Feature 3 | ProPEllErs and ThrusTErs


system-level propeller sizing and analysis, this is managed using correlation strategies (such as “aligned prediction” to model tests). Tese propeller geometries can be explicitly evaluated in PropElements, with additional considerations such as skew and nozzles.


Figure 4: average axial and tangential velocities.


is not always necessary. Particularly for “semi-custom” designs, representative wake distributions for vessel “types” can be effectively applied to propeller calculations. Representative wake field data can be found in various hydrodynamic technical papers, reports, and texts.


Wake-adapted calculations Using specialised software, a custom propeller can then be optimally designed to match the unique inflow properties of the vessel (or a semi-custom propeller designed for a vessel “type”). Tis offers a propeller that is “custom tailored” to fit, or “wake-adapted”. Such soſtware is able to consider axial and tangential inflow properties, and ascertain optimised distributions of pitch and camber for prescribed foil characteristics. Of course, the propeller design process would also take into account blade strength, tip and hub loading, and cavitation. HydroComp PropElements is an example


of wake-adapted propeller detail design and analysis software. Unlike general- purpose CFD or more complex codes, PropElements was designed to be readily employed by practicing naval architects, as well as propeller designers and builders. Its analytical core is a unique implementation of a vortex lattice liſting-line calculation, with empirical connections that allow analyses to be viscous and fully-scalable. PropElements calculation modules include Geometry, Performance (Figure 5), Strength, and KT-KQ (curves). Contemporary propeller geometries –


pressure-equalised designs, for example, with variable pitch distribution and camber established for “shockless entry” – are substantially different from series geometries (B-series, AU, Segmental). In


52 The Naval Architect July/August 2010


Performance versus stock propellers So how much “green” might be achieved with an optimised wake-adapted propeller? Te following example is for the propeller design of a twin-screw vessel using the axial and tangential distributions shown in Figure 4 above. Te original propeller was a popular stock model, with a prescribed camber (of flat-faced segmental design) and constant pitch. Te newly optimised propeller (with the same basic foil section) was designed with variable camber and pitch to better match the wake, as shown in the plot below (Figure 6). A comparative summary between the


original and an optimised propeller designed by HydroComp PropElements (Table 1) indicates a power reduction and efficiency gain of more than 6%, just by better matching the propeller to a representative wake field. Other potential design benefits that


could be attained with a wake-adapted propeller design tool include unloading the tip (i.e., reducing the pitch into the tip) to mitigate potential noise and vibration


problems, and evaluating changes in the outline (chord distribution) or thickness for even better performance.


detailed analysis of propeller performance Wake-adapted calculation soſtware such as PropElements can also be applied to analysis, as well as design. Te ability to investigate radial values of foil lift and cavitation number, for example, can help identify potential sources of root cavitation or blade impulse excitation. It can help evaluate tip loading (for hydro-acoustics), and also be employed in forensic investigations of blade strength or failure. Calculation of KT-KQ curves can be


applied to system level calculations in replacement of direct propeller series predictions. For example, KT-KQ curves from PropElements can be exported in a form that can be used with HydroComp’s NavCad speed-power software for propulsion analysis. The following plot (Figure 7) shows the results of a validation study for PropElements, which clearly demonstrates the quantitative accuracy of these calculations.


summary As custom-tailored suits offer greater comfort and better fit, a wake-adapted propeller can provide a variety of performance benefits over stock propellers.


Figure 5: hydroComp PropElements wake-adapted propeller design [Performance page].


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