An unconventional small-waterplane-area-twin-hull(SWATH) ship was designed, whose twin hulls were slightly curved in an S-shape and arranged at a mean yaw angle but mirror symmetric to their common longitudinal center plane. Based on the “zero wave resistance” theory, a viscous flow theory which could solve the Reynolds-averaged Navier-Stokes (RANS)  equations was used to study the resistance performance of the unconventional SWATH ship in calm water. In comparison with a benchmark conventional SWATH ship, which features a typical torpedo-shaped body, the simulation results prove the effectiveness of the S-shaped design. The unconventional SWATH ship has much less low-speed drag than its conventional SWATH counterpart and comparable total drag at high speeds.