9.5 evinrude prop and elevation

[boatndog]

I've got a 1972 Evinrude 9.5 Sportwin. The original prop was 8 1/8 X 8, 3-blade. I used it in Ohio at about 700' elevation on a basic 12' aluminum fishing boat and it would plane just fine on a calm lake with 2 people aboard and gear. It was in storage for 10 years and then I moved to Colorado at over 5000' elevation. I tuned it up and it runs just fine, but it doesn't have the power it did and I now I can't get it to plane with just one person and gear on the same boat. I read that I could compensate for the elevation by getting a smaller pitch prop.

I've got several questions....

Would the change in elevation cause that great a power loss, or would not using it for 10 years contribute to this as well?

Would changing to a lower pitch prop resolve this? And if so what size prop should I use without exceeding the rated operating range of 4000-5000 rpm?

I've been looking for 7 pitch props, but can't seem to find one - they always seem to be out of stock or are 2-blade props. Is a 2-blad prop worth considering?

Any advice on how I should proceed to get the power back up would be greatly appreciated.

Thanks,
boatndog

 

[billy4hp]

Re: 9.5 evinrude prop and elevation

I've got a 1972 Evinrude 9.5 Sportwin. The original prop was 8 1/8 X 8, 3-blade. I used it in Ohio at about 700' elevation on a basic 12' aluminum fishing boat and it would plane just fine on a calm lake with 2 people aboard and gear. It was in storage for 10 years and then I moved to Colorado at over 5000' elevation. I tuned it up and it runs just fine, but it doesn't have the power it did and I now I can't get it to plane with just one person and gear on the same boat. I read that I could compensate for the elevation by getting a smaller pitch prop.

I've got several questions....

Would the change in elevation cause that great a power loss, or would not using it for 10 years contribute to this as well?

Would changing to a lower pitch prop resolve this? And if so what size prop should I use without exceeding the rated operating range of 4000-5000 rpm?

I've been looking for 7 pitch props, but can't seem to find one - they always seem to be out of stock or are 2-blade props. Is a 2-blad prop worth considering?

Any advice on how I should proceed to get the power back up would be greatly appreciated.

Thanks,
boatndog

A change in elevation from 700' to 5000' would cause an approximate loss of 27% all other weather conditions being equal (temperature, humidty)...

I know when I lived in Arizona that our bass boat would run @ 68mph on Lake Pleasant (1300' elevation) and would barely make 54mph on Lake Powell (4200' elevation) with a 200hp Mercury OB...

HTH

 

[ezeke]

Re: 9.5 evinrude prop and elevation

The propeller change will definitely help, especially in getting on plane. The part number for the new OEM/BRP 8 X 7 - 3 blade aluminum is 0763588, superseding part # 387334.

You could also change the high speed jet down from the stock 48.

Because the problem is lack of air density, you may also find that your lean idle setting is too rich if you still have the setting as it was at the lower altitude.