Sec. 52.1S 



FLOW PATTERNS AROUND SHIPS 



259 



encountered on certain not-too-large vessels 

 where a relatively high power is delivered to a 

 single screw propeller on a vessel having a small 

 displacement-length quotient or fatness ratio 

 V/{0.10LY and a large keel drag. Examples are 

 river and bay passenger steamers, tugs, trawlers, 

 and whale catchers. Because of the projection of 

 the propeller below what might be called the 

 main body of the hull it could be expected that 

 the flow would have only a small degree of non- 

 axiality. However, a considerable dechvity in the 

 shaft, downward and aft, is sometimes necessary 

 to accommodate the machinery position inside 

 the hull. 



It is to be expected that, on single-screw ships, 

 much of the water moving toward the blades of 

 the propeller, in their lower positions, will be 

 unaffected by the presence of the hull, lying 

 mostly at an upper level. The present trend (in 

 the 1950's) of eliminating the rudder shoe and 

 cutting away the aftfoot on vessels of this typQ, 

 means that the flow to the lower blades is almost 

 entirely free of hull influence. Although uniform 

 and regular, it may be expected to have little or 

 no positive wake velocity unless the vessel 

 "pulls" a large stern-wave crest above it. 



52.17 Analysis of the Observed Flow at a 

 Screw-Propeller Position. Although not a part 

 of the estimating or predicting procedure, strictly 

 speaking, it is still necessary to analyze graphic, 

 tabulated, and other records from flow-indicating 

 devices at screw-propeller positions to determine 

 the principal characteristics of the flow. Simply 

 making a flow record does not tell whether the 

 nature of the flow is acceptable or not. In fact, 

 there are at least four reasons for analyzing the 

 observed flow at a screw-propeller position, 

 when determined by suitable instrumentation on 

 a model, before the model is fitted with or driven 

 by its model propeller (s). The record in this case 

 is assumed to be a 3-diml wake-survey diagram 

 similar to Figs. 11. F and 60.D: 



(1) To determine, by visual inspection, whether 

 there are any longitudinal eddies passing through 

 the disc, whether there are obvious differences 

 in flow direction at two or more points near each 

 other, and whether there are obvious large 

 differences in the longitudinal or transverse 

 components of flow for two or more such points 



(2) To estimate the probable magnitude of the 

 wake fraction for a screw propeller occupjdng a 

 disc region of given size (diameter) and position 



(3) To determine a systematic wake-fraction 

 variation with radius if one exists, and to embody 

 it, if desired, in the design of a wake-adapted 

 propeller (or selection of such a propeller from 

 stock) 



(4) To ascertain, at an early stage in the design, 

 the liabiUty and the magnitude of objectionable 

 variations in thrust and torque, per blade and 

 per wheel, for all angular positions in one revolu- 

 tion. This latter feature is discussed further in 

 Sec. 59.17. 



It is assumed, in the foregoing, that the advance- 

 velocity vectors for the proposed propeller-disc 

 position are 3-diml in nature and are determined 

 by the method described and illustrated in Sees. 

 11.6 and 11.7 and Figs. ll.E and ll.F of Volume I. 

 This method is, by the instrumentation such as 

 that currently (1955) in use at the David Taylor 

 Model Basin [Janes, C. E., "Instruments and 

 Methods for Measuring the Flow of Water 

 Around Ships and Models," TMB Rep. 487, 

 Mar 1948], somewhat artificial. For example, the 

 additional velocity induced by the action of the 

 propeller when it exerts thrust is not represented, 

 nor is the straightening effect of the propeller 

 jet acting upon the water flowing into the propeller 

 position. However, a wake determination of this 

 kind is most revealing, and many features of the 

 propeller action can be predicted from a careful 

 study of it. Instructions for conducting such a 

 study, and for determining quantitative values 

 from it, are found in Sees. 60.6, 60.7, and 60.8. 



52.18 Flow Abaft a Screw Propeller. The 

 water in the outflow jet of a screw propeller 

 producing thrust is known to be contracting at 

 the point where it leaves the propeller disc, 

 illustrated by Fig. 16.E in Sec. 16.6 of Volume I. 

 It is known to be increasing in velocity at that 

 point, and there are rotational or tangential 

 velocity components in it, additional to the 

 axial component of induced velocity. The brief 

 discussion of Sec. 17.17 reveals that the screw- 

 propeller outflow is unusual among submerged 

 liquid jets in that it maintains its identity as a 

 jet for many propeller diameters astern of the 

 disc position. 



The first quantitative observations on the 

 nature of the actual flow abaft a screw propeller, 

 in and around an outflow jet, appear to have 

 been made in about 1865 by Arthur Rigg of 

 Chester, England, in connection with his develop- 

 ment of the first contra-rudder [Inst. Engrs. 



