SDV - Propeller Design Project
This report entails the design of a propeller for a notional Seal Delivery Vehicle (SDV). The objective is to use OpenProp to best design a propeller capable of a relatively high speed (4 m/s) for the SDV. Figure 1 on the right shows a few examples of propeller and nominal hull designs for an SDV, while they are unrelated to this project, they are provided for conceptual reference. This report includes a parametric study which includes sizing, powering, and efficiency calculations. In addition, blade designand cavitation analysis are also considered in this report.
- Design speed: 4 m/s
- Hull length: 9.4 m
- Hull diameter: 1.5 m
- Design depth: 3 m to hull centerline
- Propeller Speed: TBD
- Hull drag coefficient Cd: 0.15 based on frontal area
To conduct a parametric study to begin a propeller design. Some additional design specifications and assumptions are required to be found. In the design specifications above, a Hull drag coefficient value of Cd = 0.15 is given based on frontal area. This drag coefficient is used to find drag and then thrust of this SDV using the following formula.
In this design we set the Thrust equal to calculated Drag.
Now that Thrust and Drag has been calculated. We are able to conduct a Parametric study for potential propeller designs. Figure 2 above shows the OpenProp Parametric Study User interface that is displayed when running the OpenProp software through MATLAB. On the left hand side we input our propeller design specifications (thrust, speed, diameter, etc). On the bottom section called “Range” we input our minimum and maximum number of blades, rotations speed as well as rotor (propeller) diameter. In the center of the User Interface we see a table of Geometric Blade Design Values. We assume the Cd for the propeller remains 0.008. We also keep r/R values the same. The c/D values inputted in this design report are from NACA66 (DTRC modified) data [1, page 15].
After inputting all the blade design values and propeller specifications, we run OpenProp. Figure 3 above shows the output of the OpenProp software. We are given a parametric study that charts an assortment of a wide variety of propeller designs depending on the number of propeller blades, their rotational speed as well as their diameter. The program then calculates each design’s efficiency. For this project, we select the propeller design with the highest efficiency.
This yielded in a propeller design of 3 blades, 60 rpm, and a rotor diameter of 1.9 m with an efficiency of 0.78 (78%).
Now that a specific propeller type is decided we move forward with a single design study. As shown in Figure 4 above, the detailed propeller design specifications are once again input into the OpenProp user interface.
Figure 5 and 6 showcase the 3D geometry of the propeller design, as well as the design performance after the single design study, is executed in OpenProp.
Figure 7 above shows the MATLAB code that is used to conduct cavitation analysis of the chosen propeller design. This code consist of an input script and a usage script. The input script is for the propeller specifications (c/D values, blade number, Kt, J, Vs, etc.). We inputted the propeller specifications into this script. The propeller usage script draws propeller specifications from the input script to conduct a variety of analyses. For this report we focus on the cavitation analysis of our propeller as shown in Figure 8 below.
As shown in Figure 8 above. OpenProp analysis shows that the propeller design is not prone to cavitation based on propeller specifications. This is ideal as it is known that cavitation is problematic for propeller blades. Known detrimental effects to propellers caused by cavitation include increased noise (especially for military equipment that is not ideal for stealth), as well as overall damage to the propeller thereby reducing its life and effectiveness.
Cavitation is known as the rapid formation and collapse of vapor bubbles within a liquid. Cavitation bubbles mainly occur when static pressure decreases below a liquid’s vapor pressure. In the case of a ship propeller, as it moves through the water, there is high pressure on the side of the propeller facing the flow and lower pressure on the opposite side. The area of lower pressure essentially causes boiling of the liquid (due to decreased pressure not related to temperature) and thus creates bubbles of steam trailing behind the propeller. It is also important to note that increasing a propeller’s RPM will increase the risk of cavitation.
When conducting propeller design, it is critical to consider the balance between a propeller’s rotational speed, thrust, and cavitation risk. Increasing rotational speed may increase thrust but also increases the risk for cavitation.
In conclusion, OpenProp analysis shows that a 3-bladed propeller with a rotor diameter of 1.9 meters serves to meet propeller design requirements as stated in the beginning. This propeller has an efficiency of 78% and is also not prone to cavitation.