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Author Topic:   Lower Unit Gear Ratio

jimh posted 05-21-2005 09:30 AM ET (US)
The lower unit gear ratio of an outboard motor acts as a torque multiplier. By gearing the engine down, more torque is created. In traditional two-stroke engines, the lower unit gearing tends to fall (numerically) as engine horsepower increases. Here is a table of some typical values of lower-unit gear ratio versus horsepower:
HP vs gear ratio, two-stroke outboard
90-HP: 2.33:1
125-HP: 2.07:1
200-HP: 1.87:1
225-HP: 1.75:1
(From classic Mercury two-stroke and OptiMax engines)
Four-stroke engines often are designed with higher (numerical) gear ratios. This tends to help them produce more torque, particularly at low engine crankcase speeds. Some examples:
HP vs gear ratio, four-stroke outboard
70-HP: 2.42:1
90-HP: 2.59:1
140-HP: 2.38:1
225-HP: 2.29:1
(From Suzuki four-stroke engines)
To reach the same speeds, the engines with the higher (numerical) gear ratios must turn propellers with higher pitch, or the engines must wind to higher crankshaft speeds. Both of these trends are seen.
Most two-stroke engines are rated at 5,500 RPM, but many four-stroke engines are rated at 6,000 RPM or higher.
Also, higher (numerical) gearing requires larger gears, and as result the lower unit size is often increased to accommodate. These are called bigfoot gear cases. As boat speed increases, a larger gear case creates more drag. At some point the drag begins to overcome the benefit of the torque increase. There has been a trend in four-stroke engines to reduce the size of the lower unit gear case. Suzuki accomplishes this by using intermediate gear reduction in the power head.
The need for higher (numerical) gearing for four-stroke engines seems to be evidence of their lower torque output compared to two-stroke engines.
LHG posted 05-21-2005 08:28 PM ET (US)
Jim - Mercury's 4-strokes use the same ratios as the 2-strokes, even for the Verados, which use the 2.5 liter gearcase ratio of 1.87. And the Verado is redlined at an even higher 6400RPM.
I have no idea what Suzuki is doing with nubers like that.
anthonylisske posted 07-09-2009 10:42 AM ET (US)
Jim,
This is a good thread. (I searched for it) Can you add this plus an intro into the speed calculator or the FAQs?
I had no idea about gear ratios in relation to lower units, but wondered many times about my 1.81 ratio on my Yamaha 225.
Thanks
jimh posted 07-10-2009 09:17 AM ET (US)
HP vs Gear Ratio, Yamaha Four-Cycle
75-HP: 2.33:1
90-HP: 2.33:1
115-HP: 2.15:1
150-HP: 2.00:1
200-HP: 2.00:1
225-HP: 2.00:1
250-HP: 2.00:1
300-HP: 1.73:1
350-HP: 1.73:1
cooper1958nc posted 07-11-2009 01:31 PM ET (US)
"As boat speed increases, a larger gear case creates more drag. At some point the drag begins to overcome the benefit of the torque increase."
"Bigfoot" gear cases are not "big" because they have "big" gears; they are "big" because they swing a larger diameter prop. Large diameter is advantageous, within limits, for reasons discussed at length in earlier posts.
There is no "benefit" to a "torque increase" per se. Power is RPM times torque, so increasing torque reduces RPM at constant power. Having said that, reduction gears in general help marine engines primarliy because swinging a larger prop slower is generally more efficient than swinging a small prop fast.
Changes in reduction ratio with increasing horsepower just reflect, I believe, the need to keep prop sizes within certain limits. Max diameter is fixed by the gearcase. Most props do not deviate greatly from certain pitch/diameter ratios, for various reason. Thus the need to vary reduction gearing with power.
jimh posted 07-11-2009 03:13 PM ET (US)
Actually bigfoot gear cases are bigger than the normal gear cases on those motors where they are called "bigfoot" gear cases.
