|If you go to http://www.cc27association.com/fixes/fixes.html and read the material on props and propellers, you’ll know as much as any of us do. What you have is probably a 7/8” shaft with an SAE taper, but read all the material on our pages and on the linked pages to get up to speed.
I’’m sorry but the page on specific props doesn’t offer much help for the Yanmar 1GM. When you’ve done the job and have a few months experience with the new prop, I’d appreciate a report (top speed in flat water, behaviour in rough water, general sense of how well the prop and engine are mated). Have fun and good luck.
C&C 27 Mk IV