Sec. 60.9 



SHIP-POWERING DATA 



373 



cessively 0.5, 1.0, and 2.0 propeller diameters 

 along the shaft, forward of the propeller position. 

 Draw other circles for stations abaft the shaft, 

 if necessary. If the shaft center is nearly or exactly 

 parallel to the centerplane and the baseplane, 

 only one circle is needed on a body plan showing 

 the hull. 



III. Mark carefully the outlines of the special 

 stations lying within their respective projected 

 disc circles. These outlines may be indicated by 

 colored lines on the working plan, or the respective 

 areas may be marked by different types of hatch- 

 ing, as in Fig. 33. A. 



IV. With a planimeter, or by any other suitable 

 means, determine the area of the propeller disc 

 covered by the section of the hull and of the rudder 

 (or horn or other appendage) at each of the 

 special stations. In the same manner, determine 

 the area of the propeller disc. As only the ratios 

 of areas enter in this analysis the planimeter 

 readings can be used directly, without converting 

 them into absolute area units. 



V. Multiply the area readings by suitable mul- 

 tiples, as indicated in the tabular portion of 

 Fig. 67.V. Total the products and divide by the 



sum of the multiples, 15 in the case of the ABC 

 ship, to obtain a weighted-average area reading. 

 Divide the weighted area reading by the propeller- 

 disc area reading to obtain the 0-diml area ratio. 



VI. With this area ratio enter Fig. 60. P and pick 

 off the estimated thrust-deduction fraction for 

 the ship. The two graphs for this figure are still 

 tentative, based as they are on the analysis of 

 data from a rather limited number of model tests. 



The procedure for estimating the thrust-deduc- 

 tion fraction for a rotating-blade propeller, and 

 for designing the hull to keep the augment of 

 resistance small, is essentially the same as for a 

 screw propeller. Instead of the imaginary cylinder 

 of circular section an imaginary rectangular tube 

 is projected forward of — or abaft — the basket 

 assembly of blades, for a distance equal to twice 

 the blade length or to the diameter of the blade- 

 axis circle, whichever is the greater. 



All the foregoing indicates that determination 

 of the thrust-deduction fraction in any given ship 

 case is still largely empirical, and as highly 

 uncertain. The problem is now (1955) being 

 attacked along analytic lines, as it should have 

 been years ago. It is hoped that this attack will 



0.30 

 0.28 



0.26 

 0.24 



Line for Area 

 Factor Includino 

 Rudder Area \ 



^ Iwilfred Sijkes^># 



Line for Area \"~ 

 Factor ExcludincjV 

 RudderX' 



^0.22 

 f0.20 



Victorvj Ship 



:v 



T-5 

 Tanker 



Fsef^ 



Victorjj.^^ Ship 



AF58^ 



Baker's 56C, EMB Mod&]J§2Z> 



T-5 

 • Tanker 



ABC Ship, Arch Stern 



J 0.18 



:o.i6 



;0.I4 

 I 0.12 



0.10 



K 



Moriner- 



iralamancQ Class 



J UJ i 



.^ 



j^950 Export Ships, 



TMB Model 3917, TwjjV Skpqs 



I'O.Oe 



+iTwin-Ske(^ Manhattan, TMB Model 3898 



1-0.06 



DE 1006 



0.04 



0.02 



T 



^ABC Ship I 

 ^ronsom Stern 



^ 



o Area Foctor Includes Rudder 

 • Area Foctor Excludes Rudder 

 Plotted Values of tare from 

 Seif-Propelled Model Tests 



^ 0.02 0.04 Q06 006 0.10 QI2 014 QI6 018 Q20 0.22 024 026 028 0.50 Q52 034 056 058 040 042 044 Q46 0.48 

 Area Factor -(Averocje Weighted Hull Area Within Disc Cvjlinder)-^ (Propeller Disc Areo) 



Fig. 60.P Graphs for Predicting Thrust-Deduction Fraction for Single-Screw Ships by the "Cylinder" 



Method 



