my old 2GM
(15 hp max, 13hp continuous) pushes my 9000lb 30'er at 6.5kts in near calm, and only loses a knot
or so, in about 15-20kt headwind (about 3-4ft chop).
I am no expert, but I read alot pf people recommending about 25-30 hp etc, for these size boats. I think that you do run a risk in being overpowered, in that you will often be under-revving your engine
. Of course, you can rev it higher every hour or so, etc, to prevent engine
damage, but diesels are meant to be owkred hard, not cruised at 1500 rpm
If you are cruising with engine driven pumps, huge alternators, etc, then you may be able to justify the larger engine!
That being said, if I were to ever repower
, I would like something a bit larger, like 18hp, or so, but often the choise goes from 14 to 20/22 with nothing in between!