Quick question on Mercruiser outdrive swap.

we just replaced the outdrive off our 4 cylinder mercruiser and put one on off of a v-8. just curious what kind of prop would be recomended to offset the gear ratio.

Re: Quick question on Mercruiser outdrive swap.

I did the same thing. I replaced a 16' Mark Twain 4 cylinder outdrive with a V8 outdrive.

If you want to keep the RPM's the same, then new gear ratio / old gear ratio = new prop size / old prop size.

So new prop size = old prop size x new gear ratio / old gear ratio.

Engines of a certain size tend to perform best with props within certain ranges. If you made a big jump in gear ratios it's sometimes going to require a prop that may put your boat out of it's most efficient range. However, sometimes you get lucky and it all works OK!

In my case, I replaced a 1.98 outdrive with a 1.5 outdrive. Boat originally had a 19" pitch prop. So my calculated replacement prop pitch is:

19 x 1.5 / 1.98 = 14.39"

I found a stainless 15" pitch prop on the local Craigs list for $75, the boat runs like new.

Re: Quick question on Mercruiser outdrive swap.

how would i find out the gear ratio?

Re: Quick question on Mercruiser outdrive swap.

cschaf said:

how would i find out the gear ratio?

Click to expand...

Ayuh,... It's off a V8, so it's 1.50:1...

You'd need a prop pitch of 9 or 11"...
They only go down to 15"...
That's WHY ya can't run that drive on that motor...

Re: Quick question on Mercruiser outdrive swap.

The gear ratio should be stamped on the port side of the upper half of the outdrive.

If it's an original 4 cylinder outdrive, it's probably 1.98, 2.00, 1.84, or 1.94.

A V8 outdrive is usually 1.47, 1.61, 1.5, or 1.65.

In any case, you're going to be reducing the gear ratio by approximately 25% (prop will spin about 25% faster) so you'll need to reduce the prop pitch by the same ratio to keep everything working correctly. For example, if you're currently using a 17" prop and went from a 2.00:1 to a 1.47:1 outdrive, you would need a prop pitch of 17 x 1.47 / 2 = 12.5".

As Bondo says, it's possible you'll have a difficult time finding an appropriate prop depending on what you started with.