increases, an increasing proportion of the jet falls onto a different cup geometry and so the agreement with the model decreases. This is illustrated by Fig. 12, which shows photos of the turbine disc in operation. Fig. 12a shows radial flow within a cup, the flow enter- ing and exiting at different radii. This increases the path length of the flow and changes the radius where the resultant force acts. This spreading out is due to the impact against the cup, as described for collision of coplanar jets [25], and the centrifugal force from the interaction of the rotating turbine and the straight jet. There is also interference between the incoming cup and the jet, as shown in Fig. 12b, which causes the jet to become more turbulent and de- flects some of the water away from the aimed position on the cup. As can be seen in both photos in Fig. 12, there is a large amount of water splashing around the turbine casing which will undoubtedly interact with the incoming jet. Some of the aforementioned flow characteristics could be mod- elled quite simply. For example the gravity droop could be de- scribed using of equations of motion. However, the more complex interactions, such as the jet spreading shown in Fig. 18 would be difficult to model analytically. Experiments with differ- ent turbine discs and alternate geometry could provide the basis for empirical functions for a more accurate model. The maximum power speed point can be estimated from basic impulse turbine theory, for example in [14]. This suggests that the maximum power of the turbine should occur when the tangen- tial rotational velocity (u1) is half the nozzle velocity (v1). In the model developed in Section 2, this ratio is 0.51. In real turbines this ratio is found to be more in the region of 0.47 [13]. In this testing, the maximum power occurred at a ratio of between 0.42 and 0.44. The experimental results in Figs. 10 and 11 show that increasing nozzle diameter for this fixed cup size reduces the efficiency; how- ever the mechanical power generated still increases. The 20 mm jet generates 75 W at its maximum efficiency point of 85% whilst the 30 mm jet generates 142 W but with only 70% efficiency. There- fore, a disproportionate increase in water flow is required to gen- erate the extra power. The model is shown to be simple and robust, and able to provide a good first approximation to the performance of the turbine. As the model is not able to accurately represent all the effects experi- enced in the turbine, an experimental study is carried out to fur- ther vary the parameters identified from the model, detailed in Table 2, to explore the design space further, in view of improving the turbine efficiency and overall turbine performance.