With a little planning fuel shortage should not be an issue, travelling at 1/3 speed you should be able to reach any mission objective with 3/4 fuel left (even from Pearl) use the other 1/4 fuel having a mooch - following contacts (dont forget to use electrics by day) - sinking convoys and such, then when down to 1/2 a tank head for refit - I know this is a bore to some simmers travelling at such slow speeds but it is the realistic way to use the sim, of course if you dont like the realistic option then the unlimited fuel check box is your friend
Hope this helps.