gearing on uppers.....4.3 vs 3.0

How does this work? I'm thinking the 4.3 is higher geared......requiring a lower pitch prop.

i.e.
I just popped a 3.0 upper on a boat that has a 4.3.......now do I need to go up from the 17 pitch prop it had to say........a 23 or so?

Re: gearing on uppers.....4.3 vs 3.0

Re: gearing on uppers.....4.3 vs 3.0

Ayuh,....

You'd need a 28" or maybe a 30" pitch prop.....

I tried that same experiment with Mine,+ a 26" wasn't enough....

Re: gearing on uppers.....4.3 vs 3.0

am I being dense?

When I do the prop shaft rpm difference for a 1.81 gear ratio vs a 1.99 gear ratio, I only get that he would need to go from a 17" to a 19" to get the same theoretical speed at a given rpm???

What am I mising?

Re: gearing on uppers.....4.3 vs 3.0

wca_tim said:

am I being dense?

When I do the prop shaft rpm difference for a 1.81 gear ratio vs a 1.99 gear ratio, I only get that he would need to go from a 17" to a 19" to get the same theoretical speed at a given rpm???

What am I mising?

Click to expand...

I think you are calculating on the prop side and not the engine side.

Re: gearing on uppers.....4.3 vs 3.0

engine rpm 4500, gear ratio 1.81, prop pitch 17", theoretical speed 40 mph

engine rpm 4500, gear ratio 1.98, prop pitch 19", theoretical speed 41mph

Re: gearing on uppers.....4.3 vs 3.0

wca_tim said:

engine rpm 4500, gear ratio 1.81, prop pitch 17", theoretical speed 40 mph

engine rpm 4500, gear ratio 1.98, prop pitch 19", theoretical speed 41mph

Click to expand...

You didn't account for the engine power.

Re: gearing on uppers.....4.3 vs 3.0

sure i did... inherently so by targeting the pitch required for approximately the same speed... they're not really that different...

Re: gearing on uppers.....4.3 vs 3.0

wca_tim said:

sure i did... inherently so by targeting the pitch required for approximately the same speed... they're not really that different...

Click to expand...

Yes if you are using the same engine with both, but in this case it is 2 different engines and totally different power ratings both running the same gear ratio.

Re: gearing on uppers.....4.3 vs 3.0

no. I assumed it was the same engine and same power. Hence approximately the same speed.

As an example, I just finished reconfiguring my drive an hour or so ago to put it back on in the morning.

This afternoon, I was running a 1.81 gear ratio, with a 29 pitch prop. turning 5000-5200 rpms at wot yielded 72-25 mph.

With the 1.47 gear ratio, assuming that the amount of power I have is sufficient to turn approximately mid 70's with this hull and nearing the peak horsepower output of my engine (approx 5500 rpms).

If I input a 1.47 gear ratio and change pitch until I get about the same speed, it tells me that the place to start with a prop for the 1.47 gear ratios a little over 23 pitch, and so a good first prop will be the laser 2 I've got on hand that has a fair amount of cup in the blades. Note that it of course has a lot different properties than the neutral / "stern lifting" hydromotive 29" I was running, but now that I've got trim tabs on I should be able to run more bow lift without the mid range porpoising I used to see with higher rake / bow lifting props.

Note that typically when it comes to top end, props in the 28" to 30" range are more effective (at 60+ mph, prop pitch about 2x diameter is optimum for top end), and so I doubt I will be able to get much better than the 75 mph speeds I was getting with the higher pitch, unless i can do some other things with the set-up or I can spin a little smaller diameter 3 blade prop at 25+ pitch faster and get into the upper 70's OR if I change to a different drive / drive height and get the prop close enough to the pad to run a surface piercing prop I can probably pick up a god bit more speed...

That being said, the higher pitch prop won't be as good out of the hole and will take more torque to accellerate the boat, putting a greater strain on the outdrive...

a little drawn out, but I think it gets the point across...

Re: gearing on uppers.....4.3 vs 3.0

thumpar said:

Yes if you are using the same engine with both, but in this case it is 2 different engines and totally different power ratings both running the same gear ratio.

Click to expand...

no, I'm using the same engine.
Just swapped out drives.
I'm not sure what the gear ration difference is.........and I didn't have the oppurtinity to run this boat with the old drive.

It's a 4.3 mercruiser with a 3.0 outdrive.

What are the old/new gears?

I was kinda thinking I would be able to compensate by changing the prop pitch by a couple or so.........

Re: gearing on uppers.....4.3 vs 3.0

that's what I thought from your post. the most common gear ratio behind a 4.3 is 1.81:1 /1.84:1 (depending on which lower you have), the most common for a 3.0 is 1.98 / 1.94 :1. I just assumed a constant speed, assuming that for the same angine you have the power to go about the same speed and figured out what prop pictch change would be required if everything else stayed the same (it won't, but this'll get you close).

I got that if wot was where you want it with the 4.3 gear and a 17 pitch prop, and you changed to a 3.0 gear a 19 would be about the right place to start... Prop design and other variables come into play, but that's probably good place to start.

I'm assuming Bond-O was in a lighter boat than yours and perhaps swapped the engine keeping the 3.0 outdrive gear ratio... and picked up a lot of power... Props above 25 or 26 are harder to find more expensive and designed for more specific applications, that's for sure. For all around use it is a lot easier if you stay in the 17-23 range...

No offense intended to anyone else whatsoever...

hot as you know what here today... boat is gassed up and ready to head for water after lunch... life is so very wonderful...

Re: gearing on uppers.....4.3 vs 3.0

Ayuh,.... There you go Assuming....
You're not taking into account the Torque Multiplication factor of the lower gear ratios....

My fishing boat is a Fully dressed Starcraft 221 Islander,...
I rerigged it about 10 years ago with a 4.3LX at 205hp,.. I've run 1.5:1 ratio drives,+ you need a 15" or 17" prop,.. With my regular outdrive at 1.65:1, I keep my torque-shift prop limited to about 20"....
This year, for an experiment, I droppped on a 1.98:1 outdrive.... With the screws backed all the way out to 26" of pitch, It'll turn 5600rpms Plus....

Sooo,.... As stated in my 1st post,....
If you mount a 3.0l geared drive onto a 4.3l motor,....
You'll be needing a Prop somewhere in the 28" to 32" Pitch range.....

Regearing the drive would be Much Cheaper,+ Awhole lot Easier......

Re: gearing on uppers.....4.3 vs 3.0

From WCA_TIM

engine rpm 4500, gear ratio 1.81, prop pitch 17", theoretical speed 40 mph

engine rpm 4500, gear ratio 1.98, prop pitch 19", theoretical speed 41mph

Click to expand...

I agree, I would guess that with the outdrive geared within 2" of propellor pitch the performance will be so close to identical that you can't tell the difference.

The 17" and 19" will be within the same efficiency range, not like gearing it so you have to go to something ridiculous like a 6" or 36" pitch to keep the same RPM range.

Same reason you change rear end gears on a truck if you go to bigger or smaller tires and want the same acceleration and top speed. The engine doesn't know what gearing is between the flywheel and the ground (or water), it just has to turn a certain amount of revolutions to move a given distance.

I just put a 1.65 upper ($27 on Ebay - hard to beat!) on a 3.0 that originally had a 1.98 upper. Factory prop is a 21", I'm looking for a 17" to get the RPM up where it should be. Don't anticipate any problems, I'll post when i find a 17" prop and try it out.