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Author Topic:   How Gear Ratio Affects Performance
Nauti Tauk posted 05-12-2009 07:04 PM ET (US)   Profile for Nauti Tauk   Send Email to Nauti Tauk  
I can't seem to get a firm grip on how gear ratios in the lower unit affect performance. A close friend has a new E-TEC 90 and I have a 2003 Mercury FOURSTROKE 90. Both boats are [share the model name] Montauk. His is a classic and mine is a 170. Our boat speeds at wide open throttle are dramatically different, 36 vs 44-MPH when similarly loaded. His engine turns a 15-inch and I turn a 21-inch pitch propeller. Would the different gear ratios in the lower units account for this? If a lower unit has a 2.33:1 ratio does that mean the prop is turning 2.33 times as fast as the tachometer indicates or the other way around and much slower than the indicated engine speed? Why would the propellor pitches be so drastically different if both are rated at 90-HP? Another big difference is my engine turns 6,000-RPM at maximum and his is rated optimal at 5,250-RPM. Maybe this is a simpleton question but I can't get my head around the theory involved. One thing for sure, the E-TEC is a flat out gorgeous piece of engineering. Thanks
fishgutz posted 05-12-2009 08:03 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
OK, not a simpleton question at all. It can be very complicated. The engineers that design outboards will use different gear ratios to make better use of the power and torque curves (the curve of the line that would make up a graph of the relationship between RPM and horsepower and RPM and torque.)

If an outboard says it has a 2.33:1 gear ratio. It means the engine is spinning faster than the propeller shaft, 2.33 times faster. It would be like gearing down on a truck. To get speed from the gearing down you can use a steeper pitched prop. The engine is spinning faster but the prop is turning less. The steeper pitch moves more water faster and moves the boat faster or at least up to speed. Also it can use a larger diameter prop. Harder to turn so you'd better be geared down. It would be like putting big mud tires on an old VW bug. In any gear you will have to change the rear gearing to turn those big tires. Otherwise you will be bogged down all the time or it will take forever to get up to speed.

Generally speaking a gear ratio of 2.33:1 will work better for a heavy boat. An example of this is the Montauk 150 that uses a 60 HP Bigfoot motor. Bigfoot is the 2.33 gear case. The standard gearing is 1.85:1. The standard will push most boats up to speed nicely. But the Bigfoot can turn a larger diameter prop to move a heavier boat.

As for why two 90 HP motors don't push the same boat at the same speed can be attributed to a lot of other things. Perhaps it isn't quite the right prop. Weight could be different. Who know?

I'll crunch a few numbers and see what I can come up with.

fishgutz posted 05-12-2009 08:13 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
I think your friend with the 90 E-TEC ought to try a 17 inch pitch prop. I think performance would improve.
fishgutz posted 05-12-2009 08:17 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
Check the Evinrude website performance reports for the 90 E-TEC. You could get some ballpark performance numbers there.
Tom W Clark posted 05-12-2009 08:27 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Which boat goes what speed?

A 13-1/4" x 15" Stiletto is correct for the E-TEC 90 on a classic Montauk. It is not correct if the prop is a BRP/OMC propeller. The 13-7/8" x 17" SST or Viper is the prop to use if a BRP model is used.

For whatever reason, it is generally true that four stroke outboards have higher RPM recomendations than two strokes.

dg22 posted 05-12-2009 08:34 PM ET (US)     Profile for dg22  Send Email to dg22     
With a 2.33:1 gear ratio, the prop only spins around once for every 2.33 rotations of your motor. Race outboard motors typically have a 1:1 gear ratio and therefore use lower pitch props. Regardless of the gear ratio, the trick is to pick the right prop to max out the RPMs on your motor's full throttle operating range. Most likely your friends setup is doing exactly that. The Classic Whaler is a lighter boat so that does help. If you want to close the gap, the first thing I would try is a slightly lower pitch prop on your boat (maybe a 19 pitch prop with the same diameter).

