This study provides a comparison of propulsion performance, with a particular focus on efficiency, by varying rake distribution at the tips of propellers. Owing to increased attention to environmental pollution, there is a significant interest in reducing the energy efficiency design index (EEDI) and SO_x emissions by improving the performance in the field of shipbuilding. The forward (Kappel) and backward tip rake propellers have been widely used to improve efficiency, as well as to reduce fluctuating pressure from the tip vortex cavitation. As there is almost no parametric and design research on tip rake propellers, this systematic parametric study was conducted to identify the optimal configuration by the potential code. For this performance comparison the KP505 (KCS propeller) was chosen as the reference propeller as the tips of that propeller have no rake. The model test and computational fluid dynamics (CFD) calculation confirmed the result by comparing the open water performances for the three optimally selected propellers (forward, backward, KP505). The differences of efficiency obtained from the potential analysis and the model test exhibit similar tendencies, but the result for the CFD is different. The difference would be investigated by changing the grid system around the tip as well as the turbulence model in the CFD analysis. An analysis of self-propulsion and pressure fluctuation is also expected to be conducted in the near future.

This study is a parametric investigation of the application of a compact type of pre-swirl duct for a slender aft-body 158k crude oil carrier. The International Maritime Organization (IMO) has created the Energy Efficiency Design Index (EEDI), which is an index related to carbon dioxide emissions, to enforce regulations on newly built ships; many solutions have been proposed, including an increase in propulsion efficiency. In the present study, a new type of energy-saving device (pre-swirl duct), the so-called ring stator, is proposed for 158k crude oil carriers, the hull form of which has been developed as a slender after-body full form to reduce the resistance by delaying separation. A conventional pre-swirl duct is no longer effective at the slender aft-body hull forms because of the small radial flow to the propeller. A parametric study was conducted through Computational Fluid Dynamics (CFD) analysis using the commercial program Star-CCM+, and an improvement of approximately 3% in the propulsion efficiency was achieved in the present study. The efficiency gain was validated through a comparison with the experimental results. The developed ring stator may become applicable to container ships through further parametric studies in the near future.

This paper presents the minimum submergence depth of an underwater vehicle that can remove the effect of free surface on the resistance of the underwater vehicle. The total resistance of the underwater vehicle in fully submerged modes comprises only viscous pressure and friction resistances, and no wave resistance should be present, based on the free surface effect. In a model test performed in this study, the resistance is measured in the range of 2 to 10 kn (1.03–5.14 m/s) under depth conditions of 850 mm (2.6D) and 1250 mm (3.8D), respectively, and the residual resistance coefficients are compared. Subsequently, resistance analysis is performed via computational fluid dynamics (CFD) simulation to investigate the free surface effect based on various submergence depths. First, the numerical analysis results in the absence of free surface conditions and the model test results are compared to show the tendency of the resistance coefficients and the reliability of the CFD simulation results. Subsequently, numerical analysis results of submergence depth presented in a reference paper are compared with the model test results. These two sets of results confirm that the resistance increased due to the free surface effect as the high speed and depth approach the free surface. Therefore, to identify a fully submerged depth that is not affected by the free surface effect, case studies for various depths are conducted via numerical analysis, and a correlation for the fully submerged depth based on the Froude number of an underwater vehicle is derived.

When an ultrasonic sensor generates an ultrasonic wave and detects an obstacle from a reflected wave, a signal transmitted by other ultrasonic sensors would be interference. In this paper, to overcome the interference, a transducer transmits a signal with a unique ID modulated. The interference is ignored by verifying that the reflected signal includes its ID. The ID verification process uses a correlation between the received signal and the ID. Therefore, the ID is selected from orthogonal codes with good cross-correlation. Long code has the advantage of being more robust to interference. However, the reflected wave from nearby obstacles might return before the transmission ends. Therefore, the 7-bit Barker code is applied for near obstacle detection and a 31-bit Gold code is used for distant obstacle detection. The modulation technique is DQPSK, which is available in a narrow bandwidth and has a simple receiver structure. In ID recognition based on correlation, a near–far problem occurs due to a large amplitude difference between the received wave and interference. The addition of a zero-crossing detector solves this problem. The hardware is implemented based on the algorithm proposed in this paper. The simulation showed a detection rate of at least 90% and the the result of the real measurement represented a detection rate of 97.3% at 0.5 m and 94.5% at 2 m.

Owing to environmental regulations, ships are equipped with a pre-swirl stator (PSS), which is one of the most effective energy-saving devices (ESDs) that is widely applied to various kinds of ships. It improves energy efficiency by recovering the rotational kinetic energy of the propeller with the aid of a PSS placed in front of the propeller. In this study, an asymmetric PSS system is applied to the 2500 TEU eco-friendly liquefied natural gas (LNG) fuel feeder container ship, aimed at optimizing the position of stator blades, using a potential-based program. Additionally, a parametric study was conducted for evaluating the optimum pitch angle and blade spacing. STAR-CCM+ was used for validating the efficiency of the final design. The Samsung towing tank and large cavitation tunnel were also utilized to verify the improvement in the performance of the proposed PSS. Although the efficiency gain is not largely affected by blade position optimization, the cavitation and pressure fluctuation issues are addressed by improving the in-flow to the propeller. Therefore, blade spacing optimization of the stator is important for container ships whose cavitation performance is very significant, especially the relatively high-speed commercial vessels.

Submarines with pumpjet propulsors have recently been used in many countries to improve their propulsion and noise performance. The pumpjet has the advantage of improving noise and cavitation performance by increasing the pressure inside the duct, and it has good efficiency at a low advance ratio. This study analyzed the propulsive performance of a pumpjet propulsor affixed to a SUBOFF submarine as a function of variations in the design parameters using computational fluid dynamics analysis. The incidence angle and camber of the duct and the pitch angle of the stator were selected as the design parameters. To investigate the propulsion performance as a function of the design parameter variations, each case’s thrust, torque, and efficiency were compared, and the velocity and pressure fields of each case were analyzed. In addition, the efficiency was analyzed using the non-dimensional mass flow rate and area ratio difference for each case. The duct incidence angle contributed most dominantly to the dimensionless flow rate and the difference in area ratio, and these two factors resulted in high efficiency at certain values. It is expected that further research will be conducted in the near future that takes into account cavitation inception speed and cavitation.