Lines of Communication

[Lexicus]

Well, within your borders supply and trade would be automatic, but you'd need to take it along when you go to enemy territory. Ships should also start losing health after a little while and have to make port (you could also do this in neutral territory) to recover and resupply. More modern ships would suffer less attrition, and Refrigeration would seriously reduce the damage taken.

 

[onomastikon]

Civ IV is not a good wargame. I feel that the Civ series is a fine TBS, of course, but do not see its strength resting in war-playing.
That said, you might want to check out the "Dominions" series (Dominions 3 is the current iteration), a very close-knit community to which I once belonged, all good people. Dominions is a TBS WARGAME, and a decent one too, since it concentrates on strategy AND tactics, albeit at the cost of graphics and sound (which are both still at the level of the early 90s). I should add that the genre is fantasy, but you can mod whatever you want.
Anyhow, they have supply lines done quite nicely, not only communication and fog of war, but also food etc. supply. Quite nice system actually. And the idea of "culture" is much more dynamic than in Civ, it's quite good actually for a wargame.
best to all