858 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77 J3 



Holf the Beom B^x ^^ Drown to a Scoie of One-Quorter the Unqth Lu/i_ 

 - B ' ■ 



Fig. 77.S Duiensionless Watebline-Offset and 



Section-Akea Cukves for Round-Bottom Tender 



OP ABC Ship 



positions of the maximum ordinates of each. 

 This completes the preUminary hydrodynamic 

 design of the ABC round-bottom tender, as 

 worked out in this chapter. 



77.33 Example of a Modem Round-Bottom 

 Utility-Boat Design. An example of a round- 

 bottom design for a motorboat somewhat larger 

 than that of the ABC tender is the 50-ft open 

 utihty boat designed recently (1954) by C. E. 

 Werback. Fig. 77. T is a body plan of this craft 

 and Table 77. j lists its characteristics for the 

 light- and full-load conditions. 



TABLE 77. j — Dimensions and Other Data for the 

 50-FT Round-Bottom Utility Boat of Fig. 77.T 



Light displacement, 24,500 lb = 10.937 t 



Speed, 13.5 kt 

 Full-load displacement, 48,000 lb = 21.784 t 



Speed, 10.5 kt 

 LoA = 50.02 ft Lk^l = 48.0 ft 



Be (over guards) = 14.48 ft Bwx = 12.02 ft 



Freeboard at FP above DWL = 6.2 ft 

 Freeboard amidships = 4.1 ft 

 Freeboard at AP = 4.02 ft 

 Draft to bottom of skeg = 3.92 ft 

 Radius at full power =145 miles 

 Brake power, one 165-horse diesel engine 

 Fuel, 170 gallons _ 



At 13.5 kt, T, = V/\/L_= 1.950 

 At 10.5 kt, T, = V/^L = 1.517 

 A/(0.010L)' at full load = 21.78/0.1106 = 197.2 

 Ratio of (Ih/Ps) at full load = 48,800/165 = 296.0 lb 

 per horse. 



This craft has buttock lines that are very 

 nearly straight in the region from Sta. 8 to Sta. 

 12, or for the aftermost third of the length. The 

 minimum rise-of-floor angle at Sta. 4, one-third 

 of the length from the bow, exceeds 8 deg. The 

 ratio of transom beam to maximum waterline 

 beam is rather large for a round-bottom boat but 

 in this case it provides additional room within 

 the hull. 



77.34 Design for a Motorboat of Limited 

 Draft. For a motorboat of normal design, the 



extreme draft is often determined by the size 

 and vertical position of the propeller(s) and 

 rudder (s). The bottom of the skeg or rudder shoe 

 may be the lowest projection but if so it is lower 

 than the bottom of the propeller tip circle to 

 give protection to the latter. 



When the draft is limited by operating con- 

 ditions, or when it is desired to give the propellers 

 some protection, they may be recessed upward 

 into one or more tunnels just as they are in larger 

 tunnel-stern shallow-draft vessels. Usually, how- 

 ever, there is insufficient length to drop the 

 tunnel roof down to the plane of the designed 

 waterline abaft the wheel, so the after end of the 

 upper portion of the tunnel is exposed. 



In certain speed-length ranges, particularly 

 just below hump speed, the change of trim and 

 squat for most motorboat forms is large. Shallow 

 water exaggerates this situation because of the 

 increased steepness of the waves, generated either 

 by the boat's own motion or by natural winds. 

 It must not be expected, therefore, that a boat 

 which has a draft of x ft when at rest may be 

 able to run, under a variety of conditions, in 

 water of x ft depth without scraping along or 

 striking on the bottom. 



When working up the characteristics of a 

 shallow-draft motorboat the designer may, with 

 what appears to be a reasonable first weight 

 estimate, convert this weight to volume of the 

 liquid in which the craft is to run. Then on the 

 basis of a tentative waterline area and shape, he 

 may determine the mean draft necessary to give 

 the displaced volume, assuming first that this 

 draft is constant under the entire waterline area. 

 As a first approximation, the maximum draft of 

 the hull proper may be taken as equal to 3 times 

 the mean draft. Additional draft needed to 

 provide dynamic stability of route, or mechanical 



Lenqth on Waterline, 48.0 ft 

 Beom, maximum on waterline, 12.02 ft 

 Displacement Full Lood, 48, 

 WL5 



Buttock Spocmq 15 l_ .. 



Wcterlme 

 Spacing 15 

 1.0 ft for all 

 YV/Vl at 'this 'speed. 1.517 



Designed Speed ot Full Lood, 10.5 kt 

 Fig. 77.T Body Plan op 50-Foot Utility Boat 



