I recently purchased a '92 Mercury 75 HP for my '60 center deck that came with a 13.5 x 15 prop . I don't know what type of boat the engine was on last, but this seems like a huge prop for the G3. That said, I don't really know how to size a prop for my boat and would appreciate opinions/tips from those who do and have experience. I will appreciate the suggestions.
I am no expert on props for outboard motors but can tell you that I have a 1960 G-3 with a newer 2009 "Mercury 50 HP"
Outboard and I currently have a 15" prop on it. I had tried a 17" but it doesn't seem to take off as well. However, with the 15, when I am at "full throttle" I am very close to the max RPM of the engine which is 5500. I hope that helps you some Sir.
It is more complicated than simply suggesting a pitch, as the results vary depending on both the "series" and material (aluminum, bronze, or stainless) of propeller in question. That said, most currently manufactured 19 inch or 21 inch pitch propellers intended for the Merc 75 are likely correct for your G3 with 1992 Merc 75. Note however that some currently produced propellers are basically a close reproduction of a 1960's Mercury propeller blade design, and the selected pitch will then likely need to change by 2 inches to then compare in RPMs. Outboard mounting height will also be especially important, and may need to be changed to better the propeller results. If you should like, you can e-mail me, and we can write or exchange phone numbers and speak. We of course can quote you our propeller offerings if wanted.
I run a1960 center deck with a 62 Merc 700. For pulling skiers I use a 15" pitch 2 blade and the motor trimmed down. For general cruising around with various loads 2 blade 17". For speed runs I have a 21" pitch 2 blade motor trimmed way up and all weight biased to the stern.
I have a pdf file of older merc motors and props reccomemded for specific purposed. Let me know and I can email it to you. for a G3 prop and trim is vital for different purposes.
While hopefully avoiding "dishing" the very kind help offered by hhaynes, still a 66 cubic inch piston displacement 1962 Merc 700 with a 2 to 1 lower unit gear ratio, utilizing nearly 60 year old Mercury propeller blade designs (being likely the specific smaller diameter propellers intended for the Merc 700), and being flywheel horsepower rated, only has limited help when the question of what propeller is to be utilized on a 86 cubic inch 1992 Merc 75 with a 2.3 to 1 gear ratio, utilizing hopefully at least 25 years newer blade designs, and being propeller shaft rated 75 horsepower. I respectfully do not ever feel that I am a "know-it-all", but I did see both the 1962 Merc 700 and 1992 Merc 75 models sold when new by our business.