posted 12-09-2007 09:07 PM ET (US)            

I don't think the gear ratio is really a significant factor in choosing an outboard motor.

posted 12-10-2007 08:13 AM ET (US)            

The gearcase on the HIGH THRUST version is larger than the gearcase on the regular version. The larger gearcase enables the use of a larger diameter propeller (13 inches versus 11 inches) which provides more blade surface area and the ability to produce more thrust at low engine speeds. However, because a larger diameter propeller is more resistant to twisting in the water due to its larger blades, it is harder for the powerhead to turn the propeller. Thus, to help the powerhead overcome that resistance, the gear ratio is increased to provide greater mechanical advantage. The downside of the larger gearcase is that it produces more drag at speed.

posted 12-10-2007 11:24 AM ET (US)            

John gives the most accurate answer: It depends. It is not as simple as a higher or lower gear ratio being better or worse. The gear ratio works in conjunction with the propeller to convert powerhead energy to forward propulsion.

Different manufactures take different courses with the gear ratios they choose for their motors. Even within a given manufacture's line of motors, the gear ratios are not all the same.

Gear ratio is usually expressed by the number of revolutions of the power head for one revolution of the propeller. Thus a motor with the common 2:1 gear ration will have its crankshaft spin twice for one full revolution of the prop.

As a general rule, lower gear ratios (higher numeric value) are used with larger propellers. But this "rule" breaks down fairly quickly once starts to examine all the offerings from different manufactures.

I have found that usually a manufacturer will use higher and higher gear ratios as the horsepower increases. Thus we may see that a manufacturer installs a 2:1 gear ratio lower unit on their 150 HP models, a 1.86:1 gear ratio lower unit on their 200 HP models and a 1.75:1 gear ratio on their 250 HP models.

The motors used in my example are all V-6 outboards using the same physical size of lower unit and thus the same range of propeller diameters. The propellers uses 150 HP motors are typically the same as used on the much higher HP models, motors as powerful as 275 HP. In order for those higher horsepower motors to deliver that power to the water with the sam range of props, the propeller simply has to spin faster, thus the higher gear ratio (lower numeric value.)

It is certainly NOT the case (as Peter asserts) that gear ratio is not important unless you are looking at a high thrust application. Lower unit gear ratios are not just for high thrust motors as evidenced by Suzuki's use of geared down lower units tuning larger diameter propellers. Suzuki outboards are not just for pontoon boats, yet they have gear ratios as low as 2.5:1 or even lower.

It is not even true that that motor models designed expressly as "High Thrust" are only to be used on Pontoon boats or other high weight, low speed craft. Sometimes the lower gearing and larger propellers are better for small light weight craft that are quite fast.

The 15 foot Whalers made by Boston Whaler are perfect examples of this phenomena. Whaler equips some of their models with the Mercury "BigFoot" 60 HP outboards instead of the smaller gearcase versions precisely because the boat performs better, plan and simple. This, in spite of the fact the BigFoot models were originally designed with the pontoon boat market in mind, not light weight skiffs.

The same will hold true of the Yamaha High Thrust models if one were contemplating buying one for a small Whaler.

I think the point Jim was trying to make was that if you are looking at a motor for your boat, you do not get a choice of optional gear ratios for a particular motor and thus you should concern yourself with the motor and not the gear ratio. But even this is not always true as evidenced by the Mercury BigFoot/Yamaha High Thrust example given above.

Certainly if one were considering repowering a boat one might look at the Suzuki outboard line precisely because they are using a novel approach of lower gear ratios and larger propellers and are dong so with great success. So much success in fact, that I suspect as the years go by more and more manufacturers will be adopting lower gear ratios for higher horsepower outboard motors.

Of course every lower unit an its gear ratio only works at all because it is spinning a propeller and here are LOTS of different propellers out there. Propeller design and selection is a VERY broad topic an one best visited in another discussion.

posted 12-10-2007 12:47 PM ET (US)            

Given that a new Whaler 150 Sport weighs in at a portly 900 lbs dry with no motor (compare that to the classic 15s 550 lbs) it's not a pontoon boat by any means but its not lightweight skiff. With a wider beam it's also got more hull to push through the water than the classic 15 does.

The evidence on CW seems fairly clear that the Yamaha F60 (small gearcase) performs better on a lightweight 15 Classic than the Yamaha F60 HIGH THRUST.

