Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  Lower Unit Gear Ratio vs Propeller Pitch

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Lower Unit Gear Ratio vs Propeller Pitch
mr_schuster2000 posted 11-29-2005 02:55 PM ET (US)   Profile for mr_schuster2000   Send Email to mr_schuster2000  
It is a fact that increasing prop size decreases RPM's as well as top speed.

Theoretically speaking, if I wanted to increase boat speed by spinning a higher pitched prop and not lose RPM, would one replace the gears in the lower unit with a higher tooth count? Is this possible?

bsmotril posted 11-29-2005 07:10 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
It is possible in some cases, the small block V6 Optimaxes comee to mind. They come with a 2:1 on the 135HP, and 1.86:1 on the 150 and 175s. Those parts will interchange, they use the same shafts, bearings, and housing. I think it is a question of economics more than anything else as to why it is not done. You can repitch, or purchase and install a different prop a lot easier and cheaper than it would be to buy and install new gears. Doing the gear swap takes some expensive gears, quite a bit of labor, special tools in many cases, and a resealing kit for the gearcase. Also, there are quite a few gearcases that only have a single ratio available, mainly the smaller 2 and 3 cylinder models. For some motors like OMC V4s, you can use the V6 gearcase on a V4 Motor to get a broader selection of ratios. Even so, when you get to bigger block motors, you are also limited in most outboard gearcases to only two, or maybe three optional ratios because the compact design of the housing(reduces hyrdrodynamic drag) limits the physical size of the gears that will fit inside.
jimh posted 11-29-2005 08:48 PM ET (US)     Profile for jimh  Send Email to jimh     
Your hypothesis: if the owner could just change the lower unit gear ratio they could make the boat go faster.

The motor turns the propeller as fast as it can. You make the boat go faster by getting a bigger motor.

If making the boat go faster was just a matter of picking the proper gears for the lower unit, I think the hundreds of thousands of man-hours of engineering that have gone into the development of the outboard motor and propellers would have discovered this path to better performance.

Chuck Tribolet posted 11-30-2005 08:42 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Or to put it another way, your outboard has two sets of
gears. One set is inside the lower unit and meshes with
each other. The other set consists of the prop meshing with
the water. What counts is the two ratios multiplied together.


jimh posted 11-30-2005 08:54 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--I like your analogy. However, the "gear ratio" of the propeller is a bit variable. It changes with speeds.

Perhaps these article will help:

See the section "Fluid Drive."

Binkie posted 11-30-2005 12:43 PM ET (US)     Profile for Binkie  Send Email to Binkie     
There is more to speed than changing gear ratios. Take for instance if you had say a class B racing runabout, with a Mark 20-H outboard installed on it. (This is a 20hp Mercury race motor, with racing lower unit). It has a streamlined gearcase, and runs 1:1 gear ratio, and is 12" from the clamp to the cavitation plate. You can probably run about 60MPH. Now take the same boat and put a Mark 20 "fishing" motor on it. It has the same powerhead as the race motor, but a normal 15" fishing lower unit. It has 2:1 gears, a non streamlined gearcase, and the prop is sitting 3" lower in the water. Your speed would be in the high 30MPH range. You have to remember that the boat is extremly lite. The difference is the race motor prop is turning at the same speed as the powerhead 5500RPM, while the fishing motor prop is turning at half that speed. To be able to turn at that speed the race prop, (2 or 3) blade has a very small diameter, and a big pitch, and is running with only the bottom half of the prop under the water. (There was another thread a few months ago, that stated the deeper the prop, the faster you go. Completly wrong idea) This whole post is not relative to any set up on a Whaler, and changing gearcases to a different ratio would only be an expensive way to do the same thing you could do with a different prop, if your current prop was not as efficient as it could be. I guess what I`m trying to say is buy changing gear ratios you are opening up a whole new can of worms to work with.
LHG posted 11-30-2005 11:53 PM ET (US)     Profile for LHG    
Gear ratios are selected by the manufacturer based on the engine's prop HP output, the lower gearcase ratios (higher numerically) being for the least powerful engines. Since props become more efficient within a given design line as pitch increases, the lower gear ratio gives the lower HP engine, like a 135 Optimax, the ability to use higher pitch props. Conversely, you can tell and engine's HP output by the gear ratio, the higher ratio (numerically lower) usually being a more powerful engine, within a given technology, such as EFI or DFI. This is why the 3.0 liter Mercs use a high ratio of 1.76, as do the big block Yamahas. They have the torque and power to handle it, while still turning high pitch props.

