A comparison of two mechanical wind pumping systems was carried out to determine windmill performance at different water depths. A 2.44 m (8 ft) rotor diameter windmill with 15 vanes, and a 4.88 m (16 ft) rotor diameter windmill with 32 delta wing vanes, were compared. The smaller windmill had vanes that filled 90% of the rotor area while the vanes on the larger one filled 41 % of the rotor area. The smaller unit used a gearbox and the larger one used a pump jack style with a counter weighted shaft. Results from this study showed that both windmills started pumping at about the same wind speed (2.5 and 3.5 m/s), but the larger rotor operated at 6 to 8 strokes per minute faster than the smaller system when wind speeds were between 4 and 10 m/s. Although both windmills were fitted with the. same size and style of pump, the larger rotor pumped more water because it had more stokes per unit of time. The delta wing rotor averaged 14,874 Llday compared to 10,974 Llday for the traditionally designed rotor. However, one must consider the difference in rotor diameters, total weight, cost and the efficiency of the two units. The larger rotor had a peak efficiency of 6.5% compared to 10.5% for the smaller rotor. The 35% more water pumped required twice as much rotor diameter.