I will say again that gearcase ratio is basically irrelevant when choosing among different outboard motor brands of equal HP. The gearcase ratios are chosen by the manufacturers to optimize the peformance of their powerheads throughout the operating range (idle to WOT).

posted 12-11-2007 01:43 AM ET (US)            

What I am trying to say is that the reason for engine gear ratios to have relatively minor differences as horsepower varies is to better suit the engines to series of propellers. Each manufacturer makes a line of propellers in various pitch increments. My feeling is--sorry I don't have much science to back this up--that by making a few gear ratio changes among the various engines the engines can be matched up this standard line of propellers.

The alternative situation would be one where all engines used the same gear ratio--let's say 1:1--and then there would have to be a wider range of propellers available.

posted 12-11-2007 09:07 AM ET (US)            

All depends on the motor manufacturer. Johnson/Evinrude have been using 1.85:1 gear ratios on their 150 through 250 HP motors forever, for example. Yamaha uses a 2:1 ratio on its 150 through 250 HP 4-strokes. On the other hand, Honda and Suzuki and Mercury use different ratios on their 150 HP 4-stroke models versus their 225 4-strokes.

posted 12-11-2007 09:37 AM ET (US)            

Come on, I don't think anybody selects an outboard based on the gear ratio alone, but it is not something you just ignore either.

posted 12-11-2007 10:45 AM ET (US)            

The proposition that a high gear ratio universally permits the use of a larger diameter propeller is unfounded. Take for example, an 18 Outrage with equipped with a conventional 2-stroke Johnson 150 with a 1.85:1 gear ratio versus one equipped with a Suzuki DF 150 and a 2.50:1 gear ratio. Assume that 18 Outrage can go 45 MPH with a 150 and 6 percent slip. Propeller calculator suggests using a 17 inch pitch for the Johnson and a 21 inch pitch for the Suzuki. The propeller calculator results are consistent with my own experience with my current 18 Outrage/Johnson 150 for it is rightly equipped with a Mercury Mirage Plus 17.

Now, according to the propeller offerings by Suzuki, the largest diameter propeller they have for the DF 150 is a 15 1/2 inch. That diameter is no larger than the diameter of the Mirage propeller that I use now with a motor having a 1.85:1 ratio and the Suzuki can't use Suzuki's 15 1/2 x 17 inch pitch propeller for the 18 Outrage application because the pitch is too low. For the 21 inch Suzuki propeller, the diameter shrinks down to 15 inches. So much for so called low ratio large diameter advantage.

posted 12-11-2007 11:35 AM ET (US)            

Doesn't a higer gear ratio mean lower efficiency for the gear box? I think the higher ratio would rob power from the overall outboard system.

posted 12-11-2007 11:59 AM ET (US)            

Come on Peter, now I know you're teasing us. You don't seriously think gear ratio is irrelevant? Sales pitch? As if the engineers at Suzuki told the marketing department: "...oh you guys pick a pretty number and we'll build the gear case with that gear ratio." hahahahahahahahaha!

I think a motor's gear ratio is a little more than merely cosmetic. It is, in fact, more significant than the color of paint used.

It is simple physics. Lower gearing allows larger propellers to be used. While you may be able to run a MIRAGEplus on a Johnson 150, you certainly cannot run a 16-1/2" diameter Suzuki prop on that same motor. If you try, you are going to have a badly mangled gearcase, prop or both.

Wild Turkey,

posted 12-11-2007 10:27 PM ET (US)         

I have been blasted for this before by the Evinrude people, but I will say it again. In the 75 and 90 HP range, the Evinrudes have the LEAST desireable gear ratio of 2.0. All other brands have better, lower (higher numerically) ratios, mostly around 2.33. Evinrude's 2.0 gear ratio forces E-tec 90 owners to run very low pitch props, 13, 15 or 17's, and is too high for a 75 or 90 HP 3 cylinder engine. Mercury last used a 2.0 gear ratio on the 1988 in-line 6, a high torque engine, but lowered it to 2.33 when the new 3 cylinder 90's came out, so they could turn more efficient, higher pitch props like the Laser II's. This shows in the engine's top perfromance among 90's, and is also showing in the new Verado block 90 EFI's, a hot performer according to Boston Whaler. OMC also used the high 2.0 ratio for it's V-4's, propably a good decision, but this engine also is more powerful than the 3 cylinder E-tec 90. In short, BRP should spend some R&D and give the E-tec 75 and 90's a better ratio gear case, rather than using the old leftover case from the 80's.