When the Bass and Walley mag shootout of the Merc 225 XS Opti agianst the Evinrude 225 HO occurred, one of the things the Evinrude guys blamed for the poor performance of the E-tec was it's lower 1.87 gear ratio vs the Opti's 1.76.
I'm not sure I completely understand why, but that's what was being said. It may have to do with how fast the prop is turning in the water for a given pitch?

All the above relates to high torque 2-strokes. The 4-strokes are a another matter, since acceleration tends to be bad, except for the Verados. Interestingly, because of the Verado supercharging and 2-stroke like acceleration, it can use the same Merc gearing of 1.87, the highest of anybody's 4 strokes. The reason even the L6's don't use the higher 1.76 ratio of the Opti's, is their higher redline of 6400. The Japanese companies are using higher gear ratios to help out with acceleration, which has the higher pitch prop turning faster in the water.

All this gear ratio stuff is way over my head, but I assume the respective engineers know what they are doing.

sosmerc posted 12-02-2005 01:36 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
You will note when comparing the performance of a 135 Opti vs. a 150 Opti on the same Whaler hull, the 135 will be faster from 0mph to "on-plane". The is because the 2:1 gear ratio of the 135 enables it to develop more torque than the 150 with the 1.87 gear ratio. Both engines are the same 2.5 litre displacement, but the greater the gear reduction, the greater the torque. (if you put a smaller pitch prop on the could make that 150 meet or beat the 135).
Increasing gear reduction is another way to compensate for lost power when running above 2500ft. in sea level.
Expensive torque-shift props like the Aerostar can really improve performance as well...but they priced themselves out of the market!!
bigjohn1 posted 12-02-2005 08:10 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Let me throw this into the mix...I own a 2004 Merc 115efi with 2:07-1 gears on my '05 170 MONTAUK. The highest pitch prop I can realistically swing would be an 18. I say this based on the fact that my engine was supplied with an aluminum Black Max 19P that would not develop more than about 5400 rpm's at WOT. Moving down to a 17P Black Max helped it get much closer to the correct rpm range at WOT. Then I switched to a 17P Trophy Plus 4-blade ss unit. WOT rpm's are nearly identicle to the 17P Black Max (maybe within 100 rpm's or so). If I had the 2:33-1 gears of the 90 4-stroke, in theory, I could easily "move up" to a 20 inch performance prop (such as the Laser-II) and go faster. For reference, CW member AQUANUT was running a 16P Vengenace on this very same rig and was just barely hitting his rev limiter (6100 rpm) at WOT and very lightly loaded. When he was running fully loaded, he would switch down to a 15P.

I think running a 15P or 16P prop on a Merc 115efi powering a light 170 MONTAUK is an indication that this engine is geared to high (at least for the 170). So, to answer the originally posed question,

"Theoretically speaking, if I wanted to increase boat speed by spinning a higher pitched prop and not lose RPM, would one replace the gears in the lower unit with a higher tooth count?"

Based on my experience with my particular rig, I think the answer is - YES. Enineering be damned, an over-powered 170 MONTAUK should be able to easily swing a 20P prop.

DaveH posted 12-02-2005 10:14 AM ET (US)     Profile for DaveH  Send Email to DaveH     
Not to be too nitpicky, but an engine develops torque in units of lbs-ft. You cannot increase torque by gearing, it is a fixed point on a curve of engine output per RPM. When you place a prop on a shaft and transfer mechanical energy to a fluid, the units are in inches of pitch like a gear. The torque output and pitch cancel "inches" out of the equation and the force is measured as thrust in pounds or Newtons. So the correct terminology is "the engine can develop more thrust with a different pitch prop or different gearing"
sosmerc posted 12-02-2005 11:33 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
I'll buy that DaveH. Thanks for the correction.
mr_schuster2000 posted 12-02-2005 06:23 PM ET (US)     Profile for mr_schuster2000  Send Email to mr_schuster2000     

Help me better understand your point that it's not mechanically possible to increase torque by gearing.

