## 6 Results and Discussion
When the deadrise angle of the model decreases, the slamming pressure increases. Elastic effects should be considered when the deadrise angle is less than 6\({}^{\circ}\)[26]. Figure 20: Air effect at a bottom thickness of 5 and 8 mm in cylindrical shape model. The rate of increase in strain was very small compared to the rate of increase in pressure. Figures 10 and 12 confirm the occurrence of the first peak and the second peak, approximately 0.03 s later. The second peak pressure was an occurrence caused by the effects of the structure's natural frequency. The natural frequency of an unstiffened plate can be estimated using Equation (1), the natural frequency of the plate is obtained from Equation (2), as proposed by [27], and the natural frequency can be obtained from its reciprocal. Figure 13 provides the basis for supporting the previous claim. Figure 13 shows the pressure time history of the free-drop test of the model with a deadrise angle of 10\({}^{\circ}\) dropped from a height of 1.7 m. The pressure values were measured in an experiment using wood, which is assumed to be rigid. When a wooden specimen was employed in the experiment, the second peak pressure seen in the elastic models could not be confirmed. This means that in the estimation of the slamming pressure of an object with elastic effects, the natural frequency of the structure must be considered. In order to prove the absence of the influence of reflected waves, a number of experiments were conducted in which a test apparatus was set up so that the influence of the reflected waves was not affected. At the time, however, it was confirmed that a second peak occurred. The results obtained by [28] from experiments using models with deadrise angles of 0\({}^{\circ}\) and 10\({}^{\circ}\) show the same tendency in the case of pressure coefficient. However, the slamming pressure coefficient was the highest when the deadrise angle was 3\({}^{\circ}\) and was not influenced by the air effect observed when the deadrise angle was 0\({}^{\circ}\). Figure 21 shows a simplification of the slamming pressure. The peak pressure (P1) and its duration (t\({}_{1}\)) can be used to determine the triangular shape of the impulse value, as described by [29]. The values of peak pressure and duration obtained from the free-drop experiments are shown in Table 8. The impulse values are obtained by idealizing peak pressure and duration, as shown in Figure 21; when the deadrise angle was 10\({}^{\circ}\), the duration tended to remain the same, even as the peak pressure increased. However, an experiment with the cylindrical model confirmed that duration increased with pressure. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Model Shape**} & **Drop Height** & **Bottom Thickness** & **Peak Pressure** & **Duration (s)** & **Impulse** \\  & **(m)** & **(mm)** & **(kPa)** & **(kPa-s)** & **(kPa-s)** \\ \hline \multirow{6}{*}{\
\begin{tabular}{c} Deadrise angle \\ of 10\({}^{\circ}\) \\ \end{tabular} } & \multirow{2}{*}{1.0} & 3 & 170 & 0.0010 & 0.085 \\ \cline{3-6}  & & 5 & 250 & 0.0010 & 0.125 \\ \cline{3-6}  & & 8 & 290 & 0.0010 & 0.145 \\ \cline{2-6}  & & 3 & 200 & 0.0010 & 0.100 \\ \cline{2-6}  & 1.7 & 5 & 320 & 0.0010 & 0.160 \\ \cline{3-6}  & & 8 & 450 & 0.0010 & 0.225 \\ \hline \multirow{6}{*}{
\begin{tabular}{c} Cylindrical \\ \end{tabular} } & \multirow{2}{*}{1.0} & 3 & 580 & 0.0015 & 0.435 \\ \cline{3-6}  & & 5 & 750 & 0.0020 & 0.750 \\ \cline{3-6}  & & 8 & 780 & 0.0020 & 0.585 \\ \cline{3-6}  & & 3 & 770 & 0.0015 & 0.578 \\ \cline{3-6}  & 1.7 & 5 & 850 & 0.0020 & 0.850 \\ \cline{3-6}  & & 8 & 980 & 0.0020 & 0.980 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Peak pressure, duration, and impulse for the cylindrical model and model with deadrise angle of 10\({}^{\circ}\) at first peak pressure. Figure 22 compares the maximum pressure values of the cylindrical model, flat model, and the model with a deadrise angle of 10\({}^{\circ}\).