please explain gear ratio Alpha 1 Gen 2

[poconojoe]

My alpha 1 Gen 2 has a gear ratio of 1:81. Does that mean every 1 rotation of the drive shaft the prop spins 81 times? If so, that's crazy! Kind of amazing! Or am I wrong about that deduction?

 

[GA_Boater]

Your almost right, but wrong. LOL

Each turn of the engine/input shaft rotates the lower unit drive shaft .81 turn or a .81% reduction. The other way around would mean prop destruction at WOT along with the flying shrapnel.

 

[poconojoe]

Let me try and understand this....so the engine turns the shaft then the drive reduces it to .81% of a turn? Wouldn't that be a 19% reduction?
Or does the drive reduce it to 19% of a turn which would be 81% reduction?
Or am I all confused here LOL!
Thanks!

 

[GA_Boater]

Whatev. Each 100 RPM of the engine = 81 RPM lower unit drive shaft.

 

[poconojoe]

Ok, got it. Thanks

 

[thumpar]

It is actually more of a reduction than that. For ever turn of the prop the engine has to turn 1.81 times. So close to 2 revolutions of the engine for 1 turn of the prop. It turns out to be about a 45% reduction.

 

[poconojoe]

Thanks!

 

[Silvertip]

The number 1.,81 means nothing by itself. Ratios are a relationship between two things. In your case, engine rpm and propeller shaft rpm. The proper expression in your example is 1.81:1. This indicates the engine turns 1.81 times to make the prop shaft (hence the prop) 1 revolution.