Sec. 55.14 



APPENDAGE-RESISTANCE CALCULATIONS 



295 



drag, as developed at the junction of two essen- 

 tially dissimilar forms. Again the marine architect 

 is referred directly to S. F. Hoerner's "Aero- 

 dynamic Drag," 1951, Chap. VII on pages 93-120, 

 as embodying the essence of most of the known 

 data in this field. Although written for the aero- 

 nautical engineer it should be intelhgible to and 

 useful for the marine architect who has read and is 

 familiar with the previous chapters of the present 

 book. 



55.12 The Resistance of Large Appendages 

 Considered as Parts of the Ship. Large append- 

 ages such as deep skegs and long bossings do not 

 resemble bodies for which appUcable and rehable 

 pressure-drag data are available. Further, a skeg 

 or bossing of a given shape may produce different 

 flow, velocity, and pressure patterns, depending 

 upon the form of the hull to which it is applied. 

 There are companion interference effects here, both 

 of the ship on the appendage and the appendage 

 on the ship. 



It is usually necessary to predict the drags of 

 these large appendages by: 



(1) Estimating the pressure drag from statistical 

 percentage data, as in Sec. 55.3 



(2) Taking account of the increased wetted area 

 of the ship; that is, the external area of the 

 appendage less the bare-hull area covered by it 

 when applied. This involves an increased friction 

 resistance Rp although, as explained in Sees. 22.9, 

 45.22, and 55.4, there are no acceptable rules for 

 estabhshing the correct R^ values for these parts. 



(3) Determining the drag from model tests, run 

 with and without the appendage(s) in question. 



55.13 The Calculation of Appendage Resist- 

 ance for Submerged Vessels. The notes of the 

 preceding sections of the present chapter apply 

 to the calculation of the resistance of all append- 

 ages on submerged vessels, irrespective of their 

 position relative to the hull. Those mounted on 

 top of the hull will, upon occasion, break the 

 water surface. In this case some pressure drag 

 due to wavemaking, of an amount as yet undeter- 

 mined, is added to the pressure drag due to forward 

 motion. It may indeed be more necessary to 

 predict this added drag as a design load on the 

 appendage, rather than as an increment of 

 resistance of the vessel as a whole, because this 

 partly awash condition is not one for which 

 resistance predictions are made. 



The ratio of appendage resistance to the bare- 

 hull resistance of a submersible is inherently 



larger than for a surface ship, notwithstanding 

 the greater total bare-hull resistance of the 

 entire vessel, including both the abovewater and 

 the underwater portions in the surface condition. 

 This is because of: 



(a) The provision of diving planes, and possibly 

 also of fixed stabilizers, and rope and cable 

 guards, in addition to steering rudders 



(b) The necessity for carrying one or more 

 periscopes 



(c) The high drag of radio, radar, and other 

 antennas, and of the masts or supports for them 



(d) The hull discontinuities embodied in large 

 main-ballast flood-valve recesses or large flooding 

 openings for the main-ballast tanks 



(e) The provisions for normal handling of the 

 vessel as a surface ship and for safety of the crew 

 when working about the superstructure deck 



(f) The provision of resting keels 



(g) The work and expense involved in fairing 

 and streamlining the abovewater portion of a 

 vessel of the submersible type which is to spend 

 only a small portion of its operating time sub- 

 merged. 



For a craft in the category of (g) preceding, the 

 appendage resistance may well reach 80 or 90 

 or more per cent of the bare-hull resistance of the 

 craft as a whole. For a true submarine which still 

 requires steering rudder (s) and some kind of deck 

 erection but must also carry diving planes, 

 guards, and other excrescences, the appendage- 

 resistance ratio submerged may be as high as 

 2.0 or 3.0. P. Mandel mentions that, in some 

 cases [SNAME, 1953, p. 466], the appendages 

 added to a submarine may cause the total drag 

 to be 5 times that of the bare hull alone! 



55.14 The Displacement of Appendages. The 

 drag of an appendage is of course related to its 

 size, although many other factors are involved. 

 It may be of advantage to the marine architect, 

 in the early stages of a preliminary design, to 

 know the approximate size and volume of the 

 average appendage, for ships of a rather wide 

 variety of tj^jes. 



Table 55. d gives some available information of 

 this kind, in the form of individual weights of 

 salt water displaced. From the data given, the 

 percentages of the overall displacement can be 

 calculated. While these data are by no means 

 modern (Jan 1924) they may serve as the begin- 

 ning of a more comprehensive and up-to-date 

 compilation. 



