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 covered
aluminum 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.