Outboard Engine GEAR RATIOS!

**Magcat 62** · 10-16-2016, 10:43 AM

Originally Posted by dwilfong

more gear reduction dose make more toque at the prop shaft. But it reduces the prop shaft speed that it is delivered.
This dose not change the HP at the shaft.
So you can spin a bigger wheel but it will spin at a slower speed. With nothing else changed with the eng output.

might reduce hp a shaft but increase torque as the trade-off!

**dwilfong** · 10-16-2016, 12:51 PM

hp is the amount of torque at a certain speed. If torque is the same at a higher speed then you have more HP. If the torque is the same at a lower speed you have less HP.

The ability of the eng to maintain as much torque as possible across a wide RPM range is what makes power. How quick the torque drops off after its peek is the true measure of the engs HP ability. Gears do not change this thy just put it where it is needed most.

**Mark75H** · 10-16-2016, 01:42 PM

Originally Posted by terrikirchner

i heard a quote once torque is doing more work slower and horsepower is doing less work faster. i think thats accurate and simple. physics.

No dwilfong's explanation is correct.

Horsepower is torque multiplied by time.

More like this: torque is how much you can deadlift (more accurately how much you can push or pull, no motion is needed)... horsepower is how much you can carry for a minute.

Horsepower measures actual work done, torque is just ability without work.

**Mark75H** · 10-16-2016, 01:49 PM

A fluid coupling can change torque up or down, but it cannot increase horsepower. Gears do the same thing.

**dwilfong** · 10-16-2016, 04:39 PM

The prop is a fluid coupling. As the set up on the boat changes it will grab more or less fluid (water). Like I stated the TIP SPEED has a lot to do with what gears you need to keep the prop shaft speed in line with prop diameter/pitch and the torque curve of the eng. That is what the gears in the lower do.
With a eng tied to a certain crank stroke gear changes will not do much as the toque curve or VE is for the most part controlled by the stroke.

**patchesII** · 10-16-2016, 05:00 PM

I do know it's very hard to beat a 1.87 gear on a 2.5. Does just about everything well

**Magcat 62** · 10-16-2016, 05:16 PM

i thi ou would loose hp at the shaft as the gearing gets lower you cant gain both torque and HP, POWERHEAD horsepower cant be increased by gearing its a fixed #, IMO?

**NICE PAIR** · 10-16-2016, 05:31 PM

Originally Posted by patchesII

I do know it's very hard to beat a 1.87 gear on a 2.5. Does just about everything well

I was surprised on my twin Talon 22, a pair of s3000 wanna be's liked 1:75's with 32 cleavers real well.

**WillySteve** · 10-16-2016, 05:43 PM

Just in case anybody wonders....The propeller hub nut on a Rolls Royce Dart turbo-prop engine requires a 720 ft. lb. torque.
The shop method to torque this hub nut is to suspend a 180 lb. man from a 4' bar......which is Legal Beagle as far as the FAA is concerned!
Other methods which you already know are the use of a verified, calibrated torque wrench or that the fastener length be measured prior to the application of torque. The fastener is considered torqued when it is stretched to a specified length.
I USE THE Torque Wrench since it's been 40 years since I weighed 180!!!!!

**NICE PAIR** · 10-16-2016, 06:02 PM

Originally Posted by WillySteve

Just in case anybody wonders....The propeller hub nut on a Rolls Royce Dart turbo-prop engine requires a 720 ft. lb. torque.
The shop method to torque this hub nut is to suspend a 180 lb. man from a 4' bar......which is Legal Beagle as far as the FAA is concerned!
Other methods which you already know are the use of a verified, calibrated torque wrench or that the fastener length be measured prior to the application of torque. The fastener is considered torqued when it is stretched to a specified length.
I USE THE Torque Wrench since it's been 40 years since I weighed 180!!!!!

I assume, like me, you are capable of much more torque now. ;-)

**dwilfong** · 10-16-2016, 06:25 PM

Originally Posted by patchesII

I do know it's very hard to beat a 1.87 gear on a 2.5. Does just about everything well

That's my point with the one crank used a gear change will do nothing. The prop is the gear change.

**WillySteve** · 10-16-2016, 08:00 PM

Originally Posted by NICE PAIR

I assume, like me, you are capable of much more torque now. ;-)

HAHAHA....I just use a shorter bar!!!

**NeedOne** · 11-17-2016, 11:33 AM

Good Info here, Thanks

How can the gear ratio of a particular lower unit be identified?
I can think of one way.. rotate input shaft and count prop revolution..
What if the lower is installed on a complete outboard, any way to determine without disassembly?

**JCSmile** · 11-17-2016, 11:49 AM

Rotate the flywheel. You might need a ratchet to do this or a breaker bar on the flywheel nut. Spin the flywheel (with lower unit in gear) until you see the propshaft rotate one time, however many rotations the flywheel made is your gear ratio (1.87 turns=1.87 ratio etc).

**FMP** · 11-17-2016, 01:26 PM

Hold the flywheel fixed and turn the entire motor, boat and trailer around until the prop/shaft turns one full rotation. Then count the rotations of the engine , boat and trailer . Holding the prop/shaft fixed and rotating the leg/ motor and boat / trailer about the prop axis one full turn while counting flywheel rotation is a bit awkward .