In this paper, in order to maintain frequency stability in the shipboard microgrid, an improved time-varying derivative FOPID controller is designed. When a sudden (step) load change occurs in the islanded microgrid, the derivative term of the FOPID controller produces a large and instantaneous signal, which causes damage to the shipboard microgrid equipment. In this paper, in order to maintain frequency stability in the shipboard microgrid and prevent damage to its equipment, an improved time-varying derivative FOPID controller is designed. In fact, the use of time-varying derivative in the FOPID controller is to prevent a large and instantaneous signal. In order to adjust the parameters of the proposed controller in the shipboard microgrid, the whale optimization algorithm is used. The whale optimization algorithm is used due to its high convergence and speed and to avoid getting stuck in local optima. Several scenarios are considered to compare the performance of the proposed method with other controllers such as the time-varying derivative FOPID controller optimized with the modified sine-cosine algorithm, the FOPID controller, and the PID controller, and the results show that the proposed method is superior. The proposed method was able to reduce the maximum frequency deviations caused by disturbances on the ship's microgrid by 48% and also improve the settling time of these deviations by 15%. The results show that the proposed controller has a significant advantage over the FOPID and PID methods and even the optimized version with the sine-cosine algorithm.