Boat in a River

v_resultant = v_boat + v_river (vector addition).

To cross river straight: point upstream at angle φ = sin⁻¹(v_river/v_boat) — only works if v_boat > v_river.

Min crossing time: point boat perpendicular to current (ignore drift).

Min drift: use angle cos⁻¹(v_river/v_boat) from perpendicular — requires v_boat > v_river.