Why the different gearing?

If most non-outboard engines want to rev to around 4500 rpm at wot, and most props are made in a range of pitches, why do different engines have different drive gearings?

My brother-in-law says it's because the drives behind larger engines need stronger parts, but that doesn't make sense - why does stronger parts equal taller gears, and why would a smaller engine that doesn't need heavier parts need a deeper/faster gearset?

I can understand why a smaller (lower hp) engine would need deeper gears (and/or a deeper prop) to get the torque needed to get a boat moving, but then what about top speed? I've seen small engines (with deeper gearing) in small boats that are faster than large engines (with taller gearing) in large boats, and the rpms aren't all that different at wot. Matter of fact, my little 16' Starcraft w/ a 2.5 and a 15" prop at 4400rpm will outrun my same bro-in-law's 19' Wellcraft w/ a 4.3 and a 19" prop at 4800 rpm.

I keep reading here that drives have different gearing for different engines, and the only reason I can find seems to be: "Because. Besides, that's the way the manufacturers make them."

Just trying to figure out exactly why...

Re: Why the different gearing?

The ideal prop is a 19 pitch and the different gear ratios allow motors of different sizes to use something around that pitch on the "average" boat. But MerCruiser is stupid in their packaging. You get the same 1.5:1 with a 5.7L regardless of what boat it's put on. Same gear ratio if it's on a ski boat or cabin cruiser? BULLSCHMIDT! Trucks vary gear ratios by engine size and vehicle size. Boats should too! But the rubes in Marketing make these decisions, not the Engineers that should. After all, sterndrive motors are basically marinized truck motors.

Re: Why the different gearing?

WillyBWright said:

But the rubes in Marketing make these decisions . . .

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With cost and inventory restrictions

Re: Why the different gearing?

My Thought are,.......

The Driveline is like a Dyno,..........
You need the proper Gearing to be able to absorb the HP,+ Stall the engine at it's rated WOT,......

The motor won't spin it's Load(aka, Gearing) anymore than the rated rpms, because it Doesn't have anymore HP.....

Props are only Effecient,+ available in a limited range of Pitches,....
Basically, from 15" to 25" are found at Most retailers, barring the fancy Hot Rod boating stuff....
Actually, if you find props from 17" to 23" you're doin' Pretty Good.....

In order to Stall Out higher horsepowered motors,.... Taller Gears are needed.....

Re: Why the different gearing?

I think Bond-O is basically correct.

Not a hydraulic or prop engineer, but maybe I can come up with an example that makes sense.

Power is Torque times RPM. Since as you say almost all I/O engines turn about 4400 RPM then the difference in a 130 HP motor and a 425 HP motor is obviously the torque. So the prop pitch has to absorb the torque the motor is putting out and transfer it to the water. If it can't, you over-rev your motor. If it takes too much torque to turn the prop then your motor can't reach the RPM where it makes max power.

Using your boat as an example, a 15" prop going through a 2.5 gear reduction absorbs all the torque your motor is capable of putting out. If you were to drop a motor with 4 times the power (which boils down to torque) in your boat it would take a 60" pitch prop (4 times the 15") to absorb the torque while turning the same RPM. Your boat would theoretically also be 4 times faster. I say theoretically because the drag from the water goes up exponentially with speed.

Now we have to look at prop theory. Early props actually looked like big woodscrews, with multiple turns on the blades. This was because people thought the prop worked like a screw and "screwed" it's way through the water. Then somebody (Robert Fulton I think?) hit something and broke off all his prop except the first couple of inches. All of a sudden the boat was much faster and accelerated much better. Turns out propellors actually work like airplane wings. They generate lift on the front side of the blades because Bernoulli's Theory and a lower pressure on the front side of the blade. Cavitation is when the pressure drops so low that the water boils on the front of the blade and you don't get any thrust.

