Alpha one gearing question ???

I have a MR470 and the outdrive was shot, corroded lower leg but internals were perfect etc.
The MR outdrive was geared 1:84

I managed to pick up a good condition but second hand leg that was geared 1:65R and took the upper gear from the MR leg and swapped if over expecting the gearing to be the same as previously but the boat now seems to be under performing in comparison to previously.

I have check the obvious prop has not come free from the bush and that's fine.

I am taking the boat out of the water this weekend and wondered if anyone could tell me:

If I rotate the drive shaft one full revolution how many times should the prop turn?
I am guessing the it should turn .84 of a turn but I am only guessing.

Any help please would be appreciated ???

Ayuh,... If ya swapped the upper gear set, the ratio ain't the problem...

if you have 1.84 gearset it takes 1.84 rotations of the input shaft gear to rotate the propshaft one turn. i am reading your post and you say upper gear, dont know if its a typo but that is a gear set that needs changed, if you just changed your input shaft gear, you may still have to low of gearing for your 470, the 1.62 gear ratio is for v8 altitude over 4000.

the 165 inline GM six cylinder used a 1.65:1 ratio, back when the dinasours roamed the earth......

If you drop down a couple of inches prop pitch, that may work for you.

Rod