I have a 73 Gulfstar 43 with twin Perkins 135s. Running one screw at 1500 I'll get 6-8 kts and burn 3.5 gph.
HOWEVER, running two screws, each at 1500, I'll get 8-10 kts and burn 2.5 gph.
Yes, I burn less diesel on two screws than I do one...
I gave up trying to figure out torque curves and angle of the rudders and just accept that Lucky wants to run on two screws