Wake Scale Effects on a 

 Twin-Screw Displacement Ship 



Arthur M. Reed and William G. Day, Jr. 



David W. Taylor Naval Ship Research and Development Center, 



Bethesda , Maryland 



ABSTRACT 



The results of a wake survey and boundary layer 

 profile measurements on a full-scale twin-screw 

 displacement ship are presented. The corresponding 

 model-scale measurements are also presented. The 

 full-scale wake measurements consist of the three 

 velocity components which contribute to the nominal 

 wake in the propeller plane, at four radii. The 

 full-scale boundary layer profile was obtained at 

 three longitudinal locations with and without the 

 propeller operating. The model-scale nominal wake 

 was determined in a towing tank using five-hole 

 pitot tubes while the model-scale boundary layer 

 measurements were made on a double model in a wind 

 tunnel using hot wire anemometers. 



In order to identify the scale effects between 

 the model and ship, the deviation of the velocity 

 in the propeller disk from a uniform axial flow has 

 been separated into the velocity field due to shaft 

 inclination in a uniform stream, the perturbation 

 due to the hull and its boundary layer, and the 

 viscous wake due to the appendages. The principal 

 contribution to this perturbation from the axial 

 flow is the effect of inclining the shaft in the 

 uniform stream. The perturbation of the flow due 

 to the potential flow about the hull is small , as 

 are the effects of the displacement thickness of 

 the boundary layer of the hull. The proposed 

 scheme for predicting the viscous wakes of the 

 shaft and struts meets with little success. Never- 

 theless, some conclusions are drawn as to how these 

 wakes will vary between the ship and model. 



1 . INTRODUCTION 



If unsteady propeller force and hull loading pre- 

 dictions are to be precise, the inflow to the pro- 

 peller must be known accurately. At the present 

 time the nominal wake of a model is measured and 

 extrapolated to full scale assuming geometric 



similarity. The extrapolation fails to take into 

 account any of the scale effects which may possibly 

 exist between model and full scale. This paper 

 presents preliminary results from a series of full- 

 scale nominal wake and boundary layer velocity pro- 

 file measurements on a high-speed transom-stern 

 ship. In addition, the corresponding model-scale 

 measurements are reported, along with a series of 

 analytical predictions, which are intended to 

 identify the principal contributions to the wake. 



This is not the first investigation of this 

 nature. However, it is the first project to suc- 

 cessfully measure the three velocity components in 

 the propeller disk of a high-speed twin-screw 

 transom-stern hull form. The British have per- 

 formed an extensive series of experiments on a 

 frigate, [Canham (1975) ] , and the Japanese and 

 Germans have performed flow measurements on 

 several full-form ships. The Japanese and German 

 experiments were conducted on single screw tanker 

 forms and are reported in an extensive series of 

 reports [see for instance: Namimatsu et al.(1973), 

 Namimatsu and Muraoka (1973), Schuster et al.(1968), 

 Takahashi et al.(1970), Taniguchi and Fujita (1970), 

 and Yokoo (1974) ] . 



While the British measurements were obtained on 

 the ship type of interest, a high-speed transom- 

 stern ship, only the longitudinal velocity compo- 

 nent in the propeller plane was obtained. This 

 resulted in the loss of the important tangential 

 and radial velocity components. In the case of 

 twin-screw transom-stern ships , these velocity 

 components are generally very significant due to 

 the inclination of the shaft to the direction of 

 the free-stream. 



The Japanese, on the other hand, were able to 

 measure all three velocity components in the wake, 

 but they had to make their measurements in a plane 

 ahead of the propeller disk. Due to the full 

 sterns of the tankers, the flow into the propeller 

 is highly influenced by viscous effects, and as a 

 consequence is highly affected by changes in 



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