I had a 21' center console with a 175 HP Mercury
Verado, boat weight about 6,000 lbs?. Best fuel consumption
was just above idle and returned about 6 NM per Gl. On plane at 20 kts, it's 2.3 MPG.
I now have a 30,000 LB sailboat powered by a 44 HP Diesel
, at 7 kts and 2,000 RPM
my burn rate is 1 gl an hour which of course is 7 MPG.
I'd say the Diesel
is more efficient as the whole boat, drive train and engine
are optimized at hull speed
One thing that I think makes a rather large difference is I believe the 44 HP Diesel is turning an 18" prop and the much larger, higher HP outboards turn much smaller diameter props, which I think means they are designed with much higher speeds in mind and I assume these small props just can't deliver high thrust at low speeds which is what we are after.