Outboard Engine GEAR RATIOS!

[WillySteve]

I'm sure at some point this has been discussed but.....Here We Go Just in Case!

There seems to be a misunderstanding about gear ratios among some people so I'll try to explain!
For lower units with a 2.0 ratio the engine turns 2 times for every revolution of the propshaft, (e.g. 2.0:1).
A 1.75 ratio the engine turns 1.75 times for every revolution of the propshaft, (e.g. 1.75:1).
A 1.62 ratio the engine turns 1.62 times for every revolution of the propshaft, (e.g. 1.62:1).
A Higher ratio = more engine rpm for the propshaft's rpm, (a Higher Ratio not a higher speed).
If you lower the gear ratio enough you'll be at 1 to 1, (direct drive).
Lower it even more and you'll be in overdrive which would put the propshaft turning faster than the engine!!
To further explain, which is the higher ratio, a 30:1 or a 1:1?

The ratios in modern Mercury lower units are 2.0, 1.87, 1.75 and 1.62. These are based on the propshaft's "ONE" revolution! E.G. 2.0:1, 1.87:1, 1.75:1 and 1.62:1
In general a higher ratio gearset will leave harder because the higher ratio gear lets the engine rev faster getting into it's torque band sooner!
The gereric terms which people have used for years for gears are Low gears and High gears but in actuality the actual gear ratios are just the opposite....i.e. a Low gear is a High Ratio and a High gear is a Low Ratio!

[phillnjack]

Low ratio has the fastest acceleration, high ratio is fastest top end.

so many people just cant grasp it .

high to low = 1 - 1.... .. 1.86-1......2.0-1......2.23-1...2.42-1...2.69-1........

[WillySteve]

Low ratio has the fastest acceleration, high ratio is fastest top end.

so many people just cant grasp it .

high to low = 1 - 1.... .. 1.86-1......2.0-1......2.23-1...2.42-1...2.69-1........

Not Hardly!!!
People have had this mixed up for years!
What you have there is High Speed to Low Speed GEARING but not High to Low Ratios!
A Gear Ratio is based on Turns of Input vs. Turns of Output! A 2:1 ratio is a higher ratio than a 1:1
It's that simple!

[FMP]

The confusion comes in from " higher gear" meaning taller so less crank RPM per prop RPM. Taller ratio is the lower numeric .

[WillySteve]

The confusion comes in from " higher gear" meaning taller so less crank RPM per prop RPM. Taller ratio is the lower numeric .

A Ratio is nothing other than a ratio! The Higher the Ratio of any gear set the slower the speed of the output gear in relation to the input!
High speed gears are always associated with Taller gears but it's false in saying they are a higher ratio!
When you order a gear set you only order a specific ratio which has nothing to do with the generic terms high speed, low speed, stiffer, taller, high or low!
If you tell a mechanic you want a lower gear than your 1.75:1's I can guarantee he won't order them without questions to find out exactly what you want. On the other hand, if you tell him you want 1.87:1's, (A HIGHER RATIO but lower speed gear), there'll be no problem!

[FMP]

I didn't say taller is higher ratio, see confusion. I know and you know but many do not.

[racervboat]

so with a 2:1 with a 28p prop be faster and better acceleration than 1:87 with a 26p prop at the same rpm say 7500 ?

[WillySteve]

so with a 2:1 with a 28p prop be faster and better acceleration than 1:87 with a 26p prop at the same rpm say 7500 ?

They'll be close to the same top end speed but the 26P 1.87 ratio should have a better holeshot because of the smaller pitch prop being easier to turn and there's not much difference in a 2.0 vs a 1.87 ratio. Put the 26P on the 2.0 gearbox and it'll rock out's the hole!

[racervboat]

That's what I always thought not much difference between the two ratios really just a pitch or two difference between the two but some guys don't want a 2:1 and think the 1:87 is better ?

[90 5.0]

Depends a lot on weight of boat, boat type how freely the engine revs etc but typically a 2.0 with a 28 will out accelerate a 1.87/26

a 2.0/22 holeshots harder and accelerated harder than a 1.87/20 in testing on my boat.

[tlwjkw]

damn, readin' all this i can see why folks get confused!... not once was acceleration, top speed and/or prop pitch was mentioned.. his post is not a question ta start.......... read it!.... jeeze, one thing atta time!.. he's jus tryin' ta explain tha difference in ratio's and how they are determined!... which he does a pretty good job..

[dwilfong]

Remember there is a point where the tip speed of the prop makes it less efficient.
So running a smaller prop at higher shaft speed may not gain any thing.
In the same respect running a larger prop at a slower shaft speed will not give you the best efficiency.
So with this said what is the best prop tip speed to achieve?

[AZMIDLYF]

Same principle as auto rear ends if that helps.

[WillySteve]

Remember there is a point where the tip speed of the prop makes it less efficient.
So running a smaller prop at higher shaft speed may not gain any thing.
In the same respect running a larger prop at a slower shaft speed will not give you the best efficiency.
So with this said what is the best prop tip speed to achieve?

I'm certainly no expert on prop tip speed but IMO a large pitch prop has to get to an rpm that it paddlewheels more than cuts the water efficiently. I would say that it would outweigh the tip speed loss but I certainly could be wrong because at a very high speed it's possible it could be cutting through the water just right!
Props were easy on my old RC boats....just bend and grind until you get it flying!!!! LOL

[JCSmile]

I think the reason most people don't run 2:1's is the RPM needed for a given speed is higher. Thus when you get in really high RPM ranges or have a rev limiter the 1.87's are more beneficial. And most of our boats don't need help with torque out of the hole because of light weights.