854 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.31 



Assuiiiiiig a tiaiisinissioii loss of 5 per cent, the 

 necessary brake power is 178/0.95 = 187 horses. 



IV. Skene's Eq. (77.iva) of Sec. 77.14 does not 

 apply to round-bottom forms. Instead he gives 

 a graph of the dimensional ratio P/W^'^ on a 

 base of 7\ = V/'\/L, where P is in horses 

 (presumably of brake power) and W is in tons 

 ["Elements of Yacht Design," New York, 1944, 

 Fig. 147, p. 295]. 



(a) For the 18-kt boat, where W is 6.071 t, 

 TF'^' is 8.20. For T, = 3.042, the value of P/W'^" 

 from Skene's graph is 25. Then P = 25(8.20) = 

 205 horses. 



(b) For the 14-kt boat, where W is 7.187 t, 

 W'^" = 9.98. For T, = 2.366, the value of P/W'^" 

 is 13. Then P = 13(9.98) = 130 horses. This 

 seems very low. 



V. The nomogram of P. G. TomaUn [SNAME, 

 1953, Fig. 7, p. 600, for displacement-type vessels] 

 gives: 



(a) For the 18-kt boat, Aveighing 13,582 lb, a 

 predicted shaft power Ps of 195 horses. Then 

 Pb = 195/0.95 = 205 horses. 



(b) For the 14-kt boat, weighing 16,099 lb, a 

 predicted shaft power Ps of 165 horses. Then 

 Pb = 165/0.95 = 174 horses. 



The chart of H. F. Nordstrom for determining 

 effective power Pe , Fig. 48 of SSPA Report 19, 

 1951, