I have two ways of approaching this question:
1. This summer I helped a CHB 45 owner move his boat which had twin 135 hp Perkins
. We used about 4-5 gph and cruised at about 8 kts. This is a bigger and heavier boat than yours.
2. Another way is that a 30,000 lb boat (my guess) will need about 50 hp to make hull speed
. Your hull speed
is about 8 kts. 50 hp will require about .06*50= 3 gph. Twin engines will burn a little more.
So 3-4 gph is my guess.
Also check your engine rpm
at that speed and take the fuel consumption
from the Cummins PROP curve.