Nauti Tauk posted 05-12-2009 08:43 PM ET (US)     Profile for Nauti Tauk  Send Email to Nauti Tauk     
Thanks to everone. It's quite abit clearer now. Incidently my 170 is the faster of the two. I think when propped correctly the older boat will run several miles per hour faster. The analogy of the VW with mudders helped a great deal along with a wonderful "mental picture"!
Dick E posted 05-12-2009 09:03 PM ET (US)     Profile for Dick E  Send Email to Dick E     
Jerry your boat running well. 44 mph and your turning at the Max rpm of 6,000. If the E-tec owner is going 5200 rpm he maxed out the rpm turning more pitch will cut is rpm down. Merc's are fast!
jimh posted 05-13-2009 01:20 AM ET (US)     Profile for jimh  Send Email to jimh     
Different gear ratios are used to produce a propeller shaft speed that is consistent with the propeller design. A particular propeller design is optimized for a particular speed range and propeller shaft RPM.

For example


6,000 RPM x 1/2.33 = 2,575 propeller shaft RPM

Evinrude E-TEC

5,250 RPM X 1/2 = 2,625 propeller shaft RPM

The E-TEC is spinning the propeller faster.

Horsepower generally increases with engine speed, but not always in an endless curve. Horsepower usually peaks before maximum engine speed is reached. The maximum engine speed is that which the manufacturer feels the engine can sustain over the long haul without tearing itself apart.

Horsepower is a function of torque times engine speed. Four-cycle engines tend to have less torque than two-cycle engines so they have to turn faster to make the same horsepower output.

Gearing down the engine speed increases the torque available to turn the propeller. This is another way the four-cycle motors make up for less torque output from the engine.

jimh posted 05-13-2009 01:23 AM ET (US)     Profile for jimh  Send Email to jimh     
There is quite a difference in the hull of a classic MONTAUK 17 and a new 170 MONTAUK, so I would not expect a head-to-head comparison of the boat speeds to be identical with the same motor.

I think a 2003 Mercury 90 FOURSTROKE probably uses an engine made by Yamaha and fitted to a Mercury mid-section and gear case. In 2003 Mercury did not have any four-cycle motors of their own design in the 90-HP range.

jimh posted 05-13-2009 01:57 AM ET (US)     Profile for jimh  Send Email to jimh     
You may find this REFERENCE article of interest to you. It discussions these two exact motors:

90-HP Motor Comparison
Two modern outboard motors compared

by James W. Hebert

ratherwhalering posted 05-13-2009 02:33 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
Your friend's performance stinks. He should be seeing 43MPH with that hull and engine. Adjust engine height and select an appropriate propeller, such as Tom sugguested, and his performance will improve.
Tohsgib posted 05-13-2009 02:40 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
I got much better than that with a 70hp on my classic-39mph. Is his GPS rreading nautical or statute mph?
Tom W Clark posted 05-13-2009 02:43 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
36 knots is 41.4 MPH which is closer to what we would expect.
Peter posted 05-13-2009 02:46 PM ET (US)     Profile for Peter  Send Email to Peter     
With the motor spinning 5500 RPM, a 15 inch BRP propeller (old school) is only good for about 36 MPH.
L H G posted 05-14-2009 06:05 PM ET (US)     Profile for L H G    
Gearing does matter, I believe. I got roundly blasted by the E-tec crowd a while ago for saying the E-tec 90 did not have optimal gearing, and now these comparisons seem to indicate that could be correct. All other modern 90's carry at least a 2.33 gearcase ratio. The engine is really not powerful enough to turn a 2.0 gearcase, which is carryover from the old OMC V-4, a more powerful engine that put out as much as a 135HP rating. Mercury's propeller data indicates props become more efficient with increased pitch, so the 21" the Mercaha is running through 2.33 gears is more efficient than the 15" the Evinrude is running through 2.0 gears.

Nautituak's data matches Whaler's data on the 170 with a 90 Mercaha, if I remember correctly, both indicating 44 MPH top speed. The E-tec is this example can run faster, no doubt, especially on a lighter weight old Montauk, but on a 170 Montauk it could not achieve the same 44 mph that the Mercaha 90 does. Not all 90's are created equal, despite what the decals say.

Tom W Clark posted 05-14-2009 07:11 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Gee. So all BRP should do is change the gear ratio of the E-TEC 90 and it will magically propel a Montauk eight MPH faster?

Boy! those BRP engineers sure are stupid.

