Try the same calculation while doing 8 - 9kts.
You need to find the optimum speed vs range.
Just like driving a car, 15mph with a full tank won't get you very far, neither will 150mph on a full tank BUT doing 70mph will.
Or, consider that if you are using SH3 Commander you may have been sabotaged or developed a malfunction.
Edit - Opps, Manners. Welcome to Subsim.
