I am again confused at this. As far as I know, specific speed of a turbine of given Shape and Size is defined as -"The speed at which geometrically similar (i.e. scaled down Version) turbine would rotate when provided with unit head and unit discharge" The general text-book derivation for specific speed gives But, I guess its written the other-way-round. I mean, most of the time, we have our turbine, and we have the specific speed for our turbine (owing to its shape & provided by the manufacturer), and we would like to find out at what speed it would rotate when we provide some head H and Discharge Q (P). So, I think, the useful way of writing the formula would be N = Ns * H^(5/4) / sqrt(P) . That is all for my understanding. Now my confusion. :) I also find that text-books gives charts for Ns Vs Head ? I thought, specific speed is dependent only on Turbine Geometry. Then how come, say a franchise turbine of given geometry have different specific speed for different heads? I feel like It kills the very own definition of specific speed. Thanks for reading.