Outboard Engine GEAR RATIOS!

[FORBESAUTO]

Are you sure a lower gear wouldn't help ....,, nah just joking. Good luck to ya hope you get wall hanger!

[smokin'joe]

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........

Late response (I just saw the post) but higher gear ratio does not necessarily lead to higher speed. The reason is that the higher tie p/D (pitch/diameter) ratio the higher the peak prop efficiency (prop efficiency=hp transmitted to the boat/shaft hp). So you want a gear ratio that gives you higher p/D. 1:1 hardly ever does that. The V-6 OMC race motor (ca. 1993) and the Optimax both run less than 1:1. SST45 with p/D≈1.2 would have been a faster class had they geared it lower than 1:1. SST60 is good, geared 15:28 and with p/D just short of 2. This is typically not know to prop people but the data have been in the literature since about 1955. For surface-piercing props since about 1965. What's the limit of going down in gear ratio? I don't know except that I know experimentally that 13:29 is slower than 15:28. Interestingly enough the Yamato racing motor runs p/D≈2.

[smokin'joe]

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!

What do you guys mean by 'taller'?

[smokin'joe]

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!

Depends on the prop, especially on the camber. You're assuming equal slip but slip depends strongly on camber.

[WillySteve]

If lower speed gear does not multiply torque, explain why a 4:1 "torque multiplier" which is nothing but a gear reduction you hook a ratchet to allows a man to torque a bolt to 400 ft lbs and otherwise couldn't torque it to 200 ft lbs?

The lower speed, (higher ratio), Multiplies torque to the end result the output, (the output sees more torque).
It does not multiply input torque....such as yer arm that's turning the wrench. If that arm dyno's 100 ft#'s then that's all ya got!
Things like this get pretty mixed up when there are so many variables involved.....same with a seesaw and fulcrum!
High/low gears, high/low gear ratio's, input/output torque, acceleration and top speed of input/output....then through in the fluid coupling of a prop in water which you have to figure in it's efficiency which depends on variables like pitch, dia., submerged/surfaced, blade shape, it's speed of rotation....I'm goin Fishin!!!!

[smokin'joe]

The lower speed, (higher ratio), Multiplies torque to the end result the output, (the output sees more torque).
It does not multiply input torque....such as yer arm that's turning the wrench. If that arm dyno's 100 ft#'s then that's all ya got!
Things like this get pretty mixed up when there are so many variables involved.....same with a seesaw and fulcrum!
High/low gears, high/low gear ratio's, input/output torque, acceleration and top speed of input/output....then through in the fluid coupling of a prop in water which you have to figure in it's efficiency which depends on variables like pitch, dia., submerged/surfaced, blade shape, it's speed of rotation....I'm goin Fishin!!!!

You're on the right track. It's physics, but requires physics of hydrodynamics. Power to prop goes like D^5xn^3 where D=prop diameter and n = propshaft RPM (=gear ratio x motor RPM). Power supplied by prop to boat goes like D^4xn^2xU where U is boat speed. The ratio of those (with the torque and thrust coefficients multiplying the factors) is the prop efficiency. Prop efficiency has been measured since the mid-1950s. The prop efficiency peak increases as the p/D ratio increases where p=pitch. For a given shaft hp the p/D peak falls as the gear ratio is increased. Higher gear ratios do not automatically produce higher speed. There is no formula for an optimal gear ratio for a given application. E.g, Dick Snyder wrote me that unlimited hydros picked up 10 mph when he convinced them to run a lower overdrive gear ratio. At the opposite extreme the old DP and SD kilo records were too low compared with EP and SE because the D motor was geared 12:29 while the Stinger was geared 15:28. The SST45 class would run faster with something like 15:17 but we're stuck with 1:1. I'm confused by your apparent nonstandard terminology: 1:1 is high gear, 12:29 is low gear.

[NICE PAIR]

The lower speed, (higher ratio), Multiplies torque to the end result the output, (the output sees more torque).
It does not multiply input torque....such as yer arm that's turning the wrench. If that arm dyno's 100 ft#'s then that's all ya got!
Things like this get pretty mixed up when there are so many variables involved.....same with a seesaw and fulcrum!
High/low gears, high/low gear ratio's, input/output torque, acceleration and top speed of input/output....then through in the fluid coupling of a prop in water which you have to figure in it's efficiency which depends on variables like pitch, dia., submerged/surfaced, blade shape, it's speed of rotation....I'm goin Fishin!!!!

