## 1 Introduction  
Glass fiber reinforced plastics (GFRPs) have been widely used for decades for building small ships, such as fishing boats and yachts [1; 2], as they exhibit good specific strength, corrosion resistance, and excellent workability. Many GFRP ships are manufactured with a large design margin, as this enables faster and cheaper production. However, it also results in heavier vessels due to the thicker GFRP laminate structures. Laminate structures for composite ships are considerably thicker than those used for aviation and automobile components, which can cause adverse structural effects such as fatigue over the ship's lifetime [3]. In addition, GFRPs have elicited environmental concerns related to their poor recyclability and issues during drying and disposal [4; 5]. Accordingly, increased research attention has been directed toward the optimal design and weight reduction of GFRP ships. The design regulations for GFRP structures of small crafts are addressed in the international standard ISO 12215-5 [6]. The laminate thickness, controlled by the number of glass fiber cloth layers (plies), is designed based on ship variables such as hull shape, displacement, speed, and hull form, as well as structural variables such as the layout of stiffeners, design loads, and the design of the composite material (type of reinforcement material, reinforcement method, and mechanical properties) [7, 8]. The glass fiber weight fraction, Gc, is a critical design element that significantly affects the mechanical properties of the laminate. The ISO standards and classification society rules provide equations for estimating the mechanical properties of laminate structures according to variations in Gc. These theoretical equations are also differentiated according to the type of glass fiber, reinforcement method, number of plies, and amount of resin used [6, 9, 10]. However, while Gc is an important indicator of the fabrication quality of a laminate structure [11], the quality of the laminate can vary widely according to the manufacturing method, environment, and operator skill level. GFRP ships are typically manufactured using the hand lay-up method, which can lead to degradation of the glass fiber or resin due to human error [12, 13]. Moreover, the laminate structures often contain fabrication defects, such as porosity or voids. These fabrication defects can have a significant impact on the physical performance of the laminate structure, even if Gc meets the design parameters [14, 15, 16]. For ships that adopt special glass fibers, special structures, or structures thinner than the rules allow, classification societies require manufacturers to disclose the Gc of the laminate structure along with the results of material tests. The mechanical properties are often verified by fracture testing according to the ASTM standards [3, 7, 11, 17], while experimental methods (e.g., resin burn-off) or theoretical calculations are recommended to determine Gc. Nevertheless, the classification rules do not provide detailed specifications on determining Gc. In addition, because of the flexible fabrication characteristics and different types and combinations of materials, it is difficult to verify that laminates are fabricated according to the designed Gc. To aid the design and manufacture of GFRP laminates for shipbuilding, it is critical to identify an accurate and consistent method of determining Gc. The method should also measure the size and volume of voids inside the laminate structure, as well as the measurement error of the outer shape. Considering the diverse methods currently in use to determine Gc and defect incidence, a comparative study of the different methods is of significant applicative interest.