# Propeller Design

VOLUME 2, JULY 2009

Article: Propeller Design

By Muhammad Husaini b Abu Bakar

There are many methods for propeller design, but in this study, propeller vortex lifting line code were analyzed using cavitation considerations to meet the AUV specifications. These methods lead to designing an optimal propeller, as well as calculating the efficiency of existing propellers, which determined performance flaws. The resulting analysis produced recommendations for altering the geometry of off-the-shelf propellers to improve overall efficiency, while considering the effects of minimum pressure coefficients and ranges of lift coefficient. Aspects of interest in propeller design included the following: the vehicle’s desired thrust, cruise speed, number of propellers, propeller location, and depth of vehicle. The design additionally considered the affects of blade number, rate of propeller rotation, chord length distribution, propeller diameter, and hub size.

This study made several assumptions. First, the propellers were located far enough away from the hulls of the AUV, which left axial velocity, Vs , unaffected. The ratio of axial velocity to design speed was such that Va/Vs = 1.0 . Hull wake and boundary layer effects were not considered in the cases studies presented. However, these effects can be accounted for by using basicPVL, which handles radial variation both axial and tangential velocities. Additionally, hull diameter was insignificant due to the location of the thrusters away from the body of the AUV; thruster diameter was small relative to the propeller diameter. Also, the propeller design code did not automatically check for cavitation effects, but was analyzed by hand using Brockett diagrams. Furthermore, the required thrust was determined for the design speed, and calculated per propeller, not as the total required thrust for the vehicle.

The code basicPVL was designed as an intermediate code to perform analysis during the time that the GUI for OpenPVL was being developed. The analysis considered both the optimal propeller design for the given design criteria, as well as the efficiency of the existing propeller. Efficiency, is determined by the ratio of required thrust times the wake to the power coefficient. The actuator disk efficiency can be calculated in addition to ?. If a value higher than the actuator disk efficiency is found, it can be eliminated as erroneous. The actuator disk efficiency is calculated as in Equation 1:

This calculation serves as an added check to determine if the calculated efficiency is correct. In finding an initial propeller design, PVL.exe was used with input variables, including the advance coefficient, J, the desired thrust coefficient, Ct, and the axial flow ratio, Va/Vs. A function was created to determine the effects of axial flow for alternative propeller placement or diameter of a body affecting fluid flow entering the propeller. The axial flow modeling considers several important factors. The first factor is in the case that the ratio of the propeller blade diameter to the hull diameter is not equal to one. The data was extended to include a case of a ratio of up to 1.8. Additionally, the practical assumption was made that axial flow is constant for all r/R values less than 0.225. Otherwise, the spline would model a value of zero or less at the hub, which is not possible. Other variables, such as the number of vortex panels over the radius, iterations in the wake alignment, hub image, hub vortex radius/hub radius, number of input radii, hub unloading factor, tip unloading factor, swirl cancellation factor, radial position(r/R), chord to diameter ratio (c/D), and blade section drag (CD), were kept at acceptable values from example input files to the PVL code. The c/D values were scaled automatically with respect to r/R values using a spline function.

Design using PVL

OpenPVL is a propeller design tool that is based on the same vortex lattice lifting line theory as basicPVL. However, OpenPVL offers a variety of advanced features, including:

• A user-friendly MATLAB GUI

• Ability to save valuable input and output text files for each propeller design

• Capacity to write a script file for 3D printing from the design geometry output

Figure 1 shows the main input screen for the OpenPVL code. Two options are available at this level, Parametric Analysis and Single Propeller Design.

Parametric Analysis

Three parameters provide the foundation for propeller design: the number of blades, the propeller speed, and the propeller diameter. Various combinations of these three key parameters result in different efficiencies. Thus, a parametric study allows for propeller parameter optimization. The Parametric Analysis GUI is a computational tool that calculates and graphically represents propeller efficiency. Figure 2 shows the parametric analysis GUI, which includes the user input fields required to run the analysis. OpenPVL is tailored to a propeller user’s design needs; therefore, the Parametric Analysis GUI requires user input for the following characteristics:

• Number of blades

• Propeller speed

• Propeller diameter

• Required thrust

• Ship speed

• Hub diameter

• Number of vortex panels over the radius

• Maximum number of iterations in wake alignment

• Ratio of hub vortex radius to hub radius

• Number of input radii

• Hub and tip unloading factor

• Swirl cancellation factor

• Water density

• Hub image flag

All of the fields within the GUI are populated with initial values, based on the US Navy 4148 propeller, as a guide to users. Each of the input fields are modifiable and Parametric Analysis can run any desired number of times without having to exit the program.

Single Propeller Design

Once the parameter for a propeller with a viable efficiency curve has been established, the desired inputs are entered into the Single Propeller Design GUI of OpenPVL. Figure 2-5 shows the single propeller design GUI. Determining the geometry for a single propeller utilizes both the results from the propeller parameterization, as well as additional inputs, resulting in a user-specific design. Input fields entered for the Parametric Analysis are populated with the same values for the Single Propeller Design. There are also several additional input fields, including: shaft centerline depth, inflow variation, ideal angle of attack, and the number of points over the chord. Additionally, two types of meanlines are available within the program: the NACA a=0.8 and the parabolic meanline. The thickness forms available include: NACA 65 A010, elliptical, and parabolic. OpenPVL is easily modified to accommodate additional meanlines and thickness forms.

A single propeller can be designed and quickly evaluated graphically. One of the file outputs of the Single Propeller Design is the blade geometry. This feature of OpenPVL automatically transforms x, y and z coordinates of the designed propeller blade geometry into a command file that can be read by a CAD program. The user opens the command file in the CAD program Solidworks, and has the option of saving their design as a useful stereolithography (.STL) file, or as an Initial Graphics Exchange Specification (.IGES) file. Although the command file includes scripting specific to Solidworks, the propeller geometry can be exported to another CAD program after it is saved in Solidworks as a file compatible to the other design software.