Experimental investigations of friction factor and thermal performance of V-trough thermosyphon solar water heater with helical twisted tapes of various twist ratios (Y = 3, 4, 5 & 6) have been performed and presented. Experimental trials were made under same operating conditions and results were compared with plain tube V-trough (PVT) solar water heater and also with plain tube flat plate (PFP) solar water heater. Experimental data for the PVT solar water heater is compared with the fundamental equations and found the discrepancy of +/- 15.06% and +/- 3.91% for Nusselt number and friction factor respectively. Results show that PVT collector offers better thermal performances when compared to PFP collector. It is due to the increase in solar concentration by the use of additional reflecting surfaces in solar collector. Moreover the usage of helical twisted tape creates the swirl flow inside the riser tube, which in turn increases the thermal performance and pressure drop. The minimum twist ratio 3 offers 19.01% higher thermal efficiency when compared to plain V-trough collector. Empirical correlations have been developed for Nusselt number and friction factor to match the experimental results and deviation fall within +/- 10.3% and +/- 6.74% respectively. (C) 2016 Elsevier Ltd. All rights reserved.