Though the disc is rotating, we will assume right now the all changes in velocity are only in the axial direction as shown in this diagram. These are very drastic assumptions, but we have to start from somewhere. So, we make these assumptions as it was done by Rankin and R E Froude. Now, what is the mass of fluid passing through the propeller disc? Let us call it m rho A 0 V A plus V 1. Is that correct? What is thrust? Rate of change of momentum though mass into V A plus V 2, please see the diagram