This paper proposes formulating conceptual-stage vessel design optimization problems as geometric programs, which can be transformed into convex optimization problems. Convex optimization offers significant advantages in efficiency, reliability and automation potential over the general nonlinear optimization approach typically used in naval architecture. Focusing on battery-electric vessels, geometric program compatible models are derived for lithium-ion cells, power converters, propulsion motors and propellers. Preliminary hull form development, stability calculation and structural design are also presented in the context of geometric programming. The modeling approach is applied to study optimal battery sizing for a coastal bulk carrier sailing in varying operational conditions. Using open-source software tools, the battery sizing problem is solved in less than a second on a standard desktop computer. Local sensitivity information encoded by optimal dual variables reveals that increasing the cell discharge upper bound by 1% decreases the optimal total number of cells by more than 1%. On the other hand, the sensitivities of cell volume and maximum discharging current parameters are zero, indicating that the constraints involving these parameters do not govern the solution.