نشریه مهندسی شناورهای تندرو
پیاپی 54 (بهار و تابستان 1398)

In the last century, with the advent of technology and the need for increased speeds in maritime transport, pay attention to the high-efficiency propulsion systems has increased dramatically. The important note in this regard is that, in order to increase speed discussions about that always have been the focus. One of the related methods is to, design and select the appropriate propulsion system and to select the propeller with remarkable efficiency. In this paper, numerical simulation of a four blade submerged propeller using the computational fluid dynamics method with the help of STAR CCM + software has been performed and the effects of these kinds of propellers on the key parameters of efficiency, torque coefficient and the thrust coefficient is evaluated in terms of the advance ratio and compared with the available experimental results and good agreement between the numerical results and the existing experimental results is observed. The largest difference between the efficiency obtained from the simulation and the experimental results was 15%, which occurred in the advance ratio of 1.4.

Nowadays, sandwich structures based on balsa core and composite skins are widely used in the production of lightweight and high-strength marine structures. However, Balsa, as a hydrophilic material, is sensitive to moisture, and penetration of moisture through damaged composite skins into the sandwich structures will result in severe water uptake, balsa core decay, and delamination in skin/core interface. In order to improve the damage tolerance of sandwich structures with balsa core, the idea of using fiber metal laminates (FMLs) instead of composite skins has been proposed in this research. In this way, following the evaluation of the water absorption behavior of these types of sandwich structures, the specimens made of balsa core and FMLs skins were subjected to compressive loading and critical buckling values were determined. In order to investigate the effect of the skin on the mechanical properties of these structures, the specimens with similar core and composite skins made of glass/epoxy were also prepared and subjected to similar experiments. Experimental results show that sandwich structures with balsa core and FMLs skins exhibit less water absorption and much better mechanical behavior than conventional sandwich structures with composite skins.

The water impact force caused by slamming is one of the important forces that the hydroelastic response of the craft's structure may be significant. Modeling of this phenomenon and the interaction between the hydrodynamic force and deformation of the structure are the challenging problems in offshore structures and water landing aircraft. In this article, the effect of kinematic characteristics including vertical, angular and horizontal velocities, as well as slope angle, on the hydroelastic response of the plate is investigated using the existing hydroelastic model presented for water impact. In addition, dimensionless parameters obtained the kinematic characteristics are employed to determine the conditions in which the horizontal velocity and inclination angle are significant or not during an oblique impact. In these conditions, the plate tends toward the vertical impact with the slope angle of zero. The findings show that increasing the vertical velocity reduces the oscillation period of deflection in both vertical and oblique impacts, and the relationships of maximum deflection with vertical velocity and angular velocity are linear.

Although the electrical discharge process is a material removal process, it is attempted to be used for surface operations as well, so considering the new method, it is important to analyze the mechanism of the electrical discharge surface alloying process. Nickel base alloys are widely used due to their high resistance to corrosive environments, especially at high temperatures, especially in environments where stainless steels and duplexes lose their performance. This paper first discusses the mechanism of surface alloying by electric discharge method and then, as an experimental work, examines the surface alloying of aluminum with nickel-copper alloy (Monel 400) to improve its surface properties. Therefore, the parameters of pulse on time and pulse current are considered as input parameters and the depth of hardened layer as output variable. The results of this study show that using Monel 400 electrode with negative polarity as an alloying electrode is a successful method to improve the surface properties of aluminum by eliminating harmful layers resulting from electrical discharge machining.

Nowadays, designers have always been interested in the use of motion stabilizer systems, such as the interceptor and trim tab, especially for planing vessels. These systems can improve crew comfort and reduce fuel consumption with favorable effects on the hydrodynamic performance of these vessels, such as reduced resistance and trim angle. In the present study, the effect of the interceptor on the hydrodynamic performance of model scaled "1/15" a monohull vessel with 11 m long that was tested in the towing tank of Sharif University of Technology compared with numerical simulation by software STAR-CCM+. The results show that the installation of the interceptor on the vessel reduces the resistance to a maximum of %9.4 when the vessel is in planing mode and in particular the dynamic trim due to the shift of the center of pressure to the transom. Keywords: motion stabilizer systems؛ interceptor؛ planing vessels؛ hydrodynamic performance؛ resistance reduce؛ trim angle؛ numerical modeling؛ software STAR-CCM+

Knowledge of water level changes is due to the tide and its prediction is very important due to its impact on various areas such as navigation, offshore and offshore engineering activities, marine transport, fisheries, and port management. Therefore, due to the importance of this phenomenon in all activities related to seas and beaches, it is necessary to properly identify this phenomenon in different locations by using appropriate techniques and methods. Therefore, in this paper, we have investigated the tidal prediction of the Persia Gulf coasts using the TMD Persian Gulf coasts. The results of predictions made by TMD software in the study domain are very accurate compared to the tide tables of the Iran National Cartographic Center of the Persian Gulf. Also, the tidal components derived from this toolbox in the Persian Gulf are entirely consistent with internationally validated research results. these results were assessed by comparing of measurement data in some ports in the northern Persian Gulf.