Water pumping windmills have been used for centuries. The multi-bladed, slowly rotating windpumps developed in the late 1800s are still common all over the world. However their aerodynamic performance is not properly understood because of blade interaction for high solidity operating at very slow speeds. Developing a numerical method to predict and improve performance of water-pumping windmills is essential to allow for design optimization and performance improvement. The choice of the aerodynamic model was made on basis of classical blade element momentum method due to its speed, simplicity and wide acceptance in the wind industry. Blade element theory and momentum theory are combined in this method to predict torque, power and thrust with the inputs of tip speed ratio, blade profile, number of blades and airfoil lift and drag coefficient data. Using blade element momentum theory (BEMT) comes with its own challenges, as certain assumptions do not hold true for high solidity and low speed operating rotors. There is no interaction between two adjacent blades which does not hold true for higher solidity wind turbines. BEMT code was developed by the author and verified with experiments. BEMT was used to characterize the effects of spars and tip loss correction on performance of rotor for six, twelve, twenty four bladed rotor. New tip loss correction was developed for very low speed running wind turbines and compared with BEMT with Prandtl tip loss correction and experiments. Experimental investigation was performed for N=6, 12, 24 bladed rotor configuration using Wegereef  blade profile to get complete performance profile of power, torque and thrust with their coefficients over range of tip speed ratios. Experimental setup was designed and structural analysis was performed by the author to ensure safe operation of the test rig. Monotonic increase in performance parameters was observed in thrust, power and torque for six, twelve, twenty four bladed rotor. From BEMT results it can be concluded that effect of tip loss and blade profile are significant on rotor performance. Although blade profile effects are less than tip loss effects. Solidity effects were found to be very small. BEMT with new tip loss predicted performance are compared with Prandtl's tip loss correction.