## 3 Results
### Comparison with Normal Propeller
#### 3.3.2 NACA66 (MOD, t/c = 0.12) Propeller  
Figure 21 shows the hydrofoil geometry with reduced thickness compared with the NACA66 (MOD, t/c = 0.24) hydrofoil used in this study. The NACA66 (MOD, t/c = 0.12) has a thickness reduced by 1/2 from the previously used hydrofoil (NACA66 (MOD, t/c = 0.24)), and an internal structure was added. The location of the slit and the tunnel in the x-axis direction, and the area of the tunnel are the same. The size of the round tunnel decreases as the gap between the tunnel and the slit decreases. All other particulars except the thickness are the same. As in the previous method, the injection volume of each slit height was adjusted to identify the thrust of the normal propeller, and the performance change of the propeller was studied.  
Figure 21: NACA66 (MOD, t/c = 0.12) geometry for each slit height.  
Table 8 shows the performance comparison between the NACA66 (MOD, t/c = 0.12) propeller and the normal propeller. As the thickness of the hydrofoil decreased, its geometry changed, and \(K_{T}\) decreased for the Coanda propeller and the normal propeller. The thrust that needs to be increased to identify the normal propeller is similar when compared to Table 7, but \(C_{j}\) has increased overall. In Figure 21, the curvature of the hydrofoil decreases as the hydrofoil thickness decreases. Therefore, the curvature that the jet can follow is reduced, the induced downwash is reduced, and the jet injection volume to the thrust is increased.  
In the case of the NACA66 (MOD, t/c = 0.24), the volume of jet injection is small, and at \(h_{S}\) = 0.6 mm, the required pump power to inject a jet with a velocity faster than the relative velocity of the propeller is relatively large. But in the case of the NACA66 (MOD, t/c = 0.12) propeller, the required power of the pump is the highest at \(h_{S}\) = 0.2 mm, where the jet speed is fast due to high injection volume. In addition, the increase in thrust and decrease in torque compared to the jet injection volume are also smallest, and the propeller efficiency was the lowest. However, in the case of \(h_{S}\) = 1.0 mm, the efficiency increases by about 26.4% compared to the case without a jet, and by about 8.6% compared to the normal propeller. As the jet injection volume increased, not only did the thrust increase, but the moment generated by the jet was also large, resulting in a relatively large increase in efficiency.  
In Figure 22, the thrust PUA, torque PUA, and \(-C_{P}\) contour of the normal propeller and Coanda propeller with reduced thickness are compared. As the propeller thickness decreases, the overall curvature of the hydrofoil decreases. As a result, it can be seen that the inflow velocity at the front of the propeller is reduced. In addition, the area where the thrust is generated has moved toward the trailing edge of the propeller compared to Figure 20. This phenomenon is caused by the movement of the curved surface of the hydrofoil, which induces downwash flow in the process of manufacturing the hydrofoil, as shown Figure 21.  
The \(-C_{P}\) distribution also shows a pressure drop in the region where the mass flow is concentrated and downwash flow is strongly induced. However, compared to Figure 20c, the pressure drop in front of the slit is relatively small. Likewise, the curvature of the hydrofoil decreases, and the downwash induction of the jet increases. As in the previous results, even if the thickness decreased, the area where the pressure drop appeared decreased as the slit height decreased.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline _Type_ & \(C_{j}\) & \(K_{T}\) & \(\Delta K_{T}\)/\(C_{j}\) & \(1  
Figure 22: Comparison of changes in hydrodynamic force on the surface of a normal propeller and Coanda propellers (NACA66 (MOD, t/c = 0.12)).