258 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 52.15 



Fig. 52.Y Pressure Distribution on Afterbody op 



EMB Model 3383 at a Speed of 3.8 kt, T, = 0.844 

 The numerals on the isobars indicate differential pressures 

 in inches of water of the same mass density as that in 

 which the model was towed 



standpoint either of hydrodynamic research or of 

 ship design. 



52.15 Use of Flow Diagrams for Positioning 

 Appendages. The comments of Sec. 52.12 apply 

 generally to the proper positioning of appendages 

 such as bossings, shaft struts, small skegs, and 

 rudders, as well as to the outer portions of roll- 

 resisting keels. 



Special model-basin techniques are available to 

 determine the proper strut-section angles for 

 shaft struts. Since the variation from straight- 

 ahead positioning of these parts seldom exceeds 

 7 or 8 deg, and almost never is greater than 10 

 deg, a prediction without model-test confirmation 

 would have to be extremely precise, relative to 

 present (1955) standards. 



When rudders (and diving planes) are of the 

 spade or horn-supported type they are necessarily 

 rather thick to achieve strength and rigidity. 

 Extended observation of flow around model 

 appendages in the circulating-water channel 

 indicates that their presence affects the flow to 

 considerable distances on either side. The ink 

 trail in Fig. 46.E shows this feature clearly. It is 

 not adequate, therefore, when making flow 

 predictions, to suppose that the object for which 



the flow directions are desired is always of the 

 thin-plate variety. Its position, shape, and bulk, 

 as affecting the flow, all must be taken into 

 account. 



52.16 Estimated Flow at Propulsion-Device 

 Positions. Extended discussion of the flow to the 

 propulsion-device positions, amounting almost to 

 duplication (it appears in Sec. 17.2, in Chap. 33, 

 and in Sees. 59.12 and 60.7), is considered justified 

 in view of the importance of this feature and the 

 insufficient attention it has received in many 

 quarters in the past. However, the extensive 

 model-basin instrumentation now available (1955) 

 is still inadequate to give the ship and the pro- 

 pulsion-device designers all the advance informa- 

 tion they should have in the design stage. 



The huU-surface traces of the flow just ahead 

 of a propeller position, particularly those on a 

 centerline skeg in front of a single screw, are 

 usually good indicators of the type of inflow into 

 the wheel positions close to the centerplane. For 

 example, in Fig. 52. R it appears that the flow 

 just ahead of the single propeller, at least into the 

 11 o'clock to 1 o'clock and 5 o'clock to 7 o'clock 

 positions, may be expected to have only a slight 

 upward component of velocity. For each offset 

 propeller position of Fig. 52.T, it is expected that 

 the flow outboard of the skeg ahead will have a 

 small upward component but that inboard of the 

 skeg it will have a large one. 



Nevertheless, in the absence of any proved 

 general rules for interpreting them, it can not be 

 said that the surface traces are completely 

 reliable indicators in this respect. Before an 

 acceptable procedure for estimating the flow at 

 propdev positions can be formulated, it will be 

 necessary to make, and to analyze, companion 

 off-the-surface flow traces such as those described 

 and illustrated in Sec. 52.9 and 3-diml wake- 

 survey diagrams of the type diagrammed in 

 Sec. 60.6. Until that time the surest and most 

 reliable prediction method is actually to observe 

 the flow in the vicinity of the propulsion device (s) 

 on a model of the vessel in a circulating-water 

 channel. If the propdev characteristics are approxi- 

 mately known, a stock model of the device can be 

 added to the ship model and observations made 

 under thrust-producing conditions. In most 

 cases these tests can be made and this study 

 completed at a sufficiently early stage in the 

 design to permit changing the hull or appendages 

 (or both), to correct or to improve the flow. 



A situation somewhat out of the ordinary is 