If an engines torque is rated at the crankshaft, then it becomes a fixed point on a curve based on the engines output per RPM. The torque of the engine will never change. But when this crankshaft torque is transmitted through the lower unit gears, the torque at the prop shaft, not the motor, will either increase or decrease, depending on the lower units gear ratio. The goal of gear reduction is to increase torque and decrease the workload of the engine.

Theoretically speaking, if an engine can achieve its maximum RPM's, say 6000, with a 17" prop, and one wants more top end speed from that engine, he/ she would have to make modifications somewhere. I would start with the prop. Lets run a 19" pitch prop. The new larger 19" prop has decreased the engines max. RPM's by 400 and the overall top speed. If it were POSSIBLE to change the lower units gearing, then why wouldn't that engine be capable of increasing the torque and rpms at the prop shaft, as well as increase overall speed from the higher pitched prop?

I may be incorrect. I am not a outboard specialist or an engineer. I do know that aside from boats, many individuals who own trucks with oversized tires change out their final drive gears to make up for the loss of horsepower and torque. I am aware that that tires are pushing asphalt and props are pushing water, but the comparison stands in the final gearing.

I take constructive critism well, so if I stand incorrect please let me know!

Binkie posted 12-02-2005 08:17 PM ET (US)     Profile for Binkie  Send Email to Binkie     
In real life,if you have a fishing or cruising boat it would be foolish to change out gears to obtain more torque or speed.All you can do is test different props and engine setups to obtain the best possible torque or speed for your boat. Bare in mind that torque and speed don`t go together, you have one or the other. If it doesn`t work out to your liking, get a bigger motor.
brisboats posted 12-02-2005 09:33 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Mercury marine will use gear ratio differences to move a larger heavier boat onto plane sooner and propel a lighter less loaded boat to a higher top speed. The V-8 mercruiser sterndrive behind a 260 horsepower motor can have several different ratios in it's upper gearcase. Depending on the size and weight of the boat and the intended use. For example a light 19 foot runabout will often run the 1:5:1 ratio while a 24 foot cruiser may be equipped with 1:65:1 or the 1:84:1 gearsets depending on the weight and elevation where the boat is used. Because a 150 horsepower outboard may be placed on a wide variety of boats in either a single or twin configuration. It stands to reason that ideally there is not a one size fits all gear ratio in the lower unit of an outboard. The maufacturer must make a decision on which ratio will serve the broadest range of use. OMC recognized this in fitting the commercial V-4's with a 1:86:1 ratio as they realized that these motors would see extreme use and be placed on heavier working boats in most cases. While the recreational V-4's typically ran I believe a ratio of 2:1. As has been stated above you are better served to experiment with different propellors in design and pitch than to start interchanging the gears in your lower unit.


bigjohn1 posted 12-03-2005 07:17 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
"In real life,if you have a fishing or cruising boat it would be foolish to change out gears to obtain more torque or speed."

That's one man's opinion...if everyone who ever turned a wrench on a car, boat, or motorcycle followed that line of reasoning, what a boring (and expensive) life it would be. In the automotive world, if somoeone says the only way to get more speed out of car would be to install a larger engine, they are over-looking many cheaper alternatives.

While we can't cheaply perform all the extensive modifications such as boring, stroking, camming, etc, etc, etc to give our outboards more power, what's so foolish
about changing a gearcase ratio? As long as you prop it right so it won't over-rev, no damage will be done right?
Ring and pinion gears in car/truck rear ends are changed all the time to alter either the high-end or low-end performance of that car or tow vehicle. If you have the special tools and mechanical aptitude, why not. If you don't have the tools and ability (but have the money to burn) have an outboard shop do it for you.