Currently Mercury correctly uses the 2.0 gearcase on the 135 Optimax, and earlier 2.0 liter 135/150's, V6 engines a LOT more powerful than the E-tec 75 or 90. The high performance 4 cylinder Verados, 135-200HP even have a brand new 2.08 ratio gearbox. This has got to tell you something. The small E-tecs, no more powerfrul than the Optimax 75 and 90 with 2.33 ratios, are badly handicapped by this high gear ratio, and forced to run low pitch, non-performance oriented props because of it. The inefficient props show up in decreased economy and less top end.

posted 12-12-2007 12:01 AM ET (US)            

Could we have a cite to where Mercury says that higher pitch is more efficient?

My understanding is that efficiency comes from the choice of the angle of attack of the blades. The angle of attack come from two factors: the speed of advance and the actual angle of the blades. Therefore, a propeller of any particular pitch can be made efficient if it is designed for a certain speed range.

You can see this in the performance of propellers like the MIRAGEplus. They run more efficiently at speeds of 30-MPH than they do at 40-MPH. That's because they were designed that way. The angle of attach was optimized for that speed range.

posted 12-12-2007 12:24 AM ET (US)            

"The very high ratio is a Suzuki sales pitch attempt to differentiate its product."

Peter, those are your words, not mine and they are simply false.

Glen makes a good point about Yamaha's new 350 HP and 300 HP V-8s. They will use new propeller castings that are larger than any current V-6 propeller offerings. Bigger power, bigger propellers. We will see more of this in the future.

Larry you ol' FUD-miester you. I won't even bother deconstructing your mish-mash of misinforamtion but I will echo what Jim has already pointed out: If higher pith props were simply better, everybody would be using lower gears and higher pitched props.

All the different factors that affect an outboard's performance can be (and are) varied for one manufacturer to another and from on model to another. Often the differences are subtle. A little more displacement here, lower gearing there, and sometimes the differences are more profound. Supercharged four stroke in this camp, simple DFI in that camp.

But no matter what choices are made by the engineers, today's modern outboards are all wonderful power plants for our boats. What is best for you will depend on your needs and resources. If it were all as simple as "this way is the best", or "that strategy is proven to be more efficient", we would all be running the same outboard, with the same gearing and the same propeller.

posted 12-12-2007 05:23 AM ET (US)            

..."all the characteristics of a propeller--diameter, pitch, number of blades, rake, even slip--may affect efficiency..."

"...efficiency peaks at approximately 80% (3-degree to 4-degree angle of attack) and begins to decline as the angle of attack increases beyond the optimum..."

and then concludes with:

"If all other variables remain unchanged, propeller efficiency increases as the pitch/diameter ratio increases."

These three statements support the points made by Tom, Me, and Larry, respectively.

posted 12-12-2007 08:10 PM ET (US)            

Is a propeller with a diameter of 15 1/2 inches and a 22 inch pitch (15 1/2 x 22) a large or small propeller?

What about 15 1/4 x 18, large or small?

What about 14 3/4 x 23, large or small?

posted 12-12-2007 09:38 PM ET (US)            

The reason I ask this is that Suzuki's marketeers (and Tom you've apparently gone along with this) have set forth the proposition that their 2.50:1 gear ratio of their DF 150 is an advantage versus Honda BF 150's 2.14:1 or the Verado 150's 2.08:1 or Yamaha F150's 2.00:1 (for whatever reasons they don't touch the Evinrude E-TEC's 1.85:1 ratio or the Mercury Optimax's 1.87:1 ratio.) In touting their high ratio, they say "lower gear ratio to swing a larger prop for improved acceleration." I simply want to understand what is considered to be a larger prop in the context of the DF 150 and its competition.

We'll leave the bigger V6 Suzukis and the 800 lb Yamaha gorilla for a later discussion.

posted 12-13-2007 03:06 AM ET (US)            

Large = more than 15"
Medium = 14-15"
Small = less than 14"

posted 12-13-2007 08:14 AM ET (US)            

First off, NO lower unit or propeller produces torque (or horsepower). The engine produces 100% of the torque (HP). The gear ratio combined with the propeller converts the torque (or HP) to THRUST in order to move the boat. To either clear up (or further muddy the waters), I will refer to the relative effectiveness of the thrust as TRACTION.

