In this study, the effect of the gel coat composition on tensile strength of glass fibre reinforced polyester (GFRP) composites was evaluated. The experiments were conducted on various gel coat composition and time. Samples were immersed in sea and river water for 6 months to investigate the differences in tensile strength. Optical observation using SEM on the composites surface and comparative analysis in terms of tensile strength were used before and after immersing. As results, the most excellent tensile strength was the gel coat composition consisting of 0.3 kg of reolosil QS-102 with 10 kg of resin SHCP 268BQTN with a value of 51.9 MPa. Tensile strength reduced about 33.4 % compared to other sample where reduced to 46.3401 %. The difference in the composite surface as a result of immersion process on each sample was also discussed in this paper.
