The U.S. Navy did not use supply ships at sea since Fleet Boats were designed with long range in mind.
On the fuel issue, there are game problems, namely:
1. the SH world is flat so many game distances can be as much as 25% farther than in RL;
2. there is a problem with fuel usage, namely ahead standard uses more fuel than in RL and ahead 2/3 uses too little.
Both RFB and TMO try to address this in their own way. Ducimus discusses this in more detail in the TMO manual. As a rule of thumb, I think about heading back to base when I have 35-40% fuel left.
more info here:
http://www.subsim.com/radioroom/show...=196788&page=2