Whirlpools. Coloured high-speed photograph of whirlpools in water caused by rapidly spinning balls. The balls have been dropped from the same height, but are spinning at different rates. Their spin rate increases from left to right. The whirlpools (or vortices) they produce depend on their speed of rotation. Study of fluid dynamics (hydrodynamics) can aid the design of underwater machinery, such as boat's propellers.