The Suzuki outboard have a larger propeller aperture and can handle propeller diameters of 16-inches. My OMC V6 can only handle propellers of 15.75-inch, and that is with the qualification that a 15.75-inch diameter propeller is practically an interference fit. The larger diameter of propeller possible with Suzuki may account for their use of higher gear ratios.
dg22 posted 07-11-2009 06:33 PM ET (US)
Noticed that the new E-TEC 50-HP uses the 2.67:1 gear ratio which is the same gear ratio OMC used on their 50-HP dating back to 1972 and possibly earlier. E-TEC also use this foot on their commercial 65-HP and Multi-Fuel Military model 55-HP.
fishgutz posted 07-11-2009 07:12 PM ET (US)
Wow, Cooper is right on the money. I have not heard anyone explain it so well. The increase in drag is equivilant to putting your thumb in the water. Does it slow you down? No. If it does, the increase in prop size compensates for it.
jimh posted 07-11-2009 08:23 PM ET (US)
Yeah, drag from the gear case is meaningless. I can't figure out why people are so keen on raising the engine mounting height to get a miniscule difference in the volume of the gear case being dragged through the water. Go figure.
fishgutz posted 07-11-2009 10:30 PM ET (US)
Here we go again. Raising the engine moves the center of effort up. It has little to do with reducing drag.
[Used a metaphor to express his belief he was much smarter than the rest of us.]
fishgutz posted 07-12-2009 09:14 AM ET (US)
I'm only smarter because I know I don't know everything.
dg22 posted 07-12-2009 09:56 AM ET (US)
Drag is a factor but what I think is more significant as you raise your motor is the prop breaking the surface increasing in RPM. From my experience, every inch up = 1 mph gain from a normal setup.
jimh posted 07-12-2009 10:44 AM ET (US)
This sidebar about drag is also a drag on this discussion. People who wish to propose that drag from the gear case is not a factor to be considered are invited to begin a new discussion on that topic with a separate thread.
To return to the topic of this discussion, the gear ratio used and how it varies with different motors, so far all the data collected and provided shows that as a general rule the four-cycle engines are set up with higher (numerical) gears than two-cycle engines of the same horsepower. This is completely consistent with my initial observation, which I repeat:
"Four-stroke engines often are designed with higher (numerical) gear ratios. This tends to help them produce more torque, particularly at low engine crankcase speeds."
If there is something wrong about that observation, I would be pleased to hear about it. If someone has data which is contrary to that, please present it.
Moe posted 07-12-2009 02:26 PM ET (US)
Do not be misled by taking few samples of two and four cycle engines. I think you will find gear ratio is more related engine displacement and even to shaft length (which is related to size/weight of boat to be powered), and perhaps even to manufacturer, as much as to two vs four cycle.
Note also that so-called "Big Foot"/"High Thrust" outboards are intended to power heavier boats at lower speeds, typically with 60 HP or smaller powerheads. Because horsepower to overcome drag is exponential with speed, the larger diameter of the foot, much less that of the larger diameter prop, are not as important in these lower-speed applications as is the increased prop shaft torque. Furthermore, increased prop shaft torque allows for steeper pitched prop blades, which in and of itself reduces drag compared to that of 7-10" pitch props that would otherwise be necessary in these applications.
--
Moe
dg22 posted 07-12-2009 10:17 PM ET (US)

Jimh, I read an interesting article that supports what you are saying in your inital post. At lower rpm DI two-strokes are more potent performers because they boast a power stroke with every swing of the crank. Four-strokes only build power on every other up and down swing of the crank. To compensate some engine builders incorporate a lower gear ratio in the gear case. the lower the ratio, the more torque is multiplied narrowing the gap between two and four strokes.
Peter posted 07-13-2009 09:16 AM ET (US)
Perhaps there is no better example of using gear ratios to compensate for lower torque than a comparison of ratios of the inline 4-cylinder 75-115 HP Mercury FourStrokes (Veraditos) with the inline 4-cylinder 150-200 HP Mercury Verados. The powerheads share the same displacement and internal parts (or used to). The powerhead for both engine families has a 6400 RPM. The major difference is the use of a supercharger to force more air into the combustion chamber so more fuel can be burned in it producing a greater HP and torque for the same displacement.
No one questions that the heavy breathing supercharged Verados produce more low RPM crankshaft torque than their weaker cousins. The naturally aspirated Veraditos are equipped with a 2.33:1 ratio. The supercharged Verados run with a 2.08:1 ratio.


