First of all, 54 hp is low for a 27 tonne (60,000) lb boat. You will unlikely be able to get up to hull speed
even at wot. Sailboats and efficient trawlers need 1-1.5 hp per 1000 lbs to reach hull speed
But given that issue and assuming a 3.06:1 transmission ratio (The highest ratio availabe from Yanmar
. The ratio is an absolutely critical parameter for figuring prop pitch) I plugged your data into boatdiesel's prop calculator. It gave a 20x15 two blade prop. It said that you would be able to cruise
at 2,800 rpm at 6 kts and maximum speed would be 7 kts.
Boatdiesel is pretty conservative for sailboats which have relatively efficient hulls so you may be able to do better, but it is in the ballpark.