hp to gear ratio

[cleve]

i have read the posts and posted the posts but i need somebody to explain in detail the relationship between horsepower and gear ratio. for example, why does a merc. 60 and a mercury 60 big foot (2 stroke, of course) not produce the same results? why does a yamaha 60 not post the speed of a mercury 60 (1.64 gear ratio)? i know h.p. varies between year models and manufactures but isn't horsepower created equal to some extent?

 

[_dan_]

Re: hp to gear ratio

first I would say that when they give the motor a hp rating the manufacture rounds it up or down or whatever so just cause they claim it to be a certian hp doesn't mean it is. further more every motor is different and depending how well the motor is broken in when in was new also how well it has been cared for will affect the actual hp. also timing, grade of fuel, out side temp, altitude, there are a few more vaiables that affect hp and there is also a thing called torque that is what actually moves the boat the heaiver the boat the more torque needed hp only helps at the top end. and comparing a 2 stroke to a 4 stroke they are two differnt motors the tw stroke fires every the piston hits tdc and the four only fires every second time the piston hits tdc bla bla bla there are lots of reasons

 

[steelespike]

Re: hp to gear ratio

I wouldn't be too concerned with gear ratio except to use a prop calculator.
The ratio has been selected to perform best with a specific motor design.
Different motors of say 60 hp on identical boats setup to their best performance.
will usually be within fractions of mph of each other.but they may be setup comepletely different,height,trim prop selection. Yet performance will be very close.A big foot uses a gearcase from a larger motor with different gear ratio prop turns slower.This allows a larger diameter prop designed to push heavy loads to be effective. It will work on lighter boats but will require different props and may be difficult to find one that will work perfectly on a light fast boat.Also the larger gearcase will hinder performance at the higher top speeds.

 

[Silvertip]

Re: hp to gear ratio

Since most outboards have an upper RPM range of 5000 - 5500 RPM, lower unit gear ratio and prop pitch is what determines how fast a given boat/engine combination will go. Theoretically, given two identical engines, a low numerical gear ratio with prop of less pitch can be equal to an engine with a higher numerical ratio running a prop with more pitch. In the end, it is propeller rpm and pitch that determines speed. Generally, small displacement motors run higher numerical ratio gear sets than do larger motors. Hence the difference in prop diameters and pitch requirements. Don't confuse the "higher and lower numerical ratio" with "higher and lower gear" as they mean opposite things. Higher ratio is the same as lower gear.

 

[QC]

Re: hp to gear ratio

Silvertip is correct.

Also, Manufacturers are bound by a 10% margin between advertised and delivered hp if they follow NMMA rules. Theoretically, same HP (actual shaft output), same weight, same hydrodynamically efficient lower unit, same hull should deliver similar results regardless of gear ratio provided one extremely unlikely thing: exact same propeller efficiency. The two theoretical propellers would have to be different to compensate for the ratio change (more pitch for the slower shaft speed) . . . The reality is that prop efficiency, design and performance are greatly affected by the speed they are spun and the relative boat speed, so finding two propellers that would perform exactly the same at different propeller speeds (result of different gear ratio) would be a life long pursuit . . .

The calculation for hp is torque x RPM/5252 . . . So if you change the gear ratio up numerically say from 1.5:1 to 2.0:1, with the same engine driving the input, then torque at the shaft goes up and speed goes down (shaft RPM), and hp stays the same. So much for theory and math as results on the water will be different. I like to say that Propellers are 50% Science and 50% Voodoo . . .

 

[180shabah]

Re: hp to gear ratio

I like to say that Propellers are 50% Science and 50% Voodoo . . .

And 50% marketing.