I dont know a darn thing about this

[RConch]

Help me out please folks.
All these years of playing GWX, I still don't know how the ships\planes appear on the map or cross my path.
Do the ships begin at a port and head to a port or is it just where I am on the map?
I guess the term is "spawn" but how?
I am trying to figure out convoy patterns if it is possible, such as 7 day\10 day ect.
Or does that not come into play?

Thanks