Outboard Engine GEAR RATIOS!

[NICE PAIR]

so with a 2:1 with a 28p prop be faster and better acceleration than 1:87 with a 26p prop at the same rpm say 7500 ?

28p x 7500 x 2:1 = 94.7 theoretical - 10% = 85.2

26p x 7500 x 1:87 = 96.7 theoretical - 10% = 88.9

[WillySteve]

I get 87-88 on both with 12% on the mercury calculator!

[tlwjkw]

don't forget "tip speed"...............

[RBT]

It's about prop speed and torque multiplication. Ratios move the amount of torque and where in the rpm band the propshaft see it.

[WillySteve]

It's about prop speed and torque multiplication. Ratios move the amount of torque and where in the rpm band the propshaft see it.

I agree with all but the torque multiplication factor. The only gear boxes that actually multiply torque contain a Torque Converter hooked to it's input.
NO regular gear box can increase torque coming from the engine!
Ratios do move the Torque up and down in the rpm band but they do not control the amount of torque avail., only the engine.
However if the engine is hooked to a torque converter, the converter can increase avail torque through diverting the angle of attack on it's internal driven fins!

[RBT]

,simply, if a motor makes 100 ft lbs at 10000 rpm with 2:1 it will see 200 ft lbs at a propshaft speed of 5000 rpm, same motor with 1.87 gears moves the prop speed up buy about 350 rpm but reduces the torque multiplier.

[Tiller Guy]

So is it wrong for me to say a 2.1 is geared lower than a 1.85.1?

[FMP]

So is it wrong for me to say a 2.1 is geared lower than a 1.85.1?

No, as long as you know what you're talking about. I would say the same.

[David - WI]

I agree with all but the torque multiplication factor. The only gear boxes that actually multiply torque contain a Torque Converter hooked to it's input.
NO regular gear box can increase torque!
Ratios do move the Torque up and down in the rpm band but they do not control the amount of torque avail., only the engine.
However if the engine is hooked to a torque converter, the converter can increase avail torque through diverting the angle of attack on it's internal driven fins!

That's not funny, some people actually think that's true.

[WillySteve]

OK Guys. If you will read what I wrote before popping off, ("NO regular gear box can increase torque coming from the engine!
Ratios do move the Torque up and down in the rpm band but they do not control the amount of torque avail., only the engine") you will see that the statement is about ENGINE Torque and is a definite fact!
A Gear ratio increases torque on the final wheel or propeller in a proportional amount depending on the combination of gearing, or the final ratio.
This equation is easy to figure IE: Engine Torque x Final Gear Ratio = Torque at the Final Wheel or Propeller.
In other words if the engine makes 200 ft# of torque and the gear ratio is 2:1 the final torque will be 400ft#.
The ONLY way you can increase the maximum torque output that an engine is making is with a Torque Converter!
Revving the engine on a standard shift and dumping the clutch does not increase the torque output of an engine!
A T-Converter uses hydraulic fluid pressure in a very unique way to accomplish it's ability to increase an engine's torque!
The key to the torque converter's ability to multiply torque lies in the stator.
In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the impeller to oppose the direction of impeller rotation, leading to a significant loss of efficiency and the generation of considerable waste heat.
Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the impeller, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied to the impeller by the prime mover. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to impeller rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is prevented by the one-way stator clutch.

I guess if I'm wrong then I've spent over 50 years owning and operating a Drive Line/Transmission business thinking I knew what I was doing the whole time!
Anybody wanna talk about Hydraulic Airplane Propellers that govern engine rpm's by changing propeller pitch???
I just still happen to work on those with an FAA License!

[Mark75H]

A torque converter does not change the torque of the engine, it changes the speed and torque on the output shaft - the same way gears do. Like gears and clutches, the gain comes from running the motor at or close/closer to its horsepower peak.


The maximum practical prop tip speed is just below the speed which wll cause cavitation boiling of water on the low pressure side of the prop (unless the prop is a special "super cavitating" prop; "super cavitating" is probably a misnomer, "controlled cavitating" is probably more correct). Do not confuse cavitation with votex blow out/ventilation.

Larger diameter props will have higher tip speeds at the same propshaft rpm; so when using gears with lower numbers/higher output speed ratios, smaller diameter props are often needed.

Conversely, back in the late 1950's one of the things letting Merc get skunked by Hot Rod, was Hot Rod's use of a higher number ratio/lower output speed propshaft and larger diameter prop that gave better acceleration. Kiekhaefer and Strang countered this with the release of an optional gear ratio set that matched the Hot Rod and let Mercs use similar larger diameter props.

[WillySteve]

A torque converter does not change the torque of the engine, it changes the speed and torque on the output shaft - the same way gears do. Like gears and clutches, the gain comes from running the motor at or close/closer to its horsepower peak.

OK, I worded my statement wrong! You are right in that a torque converter does not change the actual engine torque! That would not be possible.....What it does is increase the torque applied to the input shaft which drives everything downwind of the engine so basically the complete gear train is seeing an engine with more torque than it would if there were not a TC!
To RESTATE the Equation with a Torque Converter Added:
Engine Torque + Torque Increase From TC x Gear Ratio Being Delivered to the Wheel or Prop = Torque at the Final Wheel or Propeller.
A person has to be very careful on this site when making statements because there are some who can't read between the lines!

[patchesII]

I just go with what makes boat accelerate the best ��

[Mark75H]

In the same way that a gear ratio would. Simply done by hydraulics instead of solid gears.

[WillySteve]

It's amazing how threads get hijacked into such interesting topics!!!
A Low Gear has a High Gear Ratio and a High Gear has a Low Gear Ratio....Low and High Gear are nothing other than generic terms!
Does anyone agree or are we still multiplying torque!!!