Peter posted 05-14-2009 07:33 PM ET (US)     Profile for Peter  Send Email to Peter     
Hmmm, using Larry's theory, the BIG FOOT Mercury 60 FourStroke should be orders of magnitude faster than the standard Mercury 60 FourStroke and the Mercury engineers must be out of their minds offering up a small foot version. LOL.
number9 posted 05-14-2009 08:14 PM ET (US)     Profile for number9  Send Email to number9     
"Different gear ratios are used to produce a propeller shaft speed that is consistent with the propeller design. A particular propeller design is optimized for a particular speed range and propeller shaft RPM"

Uh? That first sentence, there's something wrong with the author's brain function.

fishgutz posted 05-14-2009 08:31 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
Larry is right...when it come to HEAVY boats or weak motors.

Think in extremes. Take a heavy Montauk 150. Put a 1:1 gear ratio on a 60 HP motor, put on a 6 inch diameter prop. You ain't going anywhere. You'll just be spinning your wheels. Now put on a standard geared 60 HP Merc. It'll run OK. But you won't get good acceleration and will probably be slippin like crazy. Now put on a Bigfoot Merc, large diameter prop and you have less slip and your moving along nicely.

Now try the standard gearing and a large diameter prop (if that were possible). The motor will have a hard time turning that big prop. You will have a hard time getting up to full speed. Hence the problem that Larry is talking about. Gear it down with those large diameter props now you'll be moving.

Go back and read my post about putting big mudder tires on your VW bug.

jimh posted 05-15-2009 12:32 AM ET (US)     Profile for jimh  Send Email to jimh     
I love critics who never explain themselves. Step right up, number9, and lead us to enlightenment.
L H G posted 05-15-2009 12:57 AM ET (US)     Profile for L H G    
Tom - I don't think the BRP engineers are stupid. Rather it might be that the BRP management people are cheap, and won't spend the money to develop a new gearcase, and ratio, for the 3 cylinder engines. Times are tough at BRP these days, like all places.

Another way of looking at it is to assume the BRP engineers are indeed brilliant putting a 2.0 ratio on their 90's, and that the Mercury, Yamaha, Honda, Tohatsu and Suzuki engineers are the real ones that are stupid for using a 2.33 gear ratio on their 90's, both 2 and 4-stroke versions.
Sometimes it happens that 5 out of 6 engineering departments, at five out of six companies, get it wrong.

jimh posted 05-15-2009 01:56 AM ET (US)     Profile for jimh  Send Email to jimh     
Read the article on the 90-HP comparison. It shows that the propeller shaft speed is quite similar between the motors. Torque is close, too. This destroys the myths or FUD being spread.
dg22 posted 05-15-2009 10:31 AM ET (US)     Profile for dg22  Send Email to dg22     
One thing for sure gear ratio affects more than just performance. Since many people are unaware of the gear ratio on their motor, it is an easy way for engineers to distinguish their brand from the others; whether it is bottom end power or top end speed. In the 70's Merc’s were know as fast but little power for skiing. If you look at the gear ratios used by Mercury they were very high 2:1 or 1.87:1. OMC, on the other hand, used the very low end gear ratios 2.67:1 on some of their models with those huge gear cases which would give tones of power. I remember as a kid my neighbour pulling 4 of us skiing with his 1967 3cyl, 55HP Johnson. On the other hand, our 6 inline, 115 Merc was screaming fast but brutal for skiing. We eventually bought a really low pitch prop to use for skiing to compensate for the high gear ratio on that motor.

jimh posted 05-17-2009 02:33 PM ET (US)     Profile for jimh  Send Email to jimh     
So far the only paradigm I can infer from the discussion is that the gear ratio used on a Mercury motor will produce better performance than the gear ratio used on an OMC or BRP motor, even when the use of the gear ratios is inconsistent or reverses the selection of gear ratios from time to time. That make little sense, so I will set it aside and present a more rational explanation.

As I said before, the choice of gear ratios is made to reach a propeller shaft speed that is in the design range for the propellers available. I will elaborate on this as it seemed to be incomprehensible to some readers.

Let's say we have an engine with a gear ratio of 2:1 that reaches 5,000-RPM. and it turns a propeller with 19-inch pitch. The propeller shaft turns at 2,500-RPM. The propeller advances 19-inches with each revolution. This implies the boat should go 45-MPH. Due to SLIP, it goes more like 40-MPH.

