Sec. 73.24 



FIXED-APPENDAGE DESIGN 



701 



along the section outline without encountering 

 abrupt discontinuities. One such section is a 

 thin ellipse; another is a strut section with an 

 elliptic trailing edge. 



(2) The probable angle of non-axial flow for any 

 appendage section may be estimated from: 



(a) The nominal non-axiality of the flow, con- 

 sidering the type of ship motion 



(b) The position of the appendage on the ship 



(c) The flow-obstructing or flow-straightening 

 features in the vicinity. 



For example, an appendage far removed from the 

 estimated position of the pivoting point in a 

 tight turn has a nominal degree of non-axial flow 

 that is far greater than the average drift angle 

 of the ship, measured at the CG. On the other 

 hand, if the appendage is in a tunnel, or directly 

 abaft a long skeg, it is protected, in a way, from 

 this cross flow. 



An example of the method of estimating the 

 vibratory characteristics of a typical streamlined 

 appendage in the form of a long strut arm is 

 given in Sec. 46.9. 



73.24 Design of Water Inlet and Discharge 

 Openings Through the Shell. This section treats 

 only of the design of inlet and discharge openings 

 for circulating water to the main propelling 

 machinery or to pumping plants large in compari- 

 son to that machinery, such as in a fireboat. 

 The design of secondary openings for taking 

 water into and discharging it from the hull of a 

 ship represents no particular problem that does 

 not occur with the major openings. The comments 

 and illustrations of Sees. 8.6, 8.7, and 36.20 apply 

 to all these openings in a general way. For the 

 discussion of the present section it is convenient 

 to assume axes of reference fixed in the ship. The 

 surrounding Avater is then considered as a stream 

 flowing past a stationary opening in the hull. 



The kinetic energy in the boundary layer may 

 be utilized for forcing water through internal 

 piping and heat exchangers of one kind or another 

 whenever there is sufficient velocity head for 

 conversion into the requisite pressure head. In 

 this case the available velocity head is that cor- 

 responding to flow in the boundary layer along 

 the shell. This may be taken as 0.5 the nominal 

 ship speed for prehminary estimates, less the head 

 corresponding to the desired velocity through the 

 internal system. In practice, it is found that there 

 is no particular advantage in using the scoop type 

 of injection unless the speed of the ship equals or 



exceeds 20 kt in service. In this case the deter- 

 mining figure appears not to be the speed-length 

 quotient or the Froude number but the absolute 

 speed of the ship. 



The longitudinal position of the main injection 

 is fixed within rather narrow limits by the position 

 of the internal heat exchangers. There is some 

 latitude in transverse position to suit service 

 conditions, with the proviso that the injection 

 must always remain submerged under the most 

 severe kinds of wavegoing. For an icebreaker the 

 logical position for the injection is under the 

 bottom, clear of as much ice as possible. For a 

 vessel to operate in shallow water it should not 

 be too near the bottom, otherwise it may be in 

 the mud. For vessels which may be required to 

 operate in waves in a relatively light condition, 

 especially at light or shallow draft forward, there 

 are problems other than that of picking up 

 water with the scoop. Air bubbles are entrained 

 by wave action or impacts under the forefoot, 

 but it is probable that they follow fairly definite 

 paths under the bottom. 



Water injections are to be kept clear of these 

 paths, using model tests to determine the air- 

 bubble routes. Particularly, injections should not 

 be instaUed close below roll-resisting keels, 

 docking and resting keels, or other longitudinal 

 appendages beneath which air is liable to be 

 trapped. 



When considering the use of a condenser-scoop 

 installation, or when selecting the type, the price 

 to be paid in resistance — and effective power — 

 always involves a combination of the scoop inlet 

 and the discharge. Since it is the discharge which 

 often creates the hydraulic (suction) head neces- 

 sary to draw water through the condensers, this 

 is the element of the pair which may be expected 

 to develop the greater resistance. 



Regardless of the merits of any one hydrody- 

 namic design of inlet scoop or discharge outlet 

 there is always the problem of finding room for 

 these fittings in the bottom or the lower corners 

 of the ship under or alongside the machinery 

 spaces. When room is assigned there is the matter 

 of cutting into main structural members. Finally, 

 the large piping has to be led to and from the 

 condenser(s), and space must be made available 

 for a stand-by power-driven circulating pump for 

 maneuvering. 



In Sir Charles Parsons' Turbinia of 1897 there 

 were two condensers with circulating water fed 

 to them in series from two scoops, one on each 