As has been said, a larger diameter prop will (generally) have more blade area and should provide better traction. The larger blade area shuld have less slippage and move a heavier load better. One down side of the larger diameter prop is more hydrodynamic drag which should result in limiting top speed. This backs up LHG's observations in this thread and others that Mercury outboards tend to have a closer gear ratio and provide a higher top speed for the same HP.

All of this is further compounded by the torque curve and horsepower curve of the motor itself. To back this up I will give a real life experience. In the fall of 1983, I purchased a (please forgive me) Baylinner 1900 OB with a 125 HP Force and 19 " pitch aluminum prop.. With the factory set up it ran 42 mph, took about 5 seconds to plane at 20 mph with just me in the boat. After adding an elevated ski pylon we tried pulling slalom skiers. I could just barely get an experienced slalom skier up in deep water on one ski. When I hit the throttle the engine would peak at 2800 rpm and struggle to get to 3500 rpm. At 3500 rpm it hit its torque curve and ran strongly. I picked up a 15" SS progressive pitch prop from Soderburg (sp?) in Duluth Minnesota. With this prop I hit 3600 rpm on the hole shot. I could get a pair of 200 pound slalom skiers up in deep water with no drag and the time to 20 mph was about 1.8 seconds. Interestingly top speed went up 7 mph! A basically crappy motor now ran well by getting it into its torque/power curve with a prop that cost well over $00 in 1984.

A Boston Whaler comparison of gear ratios and lower unit sizes is my current 1988 15' Center Console. I git it used with its original 1988 70 HP Mercury, and a 22" Laser 2 prop. This ran well, 43 mph GPS, but with the large lower unit 2.33 gear ratio and about 13.5" diameter was almost impossible to steer at speed. I had to alter the power trim to change directions. This very light hull was totally overpowered by the torque, the large lower unit (relative to the boat) and a propeller that gave tremendous traction. Also the weight of this motor, over 300 pounds with SS prop was too much.

The 15' CC now has a Yamaha F-60 with a 1.85 gear ratio, a 11' x 15" pitch Solas SS prop. It runs 42 MPH, and can be steered with one finger at any speed.

Getting a much smaller lower unit in the water vastly improved the performance of this hull.

Back to the question at hand, gear ratio does not matter without a proper set-up and correct prop to get the best possible traction matching the torque/power curve for the motor. If this is done with a larger propeller blade area, it should result in a slightly better ability to move a heavy load. However it is far more propeller dependent than gear ratio dependent.

posted 12-13-2007 11:33 AM ET (US)            

I don't think anybody thinks the propeller itself if producing the power; we know what the powerhead does. When we talk about torque we are referring to the torque at the propeller shaft because that is where it is relevant to the propeller chosen. Where the power is produced is neither here nor there to this discussion.

posted 12-13-2007 11:36 AM ET (US)            

I'm glad to see you are trying to make a point. I have always found your reasoning (for the most part) sound and logical. If you are trying to support a position, I invite you to articulate it instead of playing twenty questions.

You have now asked three questions. I'll start with the easy one first:

"...what is considered a 'large prop'?"

This is considered a large prop:

http://cache.viewimages.com/xc/3069873.jpg?v=1&c=ViewImages&k=2& d=32F168F2F013CA9A52DC430BBD28B307A55A1E4F32AD3138

Then there is the different question:

"... what is a larger prop?"

Well, larger is relative. Larger can be looked at different ways and not just by diameter. Weight, blade area, pitch are all factors that are relevant to a propeller's size. Let me offer some examples:

A BRP 14-3/4" x 19 SST is a larger prop that a 14-3/4" x 17" SST. The greater pitch will require more torque to spin it to the same RPM.

A 14-5/8" x 19" Mercury Revolution 4 is a larger prop than a 15-1/4" x19" Mercury MIRAGEplus even though the diameter of the Rev 4 is smaller. Heavier weight, larger blade area, more aggressive grip all requiring more torque to spin it to the same RPM as the MIRAGEplus.

A 14" x 18" Stiletto Advantage I for Suzuki is a larger prop than a 13-1/4" x 19" Advantage I for OMC/BRP. Even though the propeller design (blade geometry) is the same, the propeller with the lesser pitch is still the larger propeller because of it larger diameter and increased blade area. It is not a coincidence that this larger propeller is made for the Sukuzi outboard.

