Vertical-axis hydrokinetic turbines, with their simpler designs and omnidirectional characteristics, have the potential to supply renewable energy cost-effectively from river streams for off-grid remote communities. However, their low performance efficiencies hinder the large-scale applications of these turbines. Previous works have suggested that applying pitch angles on the blades of these turbines can improve their efficiencies. This work aimed to numerically investigate the effects of pitch optimisation on the performance of a straight-bladed vertical-axis turbine, in terms of its torque and power coefficients. Two pitch-optimisation modes, fixed and variable-pitch angles, were employed in addition to the baseline configuration of the turbine at different tip-speed ratios. The dynamics of the turbine were studied in terms of the blade incidence angles and the resultant aerodynamic and rotary forces on the blades. A semi-empirical model was developed herein using to model a simple H-type vertical-axis turbine with three blades, using real aerofoil data to represent the aerodynamic characteristics of the blades. Analyses of the results at a tip-speed ratio of 1 showed that variable-pitch optimisation was far more effective, compared to the fixed-pitch mode, in improving the torque and power coefficients of the turbine. This dynamic mode improved the turbine performance by eliminating, almost entirely, the torque-suppressing drag-induced component of the tangential force throughout the full rotation of each blade. Fixed-pitch optimisation, on the other hand, had competing gains and losses in the downstream and upstream regions, resulting in marginal increments of the turbine performance. This competing effect was eliminated at a higher tip-speed ratio of 2, effectively improving the impact of fixed-pitch optimisation on the turbine performance. In both cases of the tip-speed ratios, positive and negative radial force coefficients on the blades were both unfavourable. Unfortunately, their average values scaled more than eight times higher than those of the tangential force coefficients.