Have you had a recent haulout/clean? That can make a big difference if the hull and prop are not clean.
What rpm are you running at? If the engine starts pushing black smoke when you try get max rpm, it (the prop) may be too "square" and a re-pitch to 12x10 or 11 may work better for you.
Presumably this set up has been satisfactory in the past....
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)