Back to your boat. A 60" pitch prop would be so steep that a low speeds it would almost be like a paddle wheel turned sideways. It would cavitate immediatly at low speeds. But if we cut the gear ratio on the outdrive down to 1.25 then we cut the torque transmitted to the propellor in half. Now we only need a 30" pitch prop to absorb the torque from the 425 HP motor. The prop spins twice as fast as it would with the 60" pitch, 2.5 ratio so the theoretical top speed is still the same. This gets us back in a usable prop range.

I think that this is why you'll see the lower gear ratios on the more powerful motors. It doesn't work exactly 1:1 as I described because of efficiency losses, but it's close. The outdrives behind the more powerful motors also have to be stronger (and therefore heavier) because since the motors all turn about the same RPM, the difference in power is the torque so the shafts, gears, etc all have to absorb this greater torque.

Sorry about the long winded post, couldn't figure how to make it shorter. Not sure what the deal is with your brother-in-laws boat, maybe a lot "draggier" hull? It's obvious from the numbers you gave that his prop has a LOT more slip than yours.

Keith

Re: Why the different gearing?

Jeez 45Auto, where you been hiding? I need some support on these bhp and torque threads once and a while . . . When I claim that peak torque and torque rise are inconsequential to a boats top speed you'd think I exposed Santa or something . . . Great post!! Oh, BTW here's the hp to speed calcs for a boat that someone here posted. I paste good stuff like your post into Word and keep them . . .

Drag is a function of the square of speed so take your old top speed, square it, then divide by the old HP. Take that number, multiply it by the new HP and then find the square root of the result.

For instance:

50 MPH top speed with 210 HP changing to 300 HP would be:

50 squared=2500, 2500/210 = 11.904761, 11.904761*300=3571.4283, The square root of 3571.4283=59.7614

The new top speed would be 59.7614 MPH assuming the new prop is exactly as efficient as the old prop and that the two engines were both accurately rated for power.

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Re: Why the different gearing?

Hi QC,

Been lurking on here for a long time, lots of good knowledge and info. Don't post too often because guys like you and Don S. usually answer any questions much quicker and better than I can! Be glad to help any time I can. Keep up the good work!

Re: Why the different gearing?

To put it simply, the maximum usable safe rpm range for an outboard is between 5000-6000rpm. The lower gearing, & prop, is designed for it to run there, regardless of the weight, & load on the motor, these two variables are adjustable to compensate for that. The same applies to an I/O motor, just at a lower rpm 4200-4600 normally. It is a fact, that either of these types of motors can produce much more rpm, but it may not be safe, or prudent for longevity to run them at that high an rpm, especially if they bounce up out of the water at times, hence they are governed to that rpm.

Re: Why the different gearing?

45Auto said:

I think Bond-O is basically correct.

Not a hydraulic or prop engineer, but maybe I can come up with an example that makes sense.

Power is Torque times RPM. Since as you say almost all I/O engines turn about 4400 RPM then the difference in a 130 HP motor and a 425 HP motor is obviously the torque. So the prop pitch has to absorb the torque the motor is putting out and transfer it to the water. If it can't, you over-rev your motor. If it takes too much torque to turn the prop then your motor can't reach the RPM where it makes max power.

...

Sorry about the long winded post, couldn't figure how to make it shorter. Not sure what the deal is with your brother-in-laws boat, maybe a lot "draggier" hull? It's obvious from the numbers you gave that his prop has a LOT more slip than yours.

Keith

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AHA! That makes sense, long-winded or not. Thanks for the replies, it's guys like you all who make this place so cool.

So ultimately, the reason lies with the range of efficient prop sizes. Makes sense, especially when you look at the old wooden runabouts. Most of them were direct 1:1 drives, but their props ranged from little 13X12 in the little skiboats up to 30X24 in some of the big Hackers and ChrisCrafts.

My bro-in-law's boat is a lot heavier, has a deeper V, and imho is underpowered with the 4.3 - now I get to tease him about his prop slip, hahaha!