Gear ratio change/prop change

pkdjr8

Recruit
Joined
Apr 3, 2009
Messages
2
I have a 79 Renken 21' cuddy, 165 hp. 250 cu.in. six, with mercruiser I outdrive. This outdrive has 1:68-1 gear ratio, but unfortunately water seeped into the top portion of the gearcase and ruined the gears. I have a very good M-I outdrive to replace it with, except it came from a boat that had a 4 cyl. and the gear ratio for this outdrive is 1:98-1. A friend told me not to change the gear ratio in the new outdrive, but purchase another prop that would offset the change in rpm to get the same performance. It currently has a MW 15-1/2 dia, 19 pitch 3 blade prop. The boat tops out at approx. 35 mph@ 4000 rpm. Any help would be appreciated.
 

cribber

Lieutenant
Joined
May 29, 2008
Messages
1,338
Re: Gear ratio change/prop change

That is quite a bit of rpm to overcome just by changing pitch. You'll gain about 200 rpm per inch drop in pitch. You should plug your numbers into a prop calculator to give you a ballpark reference to start from for what you want to achieve.

http://www.hillpropellers.com/propcalc.html

http://www.rbbi.com/folders/prop/propcalc.htm

Try one of these prop calculators to see what you can achieve based on your gear ratio.
 

45Auto

Commander
Joined
May 31, 2002
Messages
2,842
Re: Gear ratio change/prop change

You might be able to get away with it if you're lucky.

Theoretically (in a perfect world) you can find the required new prop pitch using:

Old Gear Ratio / New Gear Ratio = Old Prop Pitch / New Prop Pitch

Which would be:

1.68 / 1.98 = 19 / New Pitch

So New Pitch = 19 x 1.98 / 1.68 = 22.39"

So a 22.39" pitch prop on a 1.98 ratio drive would supposedly deliver the same performance as a 19" pitch prop on a 1.68 ratio drive.

Your 4000 RPM at WOT is a little low with your current set-up, so if you could get a 21" pitch prop for the new drive it would probably put you right where you want to be.

The problem with the "unusual" ratio/prop combinations is that in the real world, you'll sometimes find that differences in prop performance due to efficiency, slip, etc, will cause the "unusual" ratio/prop combination to deliver performance other than what is expected. Only way to find out is to try it.

I have personally replaced a 2.0 ratio drive on a 120 Mercruiser with a 1.5 ratio drive and substituted a 15" pitch prop for the original 19" pitch prop (calculated replacement would be 14.25") and you couldn't tell any difference in the way the boat performed. Acceleration, fuel economy, and top speed remained identical.
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Gear ratio change/prop change

On my Force and Chrysler outboards, I regularly substitute the 1.76 gears with 2.0 ratio. I find that with the lower ratio, (2.0) I need to increase pitch by 2 to maintain the same engine RPM. Because the prop is actually spinning slower through the water, there is less parasitic drag, more horsepower is available to drive the boat, and I gain a couple of MPH even though the theoretical top speed is the same.

Make of this what you will, but in your case, I would try a prop with 2 inches more pitch and see what happens. A prop change is relatively easy and cheap and you just might be pleased with the result.
 

toddwebguy69

Recruit
Joined
May 31, 2012
Messages
1
Re: Gear ratio change/prop change

I was considering putting a 4.3 omc with 1.68.1 on a 5.0 Chevy with original drive 1.50.1. What prop should I use without causing damage or sacrificing performance?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Gear ratio change/prop change

You shouldn't resurrect an old thread.
Start your own thread. There is a new thread tab at the top of the prop main page.
The best way to figure a prop is to run your present prop and get us the wot rpm and speed with a gps.
To guess a prop,not a good ideas, we would need a lot more info bout your setup.
read "read this first" at the top of the main prop page.
 
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