I think 1.5 gph is achievable in lightly loaded calm sea conditions, with the bottom and running gear
spotless, and the engine
and running gear
in perfect condition. Much more likely to happen with the Cummins motor
than the DD, and at something below 7 knots speed.
These boats, although obviously heavy built, had no superstructure (wind resistance) and relatively small tankage (weight).
Of course the fuel
burn test could be done with just a minimal amount of fuel
in the tank and the operator as total load.
Loaded up with amenities, that burn rate is sure to increase.
That being said, one of my favorite fantasies is to convert a Willard 50 into a sportfishing/ island hopper for warm weather
use, making extensive use of canvas
frames to provide shelter and a raised command platform midships.
The quandry is stability, the roll factor resulting from the rounded stern profile will increase steeply as weight is added above the waterline.
The quick and dirty solution would be bilge
keels, but the drag penalty might be steep.