A Suzuki 16" x 17" stainless steel three blade is a larger propeller than a 15-1/4" x 17" MIRAGEplus. The latter you may fit to your Johnson 150; the former will NOT fit your Johnson 150...because it is too large.

Then you ask:

What do mean by "[i]Lower gearing allows larger propellers to be used?"

Well, that is obvious and has been answered twice. Lower gearing means higher torque. Simple physics. With higher torque a larger propeller may be used.

Now to the heart of what I suspect is your argument. [Feel free to jump in here and articulate your own thoughts. It would probably be better than playing a game of of laying clues for the rest of us to try to piece together.] Gear ratio is irrelevant to the ultimate performance of an outboard motor. The gear ratio is merely cosmetic and has nothing to do with the size of propeller used. In the case of Suzuki, their choice of relatively low gear ratios for their motors is merely marketing hype.

I am in disagreement with that argument. It is illogical and defies simple physics. The gear ratio is relevant, lower gearing allows larger propellers to be used.

As to whether this lower gearing is an advantage or not, that can be debated. You will note that I never said that it was an advantage just that it is very relevant and should be considered,ot dismissed. Certainly the evidence clearly suggests that Suzuki's unique gearing is an advantage in many cases.

Outboard manufacturers will always tout their technology as being superior and use examples to prove their point. This is nothing new. The truth is often found somewhere in between and will vary from one situation to another.

Shall I argue that Evinrude's E-Tec DFI technology is of no advantage? It is merely a marketing ploy to make consumers think their product is better? Did the marketing guys at Mercury say to the engineers: "Hey! Pocket protector, slap a Supercharger on that puppy, consumers will dig the sound of Supercharger!"

I think more of Suzuki and the choices they have made for their motors. If you want to argue about the performance of the Suzuki outboards and the merits of the gear ratios chosen for them, that is a good topic for another thread. If you want to demonstrate that gear ratio is irrelelvent please support your argument with some data.

posted 12-13-2007 12:52 PM ET (US)            

Propeller diameter influences the volume of the water column discharged. Pitch influences the speed of discharge. Newton's Third Law states that the reaction force is equal and opposite to the primary force. The primary force results from momentum change of the water column. Momentum is speed times mass. Higher mass means equal momentum change with less speed change. Kinetic energy, unlike momentum, is equal to the square of the speed times the mass. That means it requires more energy to accelerate a small volume of water to higher speed than a big volume of water to lower speed. For the same energy expended, you can get more momentum change by going to a large volume of water accelerated less. High diameter does exactly that.

Why not have 50" diameter props on outboards? Realistic limits on lower unit size. Big direct drive boats commonly run as large a diameter prop as will fit within the space.

(When it is necessary to trade off other things to run a really big diameter prop, you have a tug boat.)

posted 12-13-2007 06:28 PM ET (US)            

Engines have different gear ratios because, among other things, they have different sizes of gear cases. The gear case and the arrangement of the propeller aperture limit the diameter of the propeller which can be used. From what I can tell, an engine manufacturer chooses a gear reduction ratio which he feels will give the best range of performance for the propellers which will fit on the gear case and operate in the aperture, and which the motor will also have enough power to turn.

The innovation of the Suzuki motors has been to enlarge the propeller aperture to permit propellers of larger diameter to be used. Generally the largest propellers were limited to about 15-3/4-inch diameter by the aperture of most other motors. Suzuki has apparently enlarged the aperture on their higher power engines and can use propellers of larger diameter, including ones 16-inches or more in diameter. It seems well correlated that the gear ratio of a Suzuki gear case will have to have a greater reduction (higher number ratio) in order to spin these larger propellers, for a larger diameter propeller is always harder to turn. This break from tradition came as engine power began to grow above 250-HP, a relatively recent development. I believe this prompted the change to greater gear reduction, not some magic potion of greater propeller efficiency in general.

posted 12-13-2007 08:05 PM ET (US)            

The larger the diameter, the lower the rpm at which the tip speed will be too high and cause problems (mostly, true water boiling cavitation -not "ventilation")

The smaller a prop is, the smaller the thrust stream behind it will be. This is acceptable if the boat is small and light and if the boat speed is so high that lower unit drag is an important factor. Smaller diameter props can be run at higher rpm because the blade tip speed will be less in inches per second, mph or whatever units you want to use. (Each trip around the circle is fewer inches than a larger prop.)

