73 large gearcase to 76 small gearcase

[FillupD]

I would like to swap a large geracase on a 1973 50 hp evinrude and put it on a 1976 55 hp evinrude that now has the smaller gear case. They look the same size dimensionally. Will this swap work OK and be beneficial to have a larger gearcase?

I need to play with some props to get RPM's in range and I have large gearcase props but only one for the smaller. I would rather run with the large gearcase on the 55 if possible.
I'm getting 4000 RPM out of a 11 3/4x17" prop and need to see if I can prop the RPM's up a little.

 

[FillupD]

Re: 73 large gearcase to 76 small gearcase

I have been reading other posts and may have come to the conclusion that a larger dia. prop would vent better? Would it make the boat go the same speed with less effert on the motor or more effort for the motor? ie... small gearcase 11 3/4 x 17 to a large gearcase 12x15
My RPM's with the small one are 5150. I am thinking the 15" would get me in the range of 5500 or so? Is this right?
I just don't want to install the large case and have it rob power from the motor.