Get yourself a nice, flat calm day (0 mt/s). Select speed by actual knots, not "Standard, ⅔ etc). Try from (say) 6 kts to 10 kts and each time the speed has settled check with your NO "Max range at current speed". You'll soon find the optimum speed for maximum range for the type of boat you're in.
ORrrrr... just d/l and install WB's Decks Awash mod that adjusts the setting for "Ahead ⅔" to give the best economy by default
and sets the "Snorkel depth" command to give you decks awash by default (can be removed later when you get one fitted). I was in the process of doing exactly what WB had already done when I came across his mod - so in a fit of pique I checked all his settings in-game and guess what? He got 'em right, lol.
