that's diameter in the 14 - 15 inch range and so equates to most efficient props being in the 28 inch range.

and so if you're starting from scratch, gear ratio should be chosen with an eye on where you expect to see max speed at wot make some assumptions and pick the ratio that puts you in the right pitch range.

the only caveat and the reason that this isn't done more from the factory is that the higher the reduction, the more of the torque that is transfered to the lower gearcase. from a historical standpoint that was the weak spot - and the main reason that merc went to a 1.3 gear in alpha drives trying to get an underbuilt drive to hold up to big block tork in heavier boats. Sorry so long winded... tired tonight...

 

[mkhammer]

Man you guys are really working your brains. I love rocket science.

 

[Big Red]

Man you guys are really working your brains. I love rocket science.

Thats what I am thinking maybe my post is invisable to them or just to basic

 

[wca_tim]

Thats what I am thinking maybe my post is invisable to them or just to basic

Not invisible... and of the two choices the 1.5 is the obvious choice, but I thought the extra information might be helpful.

The boat would probably be faster with a 1.6 ratio depending on how fast already going...

What do you expect from a scientist...

 

[happy]

The 1.5 is the better choice. Red is correct 1.5 to 90 then drop closer to 1. You also have to calculate your torque which relates to the pitch on the prop, and the horsepower, which relates to your prop diameter these must be balanced also to achieve productive thrust. the other missed calculation in here is prop slip.

 

[happy]

http://sites.mercurymarine.com/port...26_49291:126_49315&_dad=portal&_schema=PORTAL

Here is Mercs website. This should answer all the fundamental questions. Should you have further questions, I'll be happy to cite equasions out of my naval architecture prop book.

 

[mkhammer]

The boat would probably be faster with a 1.6 ratio depending on how fast already going...

What do you expect from a scientist...

WCA was just making a little light out of it I didn't mean it for a slam, you guys were just going back and forth pretty much

I just want to shed some light on the subject this is gear reduction.
Lets use a 36 pitch prop for example since its the biggest one Mecury has to offer for the bravo drive. lets say your maximum rpm your engine can turn is 5000 rpm all prop slippage is realitive

1.50:1 gear ratio at 5000 rpm with a 36 pitch prop the speed would be 96 MPH
you are maxed out you can't go any faster with this combination of gear set and prop.
You must go to the next taller gear set to gain more speed.

Next taller gear ratio.say your engine has the power to pull same rpm.
1.36:1 gear ratio at 5000 rpm with 36 pitch prop the speed would be 106 mph

!.36:1 gear ratio at 5000 rpm with a 34 pitch prop the speed would be 100 mph

1.36:1 gear ratio at 5000 rpm with a 32 pitch prop the sped would be 95 mph

If you went to a lower gear ratio 1.62:1
1.62:1 gear ratio at 5000 rpm with a 36 pitch prop the speed would be 89 mph
you would be maxed out and you would have to go to the next taller gear set to gain speed which would be a 1.50:1.

Gear selection is based on your engine's power,maximum rpm , weight of boat,runnig bottom
A less powerful engine would have to use a lower gear ratio to turn the the largest most efficient prop for that boat which wouldn't be as fast.
This works the same for all drives in a higher or lower gear selection range.


MK

 

[happy]

Well said MK.

 

[wca_tim]

Thanks and no worries here...

well said mk, we're pretty much on the same page.

When I said 1.6 would be good to approaching 90, I was assuming a max rpm of 6,000 rpms and I based what I said on a quick back of the envelope guestimate 12 percent clip and a 29" prop with a 1.62 gear ratio.

the reality is that the straight theoretical relationship we all use to estimate where to go with pitch when changine gear ratio at a given level of power is only an estimate and fails to take into account that props in the 1.8 2.0 pitch to diameter range are / can be a good bit more efficient in turning torque into forward thrust.

soo... the theoretical change when going form there to a 1.5 assuming the same power would be to run a 27" prop and probably get pretty close to the same max speed, but still probably little shy of as fast as it would go with 1.6, maybe a few mph.

going to 1.25 would more likely make a big difference because now the theoretical pitch required would be a 22" and we're way off the peak performance range centered around 28" - 30" for 14-15" props and it would be a good bit slower - maybe 5 mph or more.

The mercury marine link and the single pdf docuent it was taken from are among my favorite general / starting references. Good stuff.

Respectfully submitted

Oh, and believe it or not, I wasn't being a smart ass in the least - I really am a scientist of all things...

 

[Big Red]

To be honest most BB engine builds I have seen lately really suck. People still dont get it, your torque numbers on the dyno must be higher to have a good strong boat. HP counts but not near as much as torque

 

[wca_tim]

To be honest most BB engine builds I have seen lately really suck. People still dont get it, your torque numbers on the dyno must be higher to have a good strong boat. HP counts but not near as much as torque

Hear that. Although horsepower is what determines max speed and so if trying to get the highest top end out of something we put gear and prop to have wot and peak horsepower, without strong torque numbers across the rpm range even a little boat will be a dog.

 

[Big Red]

Hear that. Although horsepower is what determines max speed and so if trying to get the highest top end out of something we put gear and prop to have wot and peak horsepower, without strong torque numbers across the rpm range even a little boat will be a dog.

I dont agree with your HP theory on top speed thats why small blocks are a flop in boats. They can have plenty of HP but no torque.