A scaling procedure for modern propeller designs

Abstract

The extrapolation procedures currently used to scale propeller characteristics tested at model scale to their full scale performances are either based on a statistical [1], the Lerbs-Meyne [2] or the recently developed strip method [3]. With the emergence of so-called unconventional propellers and different design strategies associated with them, it has been questioned whether the assumptions used in these scaling methods are still universally valid. E.g. with tip and root unloading employed, the circulation distribution deviates from the optimum, which is assumed by the Lerbs-Meyne method; more modern profiles show a different camber distribution and hence the drag coefficient must be aligned with the hydrodynamic inflow angle and not with the pitch to diameter ratio as assumed by the strip method (and implicitly by the ITTC 1978 method [4]). The work presented still uses the assumption of the equivalent profile and will explain a modified scaling procedure showing a way to calculate the hydrodynamic inflow angle solely from one open-water test conducted at a constant Reynolds number. Finally exemplary results comparing a propeller of conventional type with a recent propeller designs will also be shown. The new proposed method shows a superior performance when compared to other scaling methods.

The Effect of Propeller Scaling Methodology on the Performance Prediction

“…Helma [4,5] showed that the mean hydrodynamic inflow angleβ i into an equivalent profile can be calculated from the open-water test as follows:…”

“…a Streckwall et al specified the Reynolds numbers of the transition point x t for the open-water condition as 4×10 5 (suction side) and 2×10 5 (pressure side), and for the behind condition as 3×10 5 (suction side) and 1×10 5 (pressure side) [9]. b In his presentation, Schulze has an obvious misprint when giving the value of 0.03 for the middle part [7].…”

“…Most authors believe that these deviations between prediction and measured performance is due to the scaling method for the open-water performance, which is needed by the ITTC 1978 power prediction method. Consequently, they either modified the ITTC method or came up with completely new methods (Praefke [3] and Helma [4,5]). …”

“…Since the performance of a full scale propeller is not easily available, Helma [4,5] suggested to scale the open-water data from tests performed at different Reynolds numbers to the full scale propeller, arguing that a good scaling method must give the same results for all model-test Reynolds numbers. It also mentions that the final validation should be done by comparing predicted with measured performance data, which is the topic of this paper.…”

Scale Effects on a Tip Rake Propeller Working in Open Water

“…Since the performance of a full scale propeller is not commonly available, novel methodologies have been proposed to scale the open-water data from tests performed at different Re numbers to the full scale propeller [5,6]. The power and shaft revolutions were predicted on a series of different scaling methods.…”