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Author	Topic: Lower Unit Gear Ratio
jimh	posted 05-21-2005 09:30 AM ET (US) The lower unit gear ratio of an outboard motor acts as a torque multiplier. By gearing the engine down, more torque is created. In traditional two-stroke engines, the lower unit gearing tends to fall (numerically) as engine horsepower increases. Here is a table of some typical values of lower-unit gear ratio versus horsepower: HP vs gear ratio, two-stroke outboard 90-HP: 2.33:1 125-HP: 2.07:1 200-HP: 1.87:1 225-HP: 1.75:1 (From classic Mercury two-stroke and OptiMax engines) Four-stroke engines often are designed with higher (numerical) gear ratios. This tends to help them produce more torque, particularly at low engine crankcase speeds. Some examples: HP vs gear ratio, four-stroke outboard 70-HP: 2.42:1 90-HP: 2.59:1 140-HP: 2.38:1 225-HP: 2.29:1 (From Suzuki four-stroke engines) To reach the same speeds, the engines with the higher (numerical) gear ratios must turn propellers with higher pitch, or the engines must wind to higher crankshaft speeds. Both of these trends are seen. Most two-stroke engines are rated at 5,500 RPM, but many four-stroke engines are rated at 6,000 RPM or higher. Also, higher (numerical) gearing requires larger gears, and as result the lower unit size is often increased to accommodate. These are called bigfoot gear cases. As boat speed increases, a larger gear case creates more drag. At some point the drag begins to overcome the benefit of the torque increase. There has been a trend in four-stroke engines to reduce the size of the lower unit gear case. Suzuki accomplishes this by using intermediate gear reduction in the power head. The need for higher (numerical) gearing for four-stroke engines seems to be evidence of their lower torque output compared to two-stroke engines.
LHG	posted 05-21-2005 08:28 PM ET (US) Jim - Mercury's 4-strokes use the same ratios as the 2-strokes, even for the Verados, which use the 2.5 liter gearcase ratio of 1.87. And the Verado is redlined at an even higher 6400RPM. I have no idea what Suzuki is doing with nubers like that.
anthonylisske	posted 07-09-2009 10:42 AM ET (US) Jim, This is a good thread. (I searched for it) Can you add this plus an intro into the speed calculator or the FAQs? I had no idea about gear ratios in relation to lower units, but wondered many times about my 1.81 ratio on my Yamaha 225. Thanks
jimh	posted 07-10-2009 09:17 AM ET (US) HP vs Gear Ratio, Yamaha Four-Cycle 75-HP: 2.33:1 90-HP: 2.33:1 115-HP: 2.15:1 150-HP: 2.00:1 200-HP: 2.00:1 225-HP: 2.00:1 250-HP: 2.00:1 300-HP: 1.73:1 350-HP: 1.73:1
cooper1958nc	posted 07-11-2009 01:31 PM ET (US) "As boat speed increases, a larger gear case creates more drag. At some point the drag begins to overcome the benefit of the torque increase." "Bigfoot" gear cases are not "big" because they have "big" gears; they are "big" because they swing a larger diameter prop. Large diameter is advantageous, within limits, for reasons discussed at length in earlier posts. There is no "benefit" to a "torque increase" per se. Power is RPM times torque, so increasing torque reduces RPM at constant power. Having said that, reduction gears in general help marine engines primarliy because swinging a larger prop slower is generally more efficient than swinging a small prop fast. Changes in reduction ratio with increasing horsepower just reflect, I believe, the need to keep prop sizes within certain limits. Max diameter is fixed by the gearcase. Most props do not deviate greatly from certain pitch/diameter ratios, for various reason. Thus the need to vary reduction gearing with power.
jimh	posted 07-11-2009 03:13 PM ET (US) Actually bigfoot gear cases are bigger than the normal gear cases on those motors where they are called "bigfoot" gear cases. The Suzuki outboard have a larger propeller aperture and can handle propeller diameters of 16-inches. My OMC V6 can only handle propellers of 15.75-inch, and that is with the qualification that a 15.75-inch diameter propeller is practically an interference fit. The larger diameter of propeller possible with Suzuki may account for their use of higher gear ratios.
