Gear Ratio Difference Question

[Texasmark]

We were over there talking about 2 vs 4 cycle torque on another thread and the question of gear ratio's came up. My natural curiousity got tweaked again so here is the question.

I have a 90 hp 3 cyl Merc. I noticed the same numbers on the older I6 Merc engines.

The gearbox is 2.3:1, as is it's lower hp twin brother (sister) the 75 hp (in the currently designed engine). No other engine (hp) in the Merc lineup has that low a gear ratio; the next lowest is 2:1 and it just goes up from there (numbers get closer to 1:1). And, I noticed that when Merc changed engine designs (from I6 to I3) in the late '80's they carried the 2.3 with the (this) engine.

Since you can prop as you choose and come out with the same result (as I see it), and other engines in that general range are higher gear ratio's, like the 115 hp I6 I had and the newer I3 both are 2:1. Why the 2.3?

Well, you might say that they are designed for high torque, like made for an installation where a big boat is used with a smaller engine; or to pull a pair of heavy weight slaloms at 35 mph on a smaller rig. Ok. I can change props and accomplish that.

Well, I have a 90 on a light boat and have to go to 24P to get my rpm's down to a reasonable level. Well, for the smaller gearbox diameter (4 or 4 1/4 inch don't remember), 24P isn't that easy to find for one thing; you can't buy one in aluminum for example.

If I were running a 115, with it's 2:1 on this rig (same size gearbox) I could find props all over the place.

Just curious.

Mark

 

[Silvertip]

Re: Gear Ratio Difference Question

This is not a unique situation for Merc. Most engines, regardless of power, have a 5000 - 6000 RPM range to play with. Johnson/Evinrude 40/50 twins had 2.3 gear sets and I believe the triples did too. I had a 50 Evinrude with a 17P prop on a 16 ft walleye boat. My fishing buddy had a nearly identical boat with a 50 Merc, 13P with a 1.83 gear set. These two boats would run wide open like they were tied to each other. The difference -- mine would jump out of the hole. His took a bit longer to plane. Merc big foot lower units have 2.3 gearsets since they find primary use on pontoon. As you said, with any two gear sets you can prop both to perform the same. In the case of J/E, those engines will run 6000 RPM with no complaint. Mercs (I3's) just don't seem as comfortable above 5200 or so. My 75 HP is rated 5250 at WOT and your 90 should be 5500. I think there is a little more "tunability" with a deeper gear set (higher numerical ratio). Higher HP engines give you much more torque so they can pull lower gears (lower numerical ratio). But they also have the 6000 RPM limit to deal with so the 2:1 and lesser gear ratios come into play. Lots of words to say -- its what the manufacturer determined is best for the type of service that engine is typically going to see.

 

[Texasmark]

Re: Gear Ratio Difference Question

Sounds like we are on the same sheet of music as to the why's.

Thanks sir.

Mark

 

[Silvertip]

Re: Gear Ratio Difference Question

I guess there is another way to look at this. A big dude can loosen a stuck bolt with a shorter wrench than a wimpy guy. On the other hand the wimpy guy may very well be able to outrun him. How's that for an application of "garage logic". o

 

[Laddies]

Re: Gear Ratio Difference Question

Mark I am a little confused when you talk about differant dia. gearcases as near as I know the prop exhaust rings and props are the same dia. and are interchangeable from the early 4 cyl 650 to the I/6 1500 the large bore 3 and 4 cyl engines of late years are the same prop and all are interchangeable if you have the right hardware the only 3 cyls to use a smaller prop were the small bore of the early 70s and the newer 50 to 70s. The differant gear ratios were so Mercury did not have to build so many differant pitch props the did play with many differant prop dia. and blade configurations over the years. Supposedly the best prop design today is the current design which is progressive pitch--Bob

 

[Texasmark]

Re: Gear Ratio Difference Question

Laddies, they are 2 different critters and I wondered that myself.

The mid range, lower hp 3 cyl engines ran a small gearbox and a 10" approx dia prop.

The upper 3 cyls (like mine) run the same dia gearbox as the current 4 cyl run (4 1/4" I think) and can turn a 13-14" dia wheel just as the engines you mentioned.

The V engines (except I'm hearing one version of the 150 is smaller) runs a larger dia gearbox (4 3/4" I think) and can turn wheels of over 15"...I think (I know they can turn up to a 15.

The gearboxes on those smaller engines was around 1.6 to 1.86:1. I guess they used the small box to keep the weight down; don't know. But like Silvertip mentioned, once you get the little suckers going, they can easily keep up with a big wheel turning slower rpm's.

Take the early days of the OMC V engines. Little 10" props with low ratios (turning real fast) and whopping big powerheads.....after the brute V50 that is. Don't know what made them change their minds.

Mark

 

[Paul Moir]

Re: Gear Ratio Difference Question

I wonder if the change to thru-hub exhaust caused the gear ratio change? Larger, slower turning propellers are inherently more efficient than small high speed ones. But they require large gear ratios which translate into larger, higher drag gearcases.

Take the '68 J/E lineup for example. The older non-thru-hub 85hp and the new, thru-hub exhaust 55hp. The 85 had a 1.15:1 gearbox (the 65 was available with that or the old 1.7:1 football) while the 55hp was 2.4:1. Now I know we're not comparing apples here, but that's a massive difference.

I don't think it was so much the loss of blade area that caused the shift since the inside of the prop blades does very little. I stand to be corrected though on that though. Rather, perhaps the increase in the diameter of the "bullet" necessary to accommodate the exhaust allowed the larger propshaft gear.

But all that is just my own imaginings.

 

[waterinthefuel]

Re: Gear Ratio Difference Question

I guess there is another way to look at this. A big dude can loosen a stuck bolt with a shorter wrench than a wimpy guy. On the other hand the wimpy guy may very well be able to outrun him. How's that for an application of "garage logic". o

I'd just let the big guy do all the work. I guess I'd make a good kicker motor! 8)

 

[Texasmark]

Re: Gear Ratio Difference Question

Well Paul I think you are onto something there. I share with you the curiousity about those early V OMC engines and the gearcase(s) used by them at the time. I know of no one that had one so I have no reference for hole shot and top speed. Like we were talking about, once you get the little critters spinning up, they probably run out pretty good.

One thing we don't know is the impact of the exhaust gasses as a drag reduction element when the thru prop hubs first came out.

On bulkiness of the bullet, take a jet airliner wing leading edge; very blunt for an aircraft traveling 600 mph. Not a hydrodynamics guy so can't add any insight' but maybe it's not all that critical. And yes I realize that an airliner is trying to get air to travel a longer distance over the top of the wing and an outboard isn't, but still.

I do however, feel that the close ratio gears were necessary for the small gearboxes they had then......but they started out with the 50 hp and that gearbox was awesome.

Wonder when OMC started the design of the 3 cyl looper for introduction in '68 and whether or not any of the sister divisions knew what they were doing. May be that they knew what was coming and just finished out the production runs of the small gearcase and all while getting tooled up for the big change in design.

Thanks for the reply.

Mark