Question about Gear Ratio's

[Texasmark]

Was surfing this am and a Mr. Rhodes wanted to upgrade his '02 from 40 to 60 hp.<br /><br />In the replies, one person mentioned that the gear ratio was different; like 2:1 for the 40 and 1.64 for the 60.<br /><br />Does the selection of gear ratio have to do with which end of the lineup the engine is on for a given gearbox? Like the engines on the low end use big ratio's and on the high end low ratio's for a given box (lower unit)?<br /><br />Like my 90 is swinging a 13" wheel with a 2.3:1 gearbox. The last engine I had also swung a 13" wheel but it was a 115 and had a 2:1. Remembering the little 60 hp 3 cyl, ('80's early '90 era) as I recall it ran like a 10" wheel but had a 1.64 ratio. I'm trying to remember, but smaller engines used that lower unit also, didn't they, but don't remember if they ran higher ratio's or not.<br /><br />Just curious.<br />Mark

 

[Laddies]

Re: Question about Gear Ratio's

Mark I think that alot of the higher HP engines having less gear reduction is to make the standard pitch props (13 to 23" of pitch) work on a whole series of engines

 

[Silvertip]

Re: Question about Gear Ratio's

Gearing selection is a matter of how the manufacturer wants to achieve a performance goal. Mercury in their midrange two-strokes typically used a 1.83:1 ratio but a relatively small diameter prop with moderate pitch. OMC on the other hand in the same series of engines used a 2.34 gear set with a larger diamter prop and more pitch. Theoretically both motors would perform basically the same. It all boils down to prop speed. A prop turning slower speeds but with higher pitch does the same job as a smaller prop with less pitch but turning faster. Generally, the higher (numerical) ratio such as a 2.33 will get out of the hole better than a 1.83, but the 1.83 may have a slightly better top end. That's why prop selection is almost an artform. It's all about leverage -- or physics 101. Work boats want deep gears. High performance boats want prop speed. Prop speed on a cruise ship is about 400 RPM wide open (but its diameter is in the "oh my gosh" range. Just a little tidbit -- our larger nuclear carries have a position sensor at each end of the drive shaft. They know when maximum power is being delivered when there is a turn and one half of twist in the shaft. Thats 540 degrees folks. Talk about being "wound up". Don't want to be standing next to it if it should snap.

 

[Texasmark]

Re: Question about Gear Ratio's

Man oh man on the 540 degree shaft torque on the carrier. Just think, some guy (gal) did the numbers, a foundary came up with the right mix and poured it, a milling shop turned it down, the ships fitters put it all together and then someone hooks a nuke to it and bazaam. Whadda deal.<br /><br />When I was a teen in high school, in the summer I decked on tugs running the Texas Coast/Miss river. We ran a 400 hp "Atlas" brand diesel of umongus cubes but it ran at about 400 rpm and turned about a 15' prop with very little pitch; maybe 8 or 10' per rev. But boy, that baby really swirled the water when we would change directions and talk about push...walla.<br /><br />Maybe you hit onto the reason; standardization of props at the dealer, if for no other reason, cause it's all gears; either gear the engine or gear the prop. I think somebody figured out that the high rake props in SS with cupping just gave better performance (half out of the water) as compared to the small guys running high revs and a small diameter (in aluminum).<br /><br />But I still remember those big 4 cyl OMC guys (in the early '60's) with those tiny winy 10" props I guess with an 85 or so power head. <br /><br />And then came the 55 hp looper and the "real lower unit" and prop setup and things haven't been the same since.<br /><br /> Nuf blabbin<br /><br />Mark

 

[Silvertip]

Re: Question about Gear Ratio's

The only way you can pull off twisting a shaft 540 degrees is to make it a verrrrry long and on a carrier it certainly is.