## 4 Analysis of the Influence of Water Depth on Air Layer Drag Reduction
### Effect on the Drag Reduction
(2) When there is no jetting, the frictional resistance rises as the water depth drops. However, rather than increasing, frictional resistance reduces when the water depth reaches the extremely shallow water condition (\(h/T=1.18\)). (3) When comparing the situations of \"Groove No Jetting\" and \"Groove Jetting\", it can be seen that the relative decrease in frictional resistance increases as the water depth decreases. However, the magnitude of the relative reduction in frictional resistance starts to diminish when the water depth reaches the extremely shallow water state. This pattern matches the pattern seen in non-jetting cases. It may be caused by the poorer coverage effect of the air layer in extremely shallow water conditions. The effects of bottom groove jetting on viscous pressure resistance at various water depths are shown in Tables 11 and 12. The following conclusions can be drawn from the tables: (1) Viscous pressure resistance increases significantly as water depth decreases, suggesting that the increase in resistance caused by shallow water is mainly attributed to viscous pressure resistance. (2) By contrasting the examples of \"Groove No Jetting\" and \"Groove Jetting\", it can be seen that jetting not only lowers frictional resistance but also lowers viscous pressure resistance. (3) As the water depth falls, the reduction in viscous pressure resistance accounts for a greater share of the overall resistance reduction than the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{_h/T_} & \multicolumn{4}{c}{\(R_{t}\) (N)} & \multicolumn{2}{c}{**Resistance Reduction (N)**} & \multicolumn{2}{c}{**Reduction Rates (\%)**} \\ \cline{2-7}  & **No Groove** & **Groove No Jetting** & **Groove Saturated Jetting** & **Relative** & **Absolute** & **Relative** & **Absolute** \\ \hline
1.18 & 10.01 & 9.33 & 7.17 & 2.15 & 2.83 & 23.04\% & 28.27\% \\
1.35 & 10.41 & 9.92 & 6.93 & 2.99 & 3.48 & 30.14\% & 33.43\% \\
2.25 & 9.19 & 9.06 & 6.39 & 2.68 & 2.80 & 29.58\% & 30.47\% \\
4.0 & 8.56 & 8.52 & 6.02 & 2.50 & 2.54 & 29.34\% & 29.67\% \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Effect of water depth on friction resistance reduction (\(F_{r}=0.091\)). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{_h/T_} & \multicolumn{4}{c}{\(R_{t}\) (N)} & \multicolumn{2}{c}{**Resistance Reduction (N)**} & \multicolumn{2}{c}{**Reduction Rates (\%)**} \\ \cline{2-7}  & **No Groove** & **Groove No Jetting** & **Groove Saturated Jetting** & **Relative** & **Absolute** & **Relative** & **Absolute** \\ \hline
1.18 & 4.32 & 3.98 & 3.21 & 0.77 & 1.11 & 19.35\% & 25.69\% \\
1.35 & 4.49 & 4.27 & 3.20 & 1.07 & 1.28 & 25.06\% & 28.73\% \\
2.25 & 3.99 & 3.92 & 2.93 & 0.99 & 1.06 & 25.26\% & 26.57\% \\
4.0 & 3.72 & 3.68 & 2.78 & 0.90 & 0.93 & 24.46\% & 25.27\% \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Effect of water depth on friction resistance reduction (\(F_{r}=0.057\)). It is possible to see that the pressure at the bottom of the ship gradually decreases as the water depth becomes shallower. This is because the water flow velocity at the bottom of the ship increases in shallow water conditions, but the sinking of the ship was constrained. There are not many differences between the pressure distribution around the ship's surface before and after jetting in deep water when comparing the pressure contour maps under different water depth situations. However, jetting causes major changes in the pressure distribution surrounding the ship's surface in shallow water. The bow pressure decreases, and the stern pressure increases and this trend intensifies as the water depth becomes shallower, what is the main source of the significant reduction in viscous pressure resistance in shallow water.