Still water speed (ship speed)+current speed (water speed) = downstream speed;

Ship speed-water speed = water flow speed;

(downstream speed+upstream speed) ÷2= ship speed;

(downstream speed-upstream speed) ÷2= water flow speed.

(2) Formula for two ships sailing in opposite directions:

Downstream speed of ship A+downstream speed of ship B = still water speed of ship A+still water speed of ship B.

(3) Formula for two ships sailing in the same direction:

Hydrostatic speed of rear (front) ship-Hydrostatic speed of front (rear) ship = the speed of narrowing (expanding) the distance between two ships.