Bigger boats need bigger props to produce the necessary volume of thrust, but these props must be run at comparatively slower rpm to avoid running the tip speed into the cavitation range.

posted 12-13-2007 09:08 PM ET (US)            

In any case, I realize this is the Suzuki marketeers speaking but Suzuki's comparative advertising for the DF 150 attempts to convince the reader that its higher reduction ratio is better because that higher reduction ratio allows it to run a "larger propeller for better acceleration" -- whatever they mean by that. Some thought "larger" simply meant diameter while others think its more than that.

What I found interesting in the published information is that despite having this higher reduction ratio, Suzuki's test reports never run a propeller with more than a 14 3/4 inch diameter (like a 14 3/4 x 21 for example). Basically, the propellers they run in the test report are within in a family that includes a 15 1/2 x 17, what I run on the Johnson 150 with its 1.86:1 ratio. Perhaps that is just how things worked out for the tests. But what is of further interest is that Honda has test reports for its BF 150 (a lower reduction ratio of 2.14:1) running a 15 1/4 x 22. That propeller is no smaller, in my opinion, than the line of propellers Suzuki offers for the DF 150. Of course we all know that Mercury's Optimax and Verado 150s, Yamaha's F150, and the Evinrude 150 can also run propellers within the same "large" size families (15+ inch diameter propellers).

So I have difficulty with the almost automatic conclusion some have reached here that the higher reduction ratio automatically means a larger propeller relative to same HP motors with lesser reduction ratios. In my opinion, a larger propeller can be used only if the powerhead has the crankshaft torque to support it. Based on the Suzuki propeller offerings for the DF 150, it does not appear to me that the Suzuki DF 150 has the crankshaft torque to support the use of a "larger propeller", that is, a propeller which is larger than what the competition can use and does use.

posted 12-14-2007 01:57 PM ET (US)            

I have an 18 outrage with a 150 evinrude and there is no way I would be able to get remotly close to that size prop. I am swinging a 15 1/2x17 mirage and top out at 5400. My performance is ok but I cant even hold a candle to hookum. CRUISE is most important to me and His cruise blows me away in fact he is 9 mph faster than me and his #'s blow away the suzuki test data with there 14 diameter prop as well and I believe it has alot to do with diameter.

I just put deposit on a suzuki DF150 and the main reason was NOT prop size but it was one of the key factors that put that motor over the edge.

posted 12-14-2007 03:31 PM ET (US)            

thx

Still poised to repower my CPD 18 with high gear ratio twins for rescue towing (seeking "traction").

Interesting results within the topic here.

Read all of Buckda's stuff.
75-HP and 90-HP E-TEC 2:1 ratio @ 640 lbs.

Suzuki DF 70, 2.42:1, 710 lbs

Twin 60-HP Big Foot EFI
2.33:1, 510 lbs.

posted 12-14-2007 08:55 PM ET (US)            

2,575 to 2,745-RPM

If a (typical 90-HP two-stroke two-stroke) motor is rated for 5,000 to 5,500-RPM for maximum horsepower and it has a gear ratio of 2:1, the propeller shaft is going to turn

2,500 to 2,750-RPM

posted 12-15-2007 05:36 AM ET (US)            

I need to find out more about the Yamaha 2 strokes and how the gears and powerband marry up. I find the gear ratio difference between the 70 & 90 interesting.
With the 90 having 2.00:1 gears, I wonder if the 2.33:1 gears on the 70 dont d-tune a 90 block down to a 70?
I am intrigued, lots to learn.

Im still not sure twin 60 Big Foots wouldnt work great on my 18. ahhh, nuther time for that.

Example of power band effect on gear ratios;
My kid had a stock Honda 450 TRX quad (4 stroke with low end torque).
His buddies built Banshee (2 stroke with high end torque, 6 speed tranny and pro clutch) would outrun the stock Honda. Same paddle setups on both quads.
OK.
We installed a Honda pro kit incl jets and front sprocket one tooth less that lowered the gear ratio on the TRX. We checked OEM prior and the TRX "pro-kit" was covered under warrantee. Scottie beat the Banshee pretty good.

Sooo the Banshee kid desides to go down one tooth on his.
He lost so much hole power he almost fried his clutch...But, he would top end faster than the TRX.
Man...that was scarry to watch. The poor kid (16) killed himself on that quad 2 weeks later. We werent there.
I dont know if we are done riding quads or not.

posted 12-15-2007 05:56 AM ET (US)            

In setting up a 18 CPD tow boat with twins, I think the Yamaha 70s are probably ideal. Great power to weight ratio. The 2.33:1 gears would allow the motors to turn 13 inch diameter propellers and so you'll get to spread the thrust out over two relatively large propellers.

posted 12-15-2007 02:21 PM ET (US)            

I only expect 20% towing/work loads.
Otherwise, 40% recreational, 40% trolling.

