A former U.S. Coast Guard Martin PBM-5G Mariner used by the U.S. Navy for radar cross section observation tests off Point Loma in 1963

The PBM was used to gather information for the design of a seaplane that would rest nearly motionless on the open ocean in all weather conditions.

According to the archivist at the Glenn L. Martin Museum:  in Project Sea Stilt, an experiment to develop "dipping sonar" for aircraft. Both Convair and Martin competed for the future contract. The intent was to design some sort of "stable platform" for using seaplanes as ASW sonar dippers. Convair won the competition, but, in the end, both firms lost out to the obvious choice for such an assignment: the helicopter

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