Sec. 76.25 



DESIGN OF SPECIAL-PURPOSE CRAFT 



793 



Brief design rules for feathering paddlewheels 

 on a double-ended ferryboat are given in Sec. 71.7. 

 It is emphasized in Part 5 of Volume III, under 

 the discussion of retardation and acceleration, 

 that to be able to start and stop properly, the 

 propulsion devices of a ferryboat must have a 

 thrust-producing area, equal or equivalent to 

 Ao in Fig. 15. G, that is large in comparison to the 

 size of the vessel. If driven by screw propellers 

 these must have a relatively large blade area as 

 well as a large diameter. Sometimes the necessary 

 blade area can be achieved only by increasing 

 the propeller diameter [DuBosque, F. L., SNAME, 

 1896, p. 103]. 



76.25 Design Notes for Ferryboat Hulls and 

 Appendages. The relatively large metacentric 

 height required for a ferryboat calls for a large 

 designed waterline, in all loading conditions, 

 with respect to its displacement volume. Because 

 the cargo to be carried is one of volume rather 

 than of weight, the length and beam are also 

 large with respect to the displacement volume. 

 This means a small maximum-section coefficient 

 and a rather large beam-draft ratio. 



It may be expected that with its large waterline 

 area, small maximum-section coefficient, and 

 relatively shallow draft the ratio of the wetted 

 surface S of a ferryboat huU to the factor \^¥L 

 will be large. This means a large wetted-surface 

 coefficient Cs , perhaps so large as to be off the 

 scale of the graph in Fig. 45.G. 



Rather clever shaping of the waterhnes is called 

 for to achieve a fineness at the ends which will 

 avoid undue pressure resistance because of wave- 

 making forward and excessive pressure resistance 

 due to separation aft, yet which will provide the 

 square moment of area about the pitching axis 

 called for by (7) of Sec. 76.23. If this can not be 

 accompUshed, it is usually the hull resistance 

 which has to suffer. Fig. 76. P is a plot of half of 



the designed waterline of the ferryboat Cincinnali, 

 described in reference (b) of Sec. 76.23 preceding. 

 A complete set of lines for a double-ended ferry- 

 boat taken from Het Schip, issue of February 

 1929, is reproduced to small scale by W. P. A. 

 van Lammeren, L. Troost, and J. G. Koning 

 [RPSS, 1948, Fig. 195, p. 289]. 



Because thrust deduction is developed both 

 abaft the bow propeller and forward of the stern 

 propeller, it is important if both work simultan- 

 eously that the hull be fined as much as prac- 

 ticable abaft and ahead of these propellers. This 

 fining should extend for at least 2 and preferably 

 3 propeller diameters abaft and ahead of the 

 respective propeller discs. Actually, the form of 

 hull adjacent to screw-propeller positions and the 

 propeller clearances are determined for each end 

 propeller, by the rules of Sees. 67.23 and 67.24, 

 on the basis that that propeller pushes from the 

 stern. The resulting design should be entirely 

 adequate for a propeller which pulls from ahead. 



The combination of length to afford adequate 

 space on deck, large waterUne area for meta- 

 centric stabifity, and fining of the ends results in 

 a block coefficient Cb that is extremely low com- 

 pared to its value for the average cargo vessel. 

 The first table accompanying reference (c) of 

 Sec. 76.23 shows a range of Cb from 0.34 to 0.42. 



Because of the full waterline endings, described 

 elsewhere in this section, and of the large speed- 

 length quotients at which modern ferryboats 

 run, the heights of the bow- and stern-wave 

 crests are factors to be reckoned with in design. 

 The necessary clearances must be provided above 

 the wave profile and under the sponsons or 

 supports for the deck overhang, as well as the 

 necessary freeboard for normal running. In 

 addition, there must be some assurance that a 

 heavily loaded vessel will not take water over the 

 main deck when encountering or passing through 



3-ft 6-ft 9-ft 12-a 15-ft Buttocks 

 I 



Fig. 76.P Half-Body Plan and Half- Waterline foe New York Ferryboat Cincinnati 



