Feature 3 | ProPEllErs and ThrusTErs
Wake-adapted design and propeller analysis for naval architects
Te hydrodynamic design and analysis of marine propellers occurs along a broad spectrum of detail and complexity. Most naval architects use some sort of “system-level” soſtware for propeller analysis, essential for the proper matching of hull, engine, transmission, and propeller, writes Donald MacPherson, VP Technical Director HydroComp, Inc.
set of technical challenges to the naval architect. Tey differ from stock “off-the- shelf ” propellers in two principal ways – they are designed using contemporary foil geometries, and they are optimised and fitted to the individual vessel (or vessel type). To fully take advantage of the benefits that custom or semi-custom propellers make available, or to evaluate them in service, naval architects must look to a different kind of propeller calculation.
C
Wake fields of velocity Te term “wake” is used in a number of different ways in maritime operation, but for our purpose, we use wake to refer to the measure of local velocity at the stern of a ship. It is how we quantify differences in the environment around the propeller from vessel to vessel. Consider the following graphic (Figure
1). Tis is a stylised schematic of a hull showing the creation of its boundary layer and flow vortices at the stern. You will note at point C that the free-stream velocity V gradually reduces and becomes very small at the hull itself. Tis is the region where the propeller lives, so the propeller will be seeing water that is typically somewhat slower than the ship’s velocity. When conducting propeller analyses, we need some measure of this reduction of speed into the propeller, and we use the “wake fraction” coefficient to provide a figure for the overall reduction (reflecting the “speed of advance” at the propeller). The critical velocities for propeller
evaluation are those found in the plane of the propeller disk area. For a given vessel, these velocities can be measured in a model
50 Figure 3: Example axial velocity wake field.
Figure 2: Velocity reduction due to appendages.
test, or predicted using computational fluid dynamics (CFD) or statistical algorithms. Te following graphic (Figure 3) is an example of a wake field plot of axial velocities for a twin-screw vessel with a single strut (P-bracket). Te iso-lines on the plot represent velocity as a ratio of the open free-stream velocity. You can easily see the reduction in water velocity nearest the hull’s boundary layer, around the propeller hub and shaſting, and particularly behind the strut.
Not only are axial velocities considered,
but tangential (or rotational) contributions due to upward and transverse flow vectors are also taken into account. Tese would have a similar plot or be represented by vector arrows on the axial wake field plot. Te wake field data is further developed
to determine averaged velocities for each radial position (along the propeller blade from root to tip). Tis data is presented as a radial distribution of velocity, shown below (Figure 4). Identifying the vessel’s specific wake
field properties or radial distributions by empirical testing or detail calculation
The Naval Architect July/August 2010
ustom and semi-custom propellers – now commonplace for new vessel designs – offer a different
Figure 1: Water flow and boundary layer.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80