Sea Trade Problem

[Victorvanwavere]

Could someone please explain all the needed criteria for trading over seas?

In my Mod, I made a "Port" that allows trade over sea. Yet this only works SOME times. Lets say I have 3 islands. It will work on 2 but not the third. Has distance got anything to do with it? Maybe city Size?

Any help will be appreciated.
Thanks

 

[Ed O'War]

Certain technologies are also required to enable trading over Sea and Ocean tiles (Magnetism, Navigation?). I think the basic harbor only allows trade over Coastal tiles.

 

[Kilroy]

Astronomy is required for trade over seas, and navigation (or magnetism) for trade over oceans.