You're on the right track. It's physics, but requires physics of hydrodynamics. Power to prop goes like D^5xn^3 where D=prop diameter and n = propshaft RPM (=gear ratio x motor RPM). Power supplied by prop to boat goes like D^4xn^2xU where U is boat speed. The ratio of those (with the torque and thrust coefficients multiplying the factors) is the prop efficiency. Prop efficiency has been measured since the mid-1950s. The prop efficiency peak increases as the p/D ratio increases where p=pitch. For a given shaft hp the p/D peak falls as the gear ratio is increased. Higher gear ratios do not automatically produce higher speed. There is no formula for an optimal gear ratio for a given application. E.g, Dick Snyder wrote me that unlimited hydros picked up 10 mph when he convinced them to run a lower overdrive gear ratio. At the opposite extreme the old DP and SD kilo records were too low compared with EP and SE because the D motor was geared 12:29 while the Stinger was geared 15:28. The SST45 class would run faster with something like 15:17 but we're stuck with 1:1. I'm confused by your apparent nonstandard terminology: 1:1 is high gear, 12:29 is low gear.

You guy's are makin my head hurt, boating never used to be this complicated.

[Dave S]

Merc used different ratios as to be able to use a limited # of props on a large range of prop shafts/ hp.....A small pinion gear wants to walk out of a cases sideways. Bigger Hp = bigger gear use same prop lower hp small pinion......That's my Opinion.....

[smokin'joe]

Merc used different ratios as to be able to use a limited # of props on a large range of prop shafts/ hp.....A small pinion gear wants to walk out of a cases sideways. Bigger Hp = bigger gear use same prop lower hp small pinion......That's my Opinion.....

Yes, and that's exactly why OMC committed a gear ratio blunder with the 55 and 60 hp motors, the D motors, they used the same props as the Stinger. A lower gear ratio actually requires a higher diameter prop. Even if you ran the correct dia. prop on a D motor (about 13.5" compared with 12-12.5" dia. on the E motor)) the speed was unreasonably far below that of the higher geared E motor. The 25% hp difference can't explain the poor performance, especially since the gearcase housings were identical.

[smokin'joe]

You guy's are makin my head hurt, boating never used to be this complicated.

Wait til my book is published, aspirin won't help: Hydrodynamics of Speed on the Water, Fast Boats and Surface Piercing Propellers. Filled with equations, including a complete mathematical prescription for racing prop design.

[Dave S]

I hope it has Pixs...... some don't read well......I don't use aspirin.... hic.... My favorite OMC case 14/23 twin pinion....15 spline p/s....

[DangerNewb]

So if you had a choice between 1.62 and 1.75 on a 3 liter Sporty on a 300 promax mounted to a 1000lb modvp hull, which would you prefer? It seems the difference is equal to about 2 inches in pitch.

30 on 1.62 is about the same top end as 32 on 1.75 at 6k rpm. within 1 mph from my calcs
28 on 1.62 is about the same top end as 30 on 1.75 at 6k.

I'm more interested in good mid range acceleration with decent top end.

Leaning towards the 1.62 so I can run a lower pitched prop. 28 or maybe 30, rather than 30 or 32.

But open to advice.

[NICE PAIR]

I hope it has Pixs...... some don't read well......I don't use aspirin.... hic.... My favorite OMC case 14/23 twin pinion....15 spline p/s....

And you said ... "I hope it has Pixs" Let's see this twin pinion whatchacallit.

[WillySteve]

I'm confused by your apparent nonstandard terminology: 1:1 is high gear, 12:29 is low gear.

SIMPLE: 1:1 is a Lower gear "RATIO" but in generic/layman terms is called "HIGH" gear being it's direct drive, 12:29 in reality is a Higher gear "RATIO" than 1:1, just as 1:50....etc., but people refer to it as "Low gear."

Many people do not consider the fact that when talking about "Gearing" and "Ratios," they are 2 different animals. Started from day 1, when all there was was Standard shift vehicles. People would pull'er down into Low gear to take off and High gear to cruise. In actuality their Low gear was the High Ratio, not High gear!!!

[WillySteve]

So if you had a choice between 1.62 and 1.75 on a 3 liter Sporty on a 300 promax mounted to a 1000lb modvp hull, which would you prefer? It seems the difference is equal to about 2 inches in pitch.

30 on 1.62 is about the same top end as 32 on 1.75 at 6k rpm. within 1 mph from my calcs
28 on 1.62 is about the same top end as 30 on 1.75 at 6k.

I'm more interested in good mid range acceleration with decent top end.

Leaning towards the 1.62 so I can run a lower pitched prop. 28 or maybe 30, rather than 30 or 32.

But open to advice.

I don't have any experience with different ratios on 3.0's but I can say for typical style props in general, that at some point on any setup you can reach a point in adding pitch that the prop will start acting more like a paddle wheel than a prop!