28 of the world's major surface currents: the Florida Current, Gulf Stream and North Atlantic Drift; the Canary, North and South Equatorial, Caribbean, North Brazil, Brazil and Benguela currents; the Agulhas, the Indian Ocean's South Equatorial, the Somali and Monsoon currents (which run the other way in the northern winter, with the monsoon), the Leeuwin and East Australian currents; the Kuroshio, North Pacific, California, Alaska, Oyashio, the Pacific's equatorial currents and its Equatorial Counter Current, the Humboldt; the Antarctic Circumpolar, East Greenland and Labrador currents. The month is the calendar's, whatever the seasonal closures setting.
Ships feel them: a lane's speed zones are split where the current's push along the lane changes, each sped up or slowed by (15 kn + the current along the way) / 15 kn, at most 35% either way. Fuel burns by the second, so a ship with the current also burns less fuel a leg, and one against it more; the fuel a ship needs for a leg counts the currents of the month it sets off in, so it never runs dry for them.
It's kept if it saves at least 2.5% of the leg's time in some months and is no more than 1.2 times as long as the stretch it leaves out; the months it's quicker in go with it. A ship setting off on a leg takes the second way round in those months, and keeps to the way it chose (and the currents of the month it chose it in) to the end of the leg, saved with it, the host's way sent to the other players. So ships from the US to Europe ride the Gulf Stream, and with the Somali Current turning with the monsoon, the way along the Horn of Africa changes over the year. Lanes through a canal, and lanes to or from the Arctic, have no second way. Its status says "Sailing to Rotterdam on the Gulf Stream".
On the map the currents flow: dashes drifting along three lines across each current's width the way the water runs (back the other way in the months it turns), warm currents red-orange and cold ones blue, brighter the stronger it runs this month; only the middle line far out; fading in from a mid zoom and out again close in.
Maritime Empire