Checked it out. I wouldn't think old style 2 stroke DD 6-71 would approach the low fuel consumption numbers I'm looking for.
IF , operated in the zone , 60% of rated as builr power the DD will create 16 HP from a gallon of fuel.
Sure in theory 20+ can be done with a DEDEC DD , but the cost of parts or a controll box would wope out the firtst 10,000 hours of "savings"
For most use 20 to 30 HP per cylinder is the range , so many trawlers would be fine with a 3-71.