Welcome to SS!
Fuel can be a right little bum in the Pacific. Once I was operating the Vaughan out of Manila, I had to go up to patrol off of Tokyo for 48 hours, then go back to Manila. I was at about 50% fuel by the time I was just about to finish the last leg of my return journey. However, I got a radio message ordering me to relocate to bleedin'
JAVA which is about a million miles thattaway.
I used about 5 knots on the surface and ran my batteries right down when they were charged and just about scraped getting there