How bad does this engine load up after a hour or two of trolling?
What oil?
Should I expect a fouled plug now and then?
----

Peter, looks like we will have similar setups. I plan to hang my engines on CMC manual adj 4" offset jackplates.
Cabela's has the lowest price Ive found.

The man who built my Tug once told me a tow winch/bit should be placed 2/5ths the distance of the hull. Too far ahead and you are CONSTANTLY steering, too far aft and you are stern bound. If the tow bit is too high, you can get into irons and roll. I fear if I set the engines further back I will need to raise the towbit or use a crash bar.

I figure 4" will put the existing CPD towbit ahead a little and should help the maneauvering (rudder power) under tow substantially. I had 1/4 alum plates custom fitted to the inside and back of the transom to eliminate using lag bolts on the lower holes. Thus, I wont need to loosen thru transom bolts to dial in the engines trim and the spill tray becomes a good place to stow towline and rigging. A 150 Merc Black Max is mounted now. Looks bad ass. Instead of plugging and filling the single eng holes when I mount twins, I am just going to calk and tighten up the bolts which will become 4 more bolts that hold the plates/engs to the transom.
These plates are probably overkill. The transom is rock solid. At the time I was going to put 90 etecs on it.

mk

posted 12-15-2007 06:58 PM ET (US)            

"Very roughly, a pitch ratio of 0.8 can be expected to produce efficiencies of around 0.65, while pitch ratios of around 1.4 can result in efficiencies as high as 0.74. At pitch ratios higher than 1.5 efficiency generally starts to fall off."

It might be interesting to look at the PITCH/DIAMETER ratio of a few Mercury propellers and see if their own propellers follow any sort of pattern:

Mercury MIRAGEplus Propellers

PITCH    DIAMETER    PITCH RATIO
 27      14.375      1.88
 25      14.500      1.72
 23      14.625      1.57
 21      14.750      1.42
 19      15.250      1.24
 17      15.500      1.1
 15      15.750      0.95
 13      16.000      0.81

posted 12-16-2007 09:34 AM ET (US)            

Back to ratios, as we have been discussing, a higher reduction gear ratio may be chosen to allow for a larger propeller diameter to be used. The higher reduction ratio also leads to the use of a greater propeller pitch if the expected performance is to be maintained and the motor is not permitted to over spin. One drawback (or advantage) I see and have experienced running larger diameter/higher pitch propeller (a much larger propeller in every dimension) on the same boat is the tendency for increased stern walking caused by the fact that the thrust coming from the propeller is asymetric, a sort of paddle wheel effect. My first hand experience with this came when I repowered my 15 SuperSport. I repowered from a Johnson 48 SPL which was turning an 11 3/4 x 17 propeller to a Yamaha 70 which was turning a 13 x 19 propeller. The stern walking of the 15 SuperSport was much more pronounced with the Yamaha than it was with the Johnson. Backing into a slip or trying to dock can be difficult if you are on the wrong side of the walk.

posted 12-18-2007 09:53 AM ET (US)            

If a propeller were designed to operate in a stationary setting, for example as in a fan, the optimization of the blade design will be different than if the propeller is moving forward as it operates. This is the case on a boat. As the boat speed changes, the speed at which the water moves into the propeller blades is changing. Because of this the design of the propeller cannot be optimized for all boat speeds. The speed at which the water flows into the blades changes the angle of attack of the blades.

Further complicating this is the concept of wake. In terms of propeller design the term wake refers to the water that the hull of the boat is dragging along with it as it moves forward. If a boat is going 25-MPH, a simplistic analysis would think that the water is flowing into the propeller at 25-MPH, but this is not always the case. As the hull moves forward, it drags some water long with it. This slows the relative speed at which water flows into the propeller. The degree to which this occurs depends on the hull design. So a particular propeller cannot be optimized for all hulls.

posted 12-18-2007 09:53 PM ET (US)            

The forward motion of the boat, seen as water flow into the prop, does affect the angle of attack, but so does the RPM of the prop. So, if RPM stays in rough proportion to speed, angle of attack is not changed as much as one would think.

