Viscosity in fluids is the analog of friction in solids. Both are methods by which the kinetic energy of moving bodies can be changed into thermal power. In the absence of friction force, a part of block could slide at constant speed with a horizontal surface in the same way; an object going by a no viscous fluid would experience no viscous drag friction. Lord Rayleigh has noticed out that, in a perfect fluid, a ship's propeller would not work but, on the other hand, a ship could not need a propeller.