In river boat problems, we come across these three terms-

v_b= velocity of boat with respect to ground.

v_r= velocity of river w.r.t ground

v_br = velocity of boat w.r.t river

There are 2 types of problems-

1. When Boatsman crosses the river through shortest possible path-

In this case the magnitude of velocity of the boat 

v_b= √(v²_br-v²_r)

and time taken to cross the river

t= d/√(v²_br-v²_r)             (d= width of river)

2. When boatsman crosses the river in shortest possible time-

In this case, time taken to cross the river,

t =d / v_br