dg22	posted 07-11-2009 06:33 PM ET (US) Noticed that the new E-TEC 50-HP uses the 2.67:1 gear ratio which is the same gear ratio OMC used on their 50-HP dating back to 1972 and possibly earlier. E-TEC also use this foot on their commercial 65-HP and Multi-Fuel Military model 55-HP.
fishgutz	posted 07-11-2009 07:12 PM ET (US) Wow, Cooper is right on the money. I have not heard anyone explain it so well. The increase in drag is equivilant to putting your thumb in the water. Does it slow you down? No. If it does, the increase in prop size compensates for it.
jimh	posted 07-11-2009 08:23 PM ET (US) Yeah, drag from the gear case is meaningless. I can't figure out why people are so keen on raising the engine mounting height to get a miniscule difference in the volume of the gear case being dragged through the water. Go figure.
fishgutz	posted 07-11-2009 10:30 PM ET (US) Here we go again. Raising the engine moves the center of effort up. It has little to do with reducing drag. [Used a metaphor to express his belief he was much smarter than the rest of us.]
fishgutz	posted 07-12-2009 09:14 AM ET (US) I'm only smarter because I know I don't know everything.
dg22	posted 07-12-2009 09:56 AM ET (US) Drag is a factor but what I think is more significant as you raise your motor is the prop breaking the surface increasing in RPM. From my experience, every inch up = 1 mph gain from a normal setup.
jimh	posted 07-12-2009 10:44 AM ET (US) This sidebar about drag is also a drag on this discussion. People who wish to propose that drag from the gear case is not a factor to be considered are invited to begin a new discussion on that topic with a separate thread. To return to the topic of this discussion, the gear ratio used and how it varies with different motors, so far all the data collected and provided shows that as a general rule the four-cycle engines are set up with higher (numerical) gears than two-cycle engines of the same horsepower. This is completely consistent with my initial observation, which I repeat: "Four-stroke engines often are designed with higher (numerical) gear ratios. This tends to help them produce more torque, particularly at low engine crankcase speeds." If there is something wrong about that observation, I would be pleased to hear about it. If someone has data which is contrary to that, please present it.
Moe	posted 07-12-2009 02:26 PM ET (US) Do not be misled by taking few samples of two and four cycle engines. I think you will find gear ratio is more related engine displacement and even to shaft length (which is related to size/weight of boat to be powered), and perhaps even to manufacturer, as much as to two vs four cycle. Note also that so-called "Big Foot"/"High Thrust" outboards are intended to power heavier boats at lower speeds, typically with 60 HP or smaller powerheads. Because horsepower to overcome drag is exponential with speed, the larger diameter of the foot, much less that of the larger diameter prop, are not as important in these lower-speed applications as is the increased prop shaft torque. Furthermore, increased prop shaft torque allows for steeper pitched prop blades, which in and of itself reduces drag compared to that of 7-10" pitch props that would otherwise be necessary in these applications. -- Moe
dg22	posted 07-12-2009 10:17 PM ET (US) Jimh, I read an interesting article that supports what you are saying in your inital post. At lower rpm DI two-strokes are more potent performers because they boast a power stroke with every swing of the crank. Four-strokes only build power on every other up and down swing of the crank. To compensate some engine builders incorporate a lower gear ratio in the gear case. the lower the ratio, the more torque is multiplied narrowing the gap between two and four strokes.
Peter	posted 07-13-2009 09:16 AM ET (US) Perhaps there is no better example of using gear ratios to compensate for lower torque than a comparison of ratios of the inline 4-cylinder 75-115 HP Mercury FourStrokes (Veraditos) with the inline 4-cylinder 150-200 HP Mercury Verados. The powerheads share the same displacement and internal parts (or used to). The powerhead for both engine families has a 6400 RPM. The major difference is the use of a supercharger to force more air into the combustion chamber so more fuel can be burned in it producing a greater HP and torque for the same displacement. No one questions that the heavy breathing supercharged Verados produce more low RPM crankshaft torque than their weaker cousins. The naturally aspirated Veraditos are equipped with a 2.33:1 ratio. The supercharged Verados run with a 2.08:1 ratio.