Is this a new problem to the boat? Was it OK last season?
If it is a new problem There are many things that can cause a diesel to run slower then slower:
Check the fuel
filter. If you have a vacuum gage an increase in vacuum will tell you it's clogging up.
Check the feed line
breads algee, and Algee clogs things, I once saw an Island freport 41 with Algee in the vent line
, caused a vacuum, the boat left the slip doing 5 knots, then 3 hours later came back at 2 knots
You can check for water topping the water seperators, But thatn shoud shut her down pritty quickley.
Put it in nuetral and turn the shaft coupler. the Stuffing box could be locking up.
Also look for an air ristriction. ( Not Likely)
If it's a new vessel or the was a prop change:
The Yanmar Service manual states:
gear ratio Blades size
2.14 3 360 x 250MM
2 380 x 230
2.83 3 blade 430 x 320 MM
2 460 x 280