The Twin Otter aircraft departed Oakland at 10:50 PST for a ferry flight to Honolulu. For this flight the aircraft had been equipped with four ferry fuel tanks in addition to the normal forward and aft internal aircraft tanks. The pilot stated that at 18:00 PST (7 hours and 10 minutes into the flight), the estimated time to fuel depletion was found to be 8 hours and 40 minutes, with a time to go to destination of 7 hours and 40 minutes. Three hours later, the pilot determined that he was fuel critical due to unforecast headwinds. He declared a low fuel emergency and changed his intended destination to Hilo, HI. He also requested Coast Guard assistance at that time. Approximately 4 hours later, the pilot elected to ditch under power rather than remain airborne until fuel depletion. PROBABLE CAUSE: “The pilot’s inadequate en route fuel consumption calculations, which led to his failure to recognize a deteriorating fuel duration versus time-to-go situation in a more timely way”