FWTIW…… I had a similar problem with a
Yanmar 2GM in a 33’ Cherubini (displacement circa 7000 lbs). At higher
rpm it felt a little bit bogged down too, kind of like a truck hitting a hill on a highway. Not huge
power loss, just kind of “boggy”. I experienced about a 1.5-2
knot loss of speed at the top end compared to what I had had before.
After prop
cleaning and removal(Nada), putting back on a smaller OEM
alternator (which helped a littttttle bit) I was told to check the mixing
elbow. It wasn’t old and looked fine from the outside but inside it was all messed up. Replaced that and normal
RPM to speed ratio returned.