516 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.9 



factor, because the section shapes largely govern 

 the nature of the flow to these devices. Near the 

 stern a secondary factor may be the tentative 

 rudder position (s). For normal-form sterns the 

 fitting of twin or multiple screws calls for sections 

 of predominantly V-shape. For single screws a 

 V-shape merging into a U-shape helps to provide 

 a greater degree of equality in the vertical 

 distribution of wake velocity, and possibly also 

 a higher average wake fraction than a V-shape. 

 With the latter it is almost impossible to obtain 

 anything but blunt endings in the upper part of 

 a centerline skeg. 



It is to be recalled, when sketching in the run 

 sections, that most of the water passing the run 

 comes up from under the bottom. Put in another 

 way, the water coming up from underneath the 

 ship covers a greater surface area of the run, as 

 projected on the body plan, than the water 

 coming around the sides. The importance of good 

 shaping in the run was appreciated many decades 

 ago and its need could well be brought to the 

 attention of naval architects every few years. 

 The following extract is taken from pages 16 and 

 17 of a thesis by Mr. H. de B. Parsons entitled 

 "Ship Design and a Systematic Method of Con- 



struction," submitted in partial fulfillment of a 

 B.Sc. degree at the Stevens Institute of Tech- 

 nology, Hoboken, N. J., in 1884: 



"This eddy resistance, produced by an 'unfairness' in 

 the run, is a fault common to many boats otherwise well 

 designed. The run is the most important part of the 

 sJiip, as well as the most difficult to design, and should, 

 therefore, receive very attentive study. If it is not given 

 a shape, capable of letting the water close in naturally 

 and smoothly under the stern, the negative pressure of 

 the water against that part of the ship, will be lessened, or, 

 what amounts to the same thing, the direct head resistance 

 will be increased, thus producing a decided loss in 

 efficiency." 



With the relatively large B/H ratio of 2.80 in 

 the ABC ship, and a transom at the stern, the 

 section lines in the run fall into a pattern which 

 resembles more nearly the orthodox twin-screw 

 rather than the single-screw hull. This shape of 

 run does not lend itself to deep U-sections in the 

 portion leading to the propeller. For this reason, 

 and to hold the thrust-deduction forces to a 

 minimum, the centerline skeg was made as thin 

 as practicable, having due regard to accessibility 

 and lateral stiffness. It was then added to the 

 main hull as a sort of appendage, indicated by 



TABLE 67.C — Data on Section Coefficients in the Entrance for Typical Merchant Ships 



