Question

why boats or an aeroplanes are given special shape

Answer 1

Objects moving in fluids ( water or air ) needs special shapes to reduce friction while moving. These shapes are inspired from fishes and other animals. The shape is also called streamlined.

Answer 2

Boats or an aeroplanes are given special shape called "streamlined shape"

as the bodies moving in the fluids like ship in water and aeroplanes in the air to reduce the fluid friction