Some Hydrodynamic Aspects of Ship Maneuverability 



The remaining portion of this paper will be devoted to a detailed discussion 

 of the relevant hydrodynamic force and moment which act on a ship hull during 

 a maneuver, including analytical methods of description and those experimental 

 results which help to further our understanding of the mechanisms involved. 

 The Appendix includes a brief statement of the nomenclature and equations of 

 motion which are the basis for most work in this field. 



QUALITATIVE DESCRIPTION OF THE FLUID FLOW 



The practical description of the flow of water past a ship hull during a ma- 

 neuver poses one of the most complex problems encountered in the field of ship 

 hydrodynamics. Included to a significant extent are most of the phenomena 

 which can be associated with an incompressible Newtonian fluid. Under the cir- 

 cumstances it is not surprising that literature on the subject of ship maneuver- 

 ability is devoted primarily to experimental investigations and techniques. 

 Fortunately at least a qualitative description of the flow and associated force 

 and moment can be provided if the disturbance of the free surface is sufficiently 

 small, the boundary layer is thin, and there is no large-scale separation or 

 ventilation. 



In order to evaluate some of the above assumptions, flow observations and 

 photographs were made in the Circulating Water Channel of the David Taylor 

 Model Basin. A model of the Mariner Class ship USS Compass Island was used. 

 The results of this investigation are illustrated in Figs. 1-6. The model is 12.6 

 feet long and the flow velocity is 3.1 knots, corresponding to a full-scale speed 

 of 20 knots. Steady drift angles of 0, 3, and 10 degrees are shown. Bilge keels 

 and a rudder were installed on the model, but the propeller was not in place. 

 The flow can be visualized by means of the wool tufts fastened on the port side 

 of the model and also by means of dye streams, which were injected through the 

 hull at stations nine and fifteen and through two external tubes at the bow. The 

 wool tufts located on integer stations were fastened directly to the hull, whereas 

 those located on half-stations were fastened to pins and separated from the hull 

 surface by a distance of about one inch. 



Over the forebody the perturbation of the basic flow by the drift angle is in 

 agreement qualitatively with the expected cross-flow; that is the streamlines 

 are curved down on the upstream side of the hull and up on the downstream side. 

 However on the afterbody the situation is reversed, to an increasing extent with 

 distance downstream, and the downward flow on the downstream side is espe- 

 cially pronounced immediately ahead of the propeller aperture. It should be 

 noted that this downward flow on the downstream side (Fig. 1, bottom photograph) 

 occurs first in the vicinity of stations 12 and 13, where it is confined to the im- 

 mediate hull surface, whereas further aft the downward flow has thickened so as 

 to be indicated on the outer (half-station) tufts as well. Thus it appears that 

 this reversal of the perturbation crossflow is confined to a boundary layer which 

 becomes increasingly thicker with distance downstream. A similar effect near 

 the bow can be noted in the bottom views of the downstream side of the hull 

 (Fig. 6) where there is a noticeable "phase difference" between the inner and 

 outer tufts which disappears with increasing distance downstream. However 

 this secondary flow occurs only close to the surface and at the bow, whereas 



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