In service the PBRs turned out to not be as fast as hoped and were maintenance intensive.
There were several programs in the late/post-Vietnam era that tried to come up with a new generation
of patrol/attack craft and propulsion was one of the big problem areas. Waterjets and tunnel drive designs
were part of the research, but at the end of the day it was determined that two to three large outboards
would be the best solution.
Of course, once they had come to this understanding they promptly cancelled all the development programs.