Coatmar is right but only posted half the picture. Max rated rpm is 2800-3200. Max torque is near 2000 rpm and Volvo states the D1 was designed as a lower reving engine that cruises at 1800-2200.
Just finished a 1500 nm delivery
and 0.5 gallons (US)/hour at 2000 rpm per engine was very close to our computed fuel burn. This was motor
sailing with port, starboard or both engines running. Sailplan was kept conservative per the reefing schedule in the sailing manual. Average speed was near 7.4 nmph (knots). That is nearly 15 nmpg.
btw, running both engines increased speed by perhaps a knot
, maybe a knot
and a half and jacking the rpms to 2800-3000 made a trifle more.
Compute the hull speed
...it's about 7.4 knots and you are way into the area of diminishing returns.
In all my reading, I've never seen a Volvo recommendation to cruise
the D1-20 or -30 engine above 2200 rpm. From the torque curve, it appears to have been designed for the 1800-2200 range.