To answer the need for a new water turbine, with lower specific speeds, that would maintain all of the advantages of Saxo turbines. We designed a new single regulated vertical tubular turbine, with a diagonal runner, that enables lower specific speeds. Rather than performing expensive and time consuming experimental tests for water turbines, we used numerical simulations for evaluation of working parameters for the new turbine. Computational fluid dynamics were used to simulate different operating points. With the simulations, we have assumed incompressible single-phase flow. We presented and explained all the used leading equations, with the SST turbulence model. From the simulated operating points, we modelled the hill diagram for the new turbine, which has a smaller area of optimal performance compared to Saxo turbines. The new configuration acquires lower specific speeds between Francis and diagonal type turbines. With a more extensive analyzation, we observed three separate operating points and present cavitation and flow conditions in the turbine. In part load operation, we observed the emergence of inter-blade cavitating vortices and vortex rope in draft tube. The observed conditions are similar to those in Francis type turbines.