**1. Introduction**

Shipyards and ship design engineering companies are continuously making numerous efforts to improve the performance of their ships, to satisfy the requirements of clients and meet various environmental regulations.

The performance of a ship is determined by various factors, such as speed, fuel oil consumption (FOC), and deadweight. In particular, speed is the major indicator of a ship's performance and is one of the performance aspects that are guaranteed, through a sea trial after construction.

Although there are various methods, including attaching an appendage to improve the ship's speed, the most basic method is to improve the resistance performance by optimizing the hull form of a ship. Therefore, shipyards and ship design engineering companies continue to invest heavily in improving the existing hull forms or developing new hull forms. In addition, various methods are used to reliably estimate the resistance performance of newly developed ships.

Traditionally, model test using a basin has been employed to estimate the resistance performance of ships. However, with the recent developments in computer technologies, numerical simulations using computational fluid dynamics (CFD) have attracted attention as a replacement for experimental methods.

In the beginning, analysis using numerical simulations was performed only on the sub-surface portion of the ship, without considering the free surface. Since then, the analysis methods have evolved to consider other aspects, such as the free surface and variation in the ship's attitude for accurate performance estimation. In addition, full-scale numerical simulation [1–3] of a ship, which is difficult to perform with the latest experimental methods, numerical simulation considering the hull roughness [4,5], and various other studies are underway.

In general, the estimation of resistance of a full-scale ship, through numerical simulations, is performed in the same method as in the experiment. First, a numerical simulation is performed for a model ship, which is a downsized model of a full-scale ship, and the total resistance value obtained from this simulation is used to estimate the resistance of the full-scale ship.

While estimating the resistance performance of a full-scale ship in the experimental method, as well as in the numerical simulation method, the air resistance acting on the superstructure, which has a relatively smaller effect on resistance performance than water, is estimated using an empirical formula without directly considering the superstructure [6].

However, for ships with large superstructures, such as container ships, LNG carriers, and car ferries, wind could not only affect the resistance acting on the superstructure but could also cause variation in the ship's attitude.

The variation in the ship's attitude is one of the factors that can directly affect the resistance performance [7–9]. As the resistance acting on the ship can increase or decrease according to the ship's attitude, an analysis that considers the superstructure is required for an accurate estimation of resistance performance.

Therefore, in this study, the effects of the presence or absence of the superstructure were evaluated by analyzing the resistance performance in two different cases; a model ship of an 8000 TEU-class container ship, with superstructures and without superstructures.
