How do I find my gear ratio?

[CaneCutter79]

I need specifics on my motor to better determine prop selection. One of these requirements is the gear ratio. How can I determine this?

Motor:
1979 85hp Evinrude 2-stroke V4

I don't have the model# but I can get it. That info will be most helpful I'm sure. I think it's 85990, 85995, or 85999 based on a chart I found online.

Is there any other information needed to determine the gear ratio?

 

[ezeke]

Re: How do I find my gear ratio?

The gear ratio is specified in the Specifications Sheet of your Owner's Manual and/or your Factory Service Manual.

The gear ratios for the 1979 V4 85, 100, 115, and 140 recreational motors was 13:26 or .50.

 

[Silvertip]

Re: How do I find my gear ratio?

The ratio is actually the inverse or 2:1 (two engine revs for one prop rev) which is 26 engine revs for 13 prop revs. A .5 ratio would be over driven by a factor of four which is not the case and would really mess up the prop calculator. If .5 was the true ratio, at 6000 engine rpm the prop would be spinning at 24,000. We are talking turbine speeds with numbers like that.

 

[ezeke]

Re: How do I find my gear ratio?

The propeller shaft turns 1/2 turn for each full revolution of the driveshaft. That's .50 at the propeller which is the way it is expressed in the 1979 service and owner's manuals (and in the rest of the OMC factory manuals as well).

I know, semantics is all we have.

 

[CaneCutter79]

Re: How do I find my gear ratio?

So for the prop calculator, I enter .50 as my gear ratio or do I enter 13:26?

Thanks for the help.


I have no owners manual. I'm the third owner and it's 30yrs old. I'm wanting to buy a motor owners manual online soon.

 

[Bob_VT]

Re: How do I find my gear ratio?

This is actually a PROP question and needs to be in that forum.

Your prop selection needs to be based on more factors than a gear ratio.

 

[ezeke]

Re: How do I find my gear ratio?

There are many "prop calculators" in existence, mostly designed to shorten the process of converting known engine RPM to theoretical maximum distance of travel based on the pitch of the propeller.

Usually, a second formula then compares the theoretical maximum to the actual speed traveled to determine the amount of slip.

Many of the common formulas are somewhat counter-intuitive in that they require that the ratio be expressed as the inverse of the reduction gearcase ratio.

If you do the math yourself, for a given engine RPM, the prop shaft on your particular gearcase is turning at half that rate, so a 17 pitch propeller would move a boat a theoretical 17" X .50 or 8.5" for each engine revolution.