I also have the same engines and saildrives (Yanmar 3GM30FC and SD20) on my Admiral 38 Cat and am looking at replacing my fixed 3 blade
props for folding props. Did you find out any more info on the correct prop size? I've look at a couple of different prop suppliers and the recommendations seem to vary between 16" and 17" depending on the props.
The manual I have for the SD20 only refers to the 3YM30
Engines (and says max prop size = 16" for folding. 16.5" inches for fixed.
Any info you found would be appreciated, thanks