Slower than what? Planing hulls going slower than planing speed are much
less efficient.

Actually, i am not so sure about that, I have a 44 marine trader with twin
270
hp lemans, I cruise at 9 mph (knots are for sailors with lots of time on
their

Your 44 Marine Trader is not a planing hull.

It's a semi-displacement trawler.

At what rpm do you make 8 kt? (about 9 mph)

Experiment with slowing down to 7 kt, and you might realize a 25-30 percent
increase in fuel economy. I have experienced that kind of results in the past,
in a smaller hull with similar characteristics.

Caveat: Running a diesel engine too slowly for an extended period of time is
not generally recommended. As you know, they thrive on exercise.