As to a planing boat "dragging" water along with it, the test of this would be whether a speedometer pitot (mounted a few inches below the bottom) shows true water speed or is affected materially by this dragged water. GPS measurements seem to show that good speedometers show true water speed within a few percent. This would seem to refute the hypothesis that much water is being dragged.

posted 12-20-2007 11:20 PM ET (US)         

If you buy a 90 with a 2.33 gear ratio, running a 20" Laser II prop (like WT's 90 Merc EFI 4-stroke), that produces a higher efficiency factor of 75%, that means you have 67.5 HP
coming off the prop to push the boat. That's a big difference for the same HP rating. Gears do count sometimes. Not always, but sometimes.

Now you know why running a new 170 Montauk with a Merc 90 EFI on it against same boat with an E-tec 90 on it, would not be pretty for the Evinrude. 38-40 MPH vs 46 MPH maybe?

posted 12-21-2007 01:20 AM ET (US)            

6400/2.33 = 2746-RPM

The E-TEC turns 5,500-RPM through 2.0:1 gears, and the propeller shaft turns

5500/2 = 2750-RPM

The propeller shaft turns the same speed in either case, the boats use the same propeller, and they both go the same speed. The propeller cannot tell who is turning the shaft.

You are forgetting that they both make 90-HP at their rated maximum RPM. But that turns out to be the same propeller shaft speed. You have to explain how the propeller is supposed to know it is more efficient when the Mercury is turning the propeller shaft.

posted 12-21-2007 06:56 AM ET (US)            

So following the above logic, should we say to the Mercury Verado 250 owner, "don't come in here with that 1.86:1 ratio looking for prop help to make [your Conquest 305] go faster"? The poor Verado 250 needs a 15 inch pitch propeller to push the massive Conquest 305 around.

posted 12-21-2007 09:44 AM ET (US)            

What really is happening in these motors with higher gear reduction is that they are using a large gear case and are suited for use with the V6 line of propellers, even though they are generally not in the 150-HP or higher range. In order to turn those larger propellers the 90-HP engines need more gear reduction.

posted 12-21-2007 11:42 AM ET (US)         

Big John: to get the more desireable 2.25 gears, you have to buy a 25" 90 E-tec. 25" 90's are as rare as sharks teeth, so it's a non-issue. If you get rid of that 4-stroke Mercaha 115 (2.07 gears) on your 170, and install a 90 Evinrude, you will be shocked with the decline in top end and power. You will need a 13" or 15" prop.

Peter, I agree, running a 15" prop on any engine, including a 250 powered Verado single on a very heavy boat, is a bad buy, and HP out the prop is lost (although I would bet a 15" Rev-4 is probably a more efficient prop than most others). This points out an interesting situation on singles vs twins, and why the twins are often, surprisingly to some, faster and more efficient than an UNDERPOWERED big single. Same old rule applies - DON'T UNDERPOWER, get close to 90% of RATING for a boat, so YOU CAN RUN A MORE EFFICIENT PROP and USE MORE OF YOUR RATED HP. Remember some recent articles that have shown triple Verados use no more fuel on same rigs than twins. The trips use higher pitch, more efficient props.
Recently saw a big Venture with triple 300's, running 22" Merc Bravo I's. Also, it is standard procedure to use a higher gear ratio as power increases. Merc's powerful 3.0 liter blocks have always had 1.76 gears vs the 2.5 liter blocks with 1.87 gears. So the higher gears on the 300 Verados makes sense to me. They have the power to still turn the higher pitch props, and to higher RPM.

posted 12-21-2007 01:29 PM ET (US)            

Into this proposition Larry has introduced unrelated factors--twin engines, much higher reserve power, different engine mounting configurations, and other variables, including different types of engine fuel induction. It is not at all reasonable or scientific to throw so many variables into the comparison, then come out with the answer that the cause is the pitch/diameter ratio of the propeller--that it alone makes all the difference.

My own comparisons of propellers are a bit unscientific on one important basis: the OMC SST propellers I have used are old, well-worn, and possibly re-worked propellers, while the Mercury propellers I have test have all been new or in new-never-used condition.

In order to test Larry's theory we need everything to be the same--same boat, same motor, same time, same weather, same load, same temperatures--except the gear ratio and the propeller pitch/diameter ratio. We would test by using several gear ratios and several different propellers, but the propellers would have to be very similar in