We could imagine that this same boat could be rigged with a 1:1 gear ratio, and then when the engine turned 5,000-RPM the propeller shaft would turn at that same speed. We'd have to drop the propeller pitch to 9.5-inches, and we'd be back in the same boat speed range, 45-MPH with no SLIP, and maybe 40-MPH with some slip. The reason this is not done is because propeller designers have most likely found that a propeller with 9.5-inch pitch is not going to work very well.

Propeller design is really quite complex. Propellers have been under continuous study, refinement, and development for 150 years. There are entire naval architecture firms that specialize in propeller design. So we don't just pick random pitch propeller out of the sky and think they'll work. The people who make it their business to design propellers for use on boats of a certain size and horsepower range have developed propeller that will work well for a certain range of speeds and loads. They present these propellers for us to use.

In the case of an outboard motor boat that needs to go around 40-MPH, we can get a propeller with a 19-inch pitch and we need to turn it at a propeller shaft speed of around 2,500-RPM. To do this we will need a certain amount of horsepower and provide that horsepower with a certain amount of torque. Let's say we need 90-HP to accomplish this.

If an engine doesn't make 90-HP until it winds up to 5,800-RPM, then we have to gear that engine down. The gear ratio will be 5800:2500, or 2.3:1.

If an engine makes 90-HP at 5,000-RPM, then we gear that engine down to turn the propeller at 2,500. The ratio is 5000:2500, or 2:1.

As far as the propeller is concerned, it cannot tell which engine is turning the propeller shaft. The shaft turns at 2,500-RPM, the propeller pushes the boat forward, and off we go. The gear ratio has no effect on performance.

jimh posted 05-17-2009 02:55 PM ET (US)     Profile for jimh  Send Email to jimh     
Another way to consider propeller design is to think of a fan, a fan like you have around the house. A fan moves air. Some fans are designed to move air at low speeds and some are designed to move air at high speeds. The fans have different shapes and different blades. A ceiling fan moves air, but it is designed to move air where the air coming into the fan is low velocity. The intake fan on a jet engine is designed to move air, too, but it is designed to move air where the inlet air speed may be 400-MPH. The fans have different shapes and different kinds of blades. The same for boat propellers.

A propeller that is on a boat going 40-MPH has water coming into the blades that is already going 40-MPH. It has to accelerate that water and push it efficiently out the back. This requires a different design than a propeller that is going to be working at 5-MPH. The water coming into the propeller is moving much slower. These effects produce changes in the design of the propeller.

In the case I gave about of a boat going 40-MPH, the propeller designer has optimized the propeller design for this speed range. It has 19-inches of pitch and likes to turn at 2,500-RPM.

Now image we have really powerful motor and we could run it at 1:1 gearing and still have plenty of power to turn the 19-inch pitch propeller. We could turn it at 5,000-RPM at the propeller shaft. Would this same propeller now push the boat to twice the speed? Instead of 40-MPH would we go 80-MPH?

Probably not. The 19-inch pitch propeller is not designed to run at a shaft speed of 5,000-RPM and it will begin to lose efficiency. What we do instead is change the design of the propeller, increase the pitch, work with the blade angle of attack so that it is effective when water is flowing into it at 80-MPH. We probably wind up with a propeller that has a 29-inch pitch and needs a propeller shaft speed or about 3,166-RPM. If we used a motor that ran at 5,500-RPM, we'd need a gear ratio of 5500:3166, or 1.74:1.

With particular combinations of motors and their gear ratios, propellers and their designed speeds, and boats with a particular loading, we find that sometimes we get a better fit between all three parameters and the result is better performance. Did that come about because a motor used a certain gear ratio? No, it just happened that the motor's gear ratio was a better fit for a certain combination of boat load and propeller design.

When outboard engine designers offer a gear ratio, when are making a guess about what kind of boat load the motor will be used with and what kind of propellers will be used.

I get a huge laugh when LHG says that BRP is "cheap." BRP offers their 90-HP motor with two choices of gear ratio. That is hardly cheap. That is just LHG with his usual pro-Mercury rhetoric. BRP offers a 90-HP in a 20-inch shaft with 2:1 gears. It offers a 90-HP in a 25-inch shaft with 2.25:1 gear. The logic seems simple. A 20-inch shaft motor is probably going on a lighter boat than the 25-inch shaft motor. The 25-inch shaft motor is geared down a bit to work better with heavier boats.

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