DaveH posted 12-03-2005 10:33 AM ET (US)     Profile for DaveH  Send Email to DaveH     

If you look at an engine at the crankshaft, for every change in RPM, there is a unique measured Torque value and Hp value.

Torque is measured in Ft-lbs
Horsepower is measured in Ft-lbs/second

If you think about what that means, Torque is the amount of force able to twist the shaft and Horsepower is how fast we can apply that twist force.

An engine creates a fixed amount of torque and horsepower at say 500 RPM throught the crankshaft, that value changes at 501 and 502 RPM. When we apply a shaft, gearing and a propeller and place that engine in water, we now have a load, or a resistance to the torque. We have not changed the engine at all, just the ability to apply the engine's output. We did not increase the torque value at all.

Now, I mentioned that you cannot increase an engine's torque by changing the gearing-that is true. My whole point of the comment was to identify that you can increase thrust, the proper terminology, through gearing, prop pitch, prop diameter, etc. By changing the gearing, you can take advantage of the torque an engine can apply, which in turn, increases the amount of thrust applied through the propeller by the water it moves.

Tom W Clark posted 12-03-2005 11:15 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I think schuster is making the point that reducing the gearing will increase torque at the prop shaft. You are making the point that a motor's power head will produce the same torque regardless of how the gearcase is geared.

You are both correct.

mr_schuster2000 posted 12-03-2005 11:29 AM ET (US)     Profile for mr_schuster2000  Send Email to mr_schuster2000     

I fully agree with your statement!

To answer my initial question, it should be possible, through gear reduction, to increase the prop's pitch, maintain the engines max. RPM's, increase thrust, and the overall speed of the vessel?

Right or Wrong?

Tom W Clark posted 12-03-2005 11:32 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The answer to your question is no. You can do everything right up to the last part "...increase thrust, and the overall speed of the vessel" There is no reason why that would necessarily happen.
DaveH posted 12-03-2005 11:47 AM ET (US)     Profile for DaveH  Send Email to DaveH     
Mr. Schuster2000 and sosmerc:

Acccck! I need to stop trying to oversimplify explanations because I just reread what I wrote, and it is not correct.

I was trying to explain that the engine puts out what it puts out power-wise. I thought that you were saying the engines torque output was increasing. Propshaft torque CAN be increased through gearing with the sacrifice being rotational speed. My point was the end result is to generate thrust, which is actually what drives the boat.

Sorry Mr. Schuster and sosmerc, you are both absoutely correct about gearing and prop torque.

mr_schuster2000 posted 12-03-2005 11:57 AM ET (US)     Profile for mr_schuster2000  Send Email to mr_schuster2000     
Last post from me on the subject, then i'm going to bury it.

Tom Clark,

If you are running a normal engine set-up, including the prop, say a 17" pitch. You re-gear the lower unit with a higher gear ratio using that same prop, the motor will produce more torque at the PROP SHAFT and most likely over-rev. So, why can't you take advantage of the additional prop shaft torque by installing a higher pitched blade, in theory, my theory I guess, it would increase the vessels speed and bring the engines RPM's back under control.

I don't know, I'll stop bugging everybody at this point and by a speed boat. Thanks to all!

mr_schuster2000 posted 12-03-2005 12:04 PM ET (US)     Profile for mr_schuster2000  Send Email to mr_schuster2000     

We're on the same page and my point is this.

Gear reduction increases prop shaft torque at the cost of increased engine RPM's. Why then would it be impossible to run a higher pitched prop, bring the RPM's back to normal, and increae the speed?

DaveH posted 12-03-2005 12:12 PM ET (US)     Profile for DaveH  Send Email to DaveH     
Mr Schuster:

To fully answer your question, here is why you sacrifice rotational speed when you obtain a torque advantage:

The equation for transformation ratio of two gears is based upon the number of teeth in the gear:

Gear 1 has N1 teeth, T1 torque, w1 rotational (angular)velocity
Gear 2 has N2 teeth, T2 torque, w2 rotational (angular)velocity

The relation ship is:

Transformation ratio= N1/N2 = T1/T2 = w2/w1

As you can see, if you increase torque through gearing, you loose angular velocity. That is why gearing and prop selection is such an artform. It is a true give-and-take.

DaveH posted 12-03-2005 12:21 PM ET (US)     Profile for DaveH  Send Email to DaveH     
You posted while I was typing. The answer is you can potentially increase boat speed with a higher pitch prop, but you cannot maintain the same RPM, which is what you initially were asking.

There are also components of drag (as well as other forces) which we haven't discussed. Drag will apply more and more as speeds increase. We have been discussing only one component of a very complicated set of dynamics which affect the boat.

sosmerc posted 12-03-2005 12:26 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
Mr_Schuster2000...You started an interesting that can be very confusing and hard to grasp.
I don't believe you ever told us what hull and motor and prop combination you have and specifically what your objective or goal is.
I think in a nutshell, you are after more speed, without over-reving the engine. This is a matter of testing with different setups (different props, different engine heights, etc). You could experiment with different gear ratios too, but due to the cost and your objective, this is rarely the coarse taken (unless you have a high altitude application).
I have run the Aerostar torque-shifting props in the past, and they are pretty amazing in that they can give you the best of both worlds...awesome holeshot with the prop in its low-pitch takeoff mode, yet great top speed at high rpm when the prop has settled into its high-pitch setting.
Before you sign off on this thread, can you tell us more about your rig?
mr_schuster2000 posted 12-03-2005 03:02 PM ET (US)     Profile for mr_schuster2000  Send Email to mr_schuster2000     

I have a 1987 15'SS with the original, worn out, Johnson 70 HP, running a 3 blade 13x19" SS prop! I have the need for speed always and would love to gain more top end! The engine will turn 5800 RPM with the 3 blade 13x19" SS prop. Increasing prop pitch at this point would ultimately lower my RPM's and top end. Havn't tried a 4 blade prop of the same pitch.

I am really not interested in spending the money on a new gear set or even a lower unit with a higher gear ratio. The cost would be absurd. I was just curious if it were "possible" to re-gear or replace the lower unit so a higher pitched prop. could be installed, ultimatley increasing speed. It was just a question of theory!
Aside from purchasing new lower unit, boat, or 90HP engine, anyone have suggestions on increasing top speed of this rig?

Mike S.

sosmerc posted 12-03-2005 08:57 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
Unless your 70hp is really tired, I think you could easily go to a 21pitch...that motor should still produce good power even it the rpm's get "dragged down to say, 5300...and you should be a bit faster. I have run 15 Whalers with comparable horsepower...and they can be made to fly! You will need either dual cable steering, or hydraulic steering to reduce the steering slop as much as possible...this slop causes chine walking and makes it hard to control the boat, thus hurting it's top speed potential. With a good stainless prop, like Merc's Laser II or Michigan Rapture, you should be able to raise your engine as high as it will go on the transom. I assume you have power trim? You need it, if speed is your objective.
Also, a hydraulic jack plate will give you the ability to really "dial in" your setup!
I once rigged a 15 Sport Center Console model with a 1985 4 cylinder Merc 75hp. We put dual cable steering on it...raised it way up on the transom and ran a Merc high performance "Chopper" prop...I think it was a 20 inch pitch. The boat ran over 50 and got very, very loose at top end. I think this was just about the limit for this particular boat.....maybe with trim tabs and solid engine mounts there may have been few more miles an hour left in it...but it felt like it was going 100!!
jimh posted 12-05-2005 09:12 AM ET (US)     Profile for jimh  Send Email to jimh     
An engine's maximum speed reacts to the load placed on it. The heavier the load the lower the maximum speed.

A boat's engine sees the load of the propeller through the gear reduction which drives the shaft. The total load on the engine is a combination of the gear reduction and the propeller load.

If you change the propeller pitch you change the load on the engine.

If you change the gear ratio you change the load on the engine.

The engine does not care where the load is coming from. It cannot tell. If the load increases due to propeller pitch increase the engine slows down. If the load increases due to a gear ratio change, the engine slows down. For heaven's sake, this is seventh-grade science class stuff!

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.