Flowers Boat Works 46 hull #1, Flowers Boat Works 36 hull #1, Wayne Beal 28 Hull #1, Repco 30 1968,
My buddys 42 Duffy, powered with a 800 Cat and weighs in at about 35,000lbs burns 28-29 gal/hr at 19kts, full fuel 700+gallons, 1000lbs ice, bait and crew. This is a flybridge boat with 2:1 reduction and 5 bladed prop.
If you had a very efficient hull, a big prop, no bridge, and a light weight basic 42', you may be able get fairly close to 1 nmpg at a 16- 18 knot cruise, but I think everything would have to be working in your favor to even get close. Sounds like the #'s you quoted for the 42' H&H are probably typical and what you could expect for most 42' boats.
This is just my opinion, and I could be wrong. But an efficient 42' boat would be a 40' Young brothers lengthened 2' by Stewart Workman; install a 500hp Deere, Volvo, Cat 9 liter with a 2.5 reduction, spinning a 32" diameter prop, build it light weight, no bridge, no extra comforts, and I bet you could hit 1 nmpg at 16-18 knots. 'Course, I could be wrong, but the 40' Young Brothers is more slim at 13'6" than the other 40's and would likely push a little easier than the wider boats.
I must say, those are impressive performance numbers for a 430hp engine in a large 42' boat. Must be a light boat and a good running engine. I was comparing the torque curves of the Diamond 430hp vs. the new QSC 500hp, and the QSC has a much more robust torque curve, peaks at 1,327 ft/lbs. The 430hp Diamond peaks at 1,029 ft/lbs. As I said before, keep the boat light and simple, install a lighter weight 500hp engine like a 9 liter or QSC, 2.5 reduction, big prop, and that will be the most efficient combo possible in a 42' boat imo. And with a 500hp engine, you should be able to beat Sockeye's numbers in speed.
Splashed the Boat today and ran her to check everything out. It's a 40 H&H with a 650 Mack. At 14-15 knots we were burning 15 -16 GPH with the tide. At 19 Knots it was 26 GPH against the tide as per the Flow Scan.