## 5 Discussion  
The Gc values of the two hull plates with different design Gc were verified through four methods. Figure 15 shows a comparison of the experimental results. The differences between the Gc values obtained by the simple direct measurement and hydrometer method were not significant, because the error in the measurement of the external volume was relatively small. Therefore, the discrepancies in the results obtained using the burn-off method are likely due to differences in the volume of inner defects, such as voids. In other words, it was found that the volumes of hull plates with design Gc values of 40 and 64 wt.% contained voids with volumes comprising 0.95 and 1.20 wt.% of the total volume. As shown in Figure 14, the larger difference in the measured results for the Gc 64 wt.% hull plate can be considered to arise from the fabrication qualities of the laminate structures. Furthermore, although these are relatively well-fabricated prototypes produced using the vacuum infusion method, the differences in the results are not negligible. For example, the 2.14 wt.% difference in Gc obtained in the burn-off measurement method for the hull plate with a design Gc value of 64 wt.% corresponds to a decrease in the bending strength of the laminate of about 4.32%, based on the ISO 12215 equations.  
Figure 14: Volume fraction of voids in the specimens, measured by the burn-off method: (**a**) design Gc 40 wt.%; (**b**) design Gc 64 wt.%.  
Figure 15: Comparison of Gc values measured by the rule calculation, direct measurement, and burn-off methods.  
The experimental results are summarized in the form of a box plot, as shown in Figure 16. In the case of the 40 wt.% Gc hull plate, the shape, including the thickness, corresponded well with the design. Nevertheless, nonuniform impregnation of the polyester resin can be confirmed by the deviation in Gc value shown in Figure 15. In particular, as shown in Figure 14, specimen #2 (with a Gc of 35.66 wt.%) contained a relatively large volume of voids and polyester. In the case of the hull plate with a Gc of 64 wt.%, it can be confirmed from the increased Gc seen with the simple direct and hydrometer measurement methods that the pressure of the air compressor was slightly excessive during infusion. This was done to achieve the high Gc level, and it was confirmed that they were fabricated with a thickness slightly below the design thickness. However, this does not constitute an increase in the actual Gc value, but rather an increase in the internal void volume. This resulted in a lower E-glass fiber content, relative to the content specified in the design, and this was confirmed in the burn-off measurement results.  
The results of each measurement method may be summarized as follows: the rule calculation can be used very quickly and efficiently if the ship design and material design are known. However, depending on the manufacturing quality of laminate structures, the errors in the measurements of physical properties may increase, and these errors cannot be confirmed. To reduce this error, it is reasonable to use the density suggested by the manufacturer or the density measured directly, rather than use the density of the fiber and resin suggested by the rule. In the simple direct measurement, the differences in shape between the design and laminate can be easily compared. In particular, it is easy to identify key errors from the comparisons of laminate thicknesses and weights. Nonetheless, it is preferable to use a device such as a hydrometer to identify apparent quality defects involving the density of raw materials or the non-uniformity of the external shape. However, with these methods, the effect of internal defects on Gc cannot be identified. Using the burn-off measurement method, suggested in the rules of classification societies, it was possible to measure Gc most accurately, and defects in laminates, including voids, could also be measured very accurately. However, this method requires specialized knowledge regarding analysis with the equipment used for combustion.