The assumptions that were made by Mitchell and later on followed till date are as follows: the fluid is non-viscous and irrotational- did I explain to you what is irrotational? Irrotational means, if the fluid particles do not have a rotational component of velocity, all velocities are linear, what does that mean, where does that fluid particle have a rotational velocity, can you tell me? A vortex which need not be in bounded layer, which can happen in non viscous fluid, that vortex or circulation- that is rotational flow. But for ships the assumption was there is no rotational flow and this gave rise to what is known as a velocity potential, this makes life much easier, this assumption gives rise to a velocity potential phi- and this is the basis of all theoretical calculations, we will not go more into this.