Diesel engines use approximately 1 gal per hour per 20 HP developed
Fuel consumed is proportional to the power developed
Power developed is approximately proportional to the cube of the engine revs
Hence fuel consumed is proportional to the cube of the engine revs
If you ½ the revs you reduce the power & hence fuel consumption by 1/8
Assume a sailing boat has 40 hp engine
Max revs 3800 rpm
At 3800rpm developes 40 hp ( consumes 2 gal per hour )
If normal cruising revs are 3000 rpm ( 80% of max )
Power developed 80% X 80% X 80% = 51.2% of the max
ie 0.51 x 40 = 20.4 hp
Fuel consumed 1 gal per hour
Therefore by reducing the engine revs from 3800 to 3000 you have halved the fuel consumption. & most probably only reduced the speed from say 7kn to 6kn.
Now reduce the engine speed to 2500 rpm ( 65% of max )
Power developed 65% x 65% x65% = 27% max
ie 0.27 x 40 = 10hp
Fuel consumed 0.5 gal per hour
The same rule apply to much larger diesel engines.
A 400hp engine will consume 20 gal per hour at full revs and only 2.5 galls per hour at ½ max revs.