Effects of the hull stern geometry to the propeller

The propeller propulsion systems which are located at the stern area of the hull, have significant relations with the strern geometry of the vessel. Lay out of the propeller and other companents related with the propeller affects the perfomance and the design parameters. In this study; a three dimensional modelling software and a computational fluid dynamics software is used in order to compare the effects of the stern geometry to a marine propeller. Primarily, in order to test the validity of the calculations, DTMB 4119 propeller is analysed. Analysis of the propeller is prepared for different mesh structures and properties for reaching the minimum error value. For verifying the solution of the hull form metodology, validation studies were performed using the DTMB 5415 hull. Reacing the miminum error value is the main objective as well as the propeller validation. Hull geometry, used in the study carried out, was designed using a three dimensional modeling program. Propeller design was prepared according to this hull geometry. First, studies have been conducted to determine the open water performance characteristics of the designed propeller. As a result of analysis, open water curves for different advance coefficients are obtained. Conventional lay out calculations were prepared in order to compare the propeller open water performance characterisitic. Also, conventional hull resulst are a reference point for making a comparison between different stern geometries. Conventional lay out was prepared by arranging the propeller, shaft,brackets and other components according to the shaft angle and propeller – hull tip clearence limitations. At the performance boats, propeller tunnels are used which can be designed in different forms. This form contains general assumptions and constraints used in the design. In this study, by appropriate modeling assumptions in the design of this tunnel forms, comparisons are made with open water and conventional lay out values of different stern shapes. In order to compare the performance values of the tunnel hull form with the other forms and open water values, the analysis was repeated on three different advance coefficient for each form. Most of the vessels have their propellers and shaft equipment below the keel. Propellers in tunnel are an alternative to conventional systems; hereby reducing the draft of the craft is the main objective of the tunnels. Moreover designers forced to use small diameter propellers in order to reduce vessel draft but these solutions bring other inefficiencies such as cavitation and poor propulsive efficiency. Tunnel design provides reducing in shaft angle and decreasing in draft, at the same time propellers which have large diameters become available by using propeller tunnels. In order to get advantages of the propeller tunnels, propeller and tunnel geometry must be designed together. According to the experimental results, propeller tunnels does not bring a negative effect on propeller efficiency. Moreover, an optimum tunnel-propeller design may lead approximate efficiency compared with conventional arrangement.. Small tip clearance, for an optimum tunnel-propeller design, generates close efficiency to open-water. Reduced clearance provides the propeller to operate with increased efficiency by the help of reduced tip losses.But main objective is reducing the operaional draft and generating extra space for propellers having larger diameter. By the usage of the propeller pockets which are shallow tunnels, propeller induced vibrations become lower. On conventional installations without tunnels, minimum clearance should be minimum 15 percent of the propeller diameter between the propeller tip and the hull; otherwise excessive vibration will occur. But tunnel usage gives flexibility to reduce the clearance to 5 percent and with an optimum design nearly zero clearance is possible without vibration. This allows a larger propeller diameter, lower cavitation without vibration. In this study; a specified motor yacht hull and a propeller is analyzed without tunnel and with tunnel geometry by using CFD solver. Propeller efficiencies are compared for each conditions in order to determine the advantages and disadvanteges of different stern geometries of performance hulls. Most of the vessels have appendages below the hull as propeller, shaft, and brackets. These appendages increases the total draft of the vessel so increased draft creates operational disadvantages for the vessel. In some cases propeller diameter reduction is a solution in order to reduce total draft of the boat. But this method leads to lower propulsive efficiency and limitations cause non optimum conditions. Propeller tunnel usage causes buoyancy loss for the hull therefore total draft increases. On the other hand, propeller tunnel brings flexibility to reduce the shaft angle. In short, shallow operational draft may be provided even though hull draft increases. Propeller tunnel usage provides efficiency conversation for most of the cases due to the shrouding effect of the propeller tunnels. Especially shallow partial tunnels lead to reduction in propeller induced vibrations. In order to reduce propeller induced vibrations for conventional propeller installations, propeller tip – hull clearance should be increased; however increased clearance brings increment in total draft. In conventional layout, hull form directly effects the nominal wake coming through the propeller zone. But boats which have propeller tunnels, nominal wake is mostly specified by the tunnel geometry. Propeller tunnel geometry have significant influence not only on suction side of the propeller. Tunnel length at the aft side of the propeller effects the wake of the vessel. According to the experiments and researches, there are some limits for the tunnel entrance zone angles, exit region characteristics, location of the propeller. Tunnel entrance region should not be longer than required dimensions. This condition causes loss in buoyancy and also increase in draft. Most critical condition is the angle of the entrance zone. Angle between the tunnel entrance the hull buttock at that section should not be more than 15 degrees. Also, diameter of the entrance zone should not have larger diameter than the propeller zone. Tunnel center and propeller center must be concentric in order to provide a constant clearance between hull and propeller blade tips. Also tunnel longitudinal axis should be parallel to the waterline. For propulsion efficiency, one of the main point is tunnel geometry around propeller zone. Location of the propeller and entrance of the tunnel become critical for hydrodynamic characteristics. Tunnel geometry accelerates the flow and lower the pressure. Therefore tunnel entrance zone should be close to the propeller zone. Tunnel hull forms which have a very low propeller – hull clearance may have higher propulsive efficiency than open water efficiency. Because this configuration reduces the propeller tip losses and provides a proper wake. But this study concentrates on having closer efficiency to the conventional lay out by using same propeller. For the vessels which have propeller tunnels, propeller rate pressures have more uniform distribution. It is possible to make an arrangement that propeller tip and hull clearance have a dimension close to zero. Avoiding mechanical interaction, propellers which have 5% d/D clearance provides higher efficiency. Tunnel exit zone mostly effects the hydrodynamic characteristics of the vessel. Especially at semi planning and planning speeds, tunnel exit geometry and lost volume effects the planning angle at that speeds. For Fnv > 2.8, shorter tunnels provides higher propulsive efficiency compared with longer ones. Propeller tunnels lead to reduction in shaft angle and total draft. Tunnel geometry and propeller location in the tunnel depend on the design and there are important points such as inflow velocity, pressure and angle. Propeller blade tip – hull clearance brings both hydrostatic and hydrodynamic advantages. Lower clearance brings possibility to reduce the tunnel diameter and increases propulsive efficiency. Smaller tunnel diameter increases the shrouding effect and leads to top speed increment. Number of propeller blades also an important selection for the vessels that have propeller tunnels. In order to minimize the torsional resonance, determining the number of blades of the propeller regarding the engine RPM range is one of the main points of this selection. For reducing the vertical blade rate forces, if one blade enter the tunnel than another blade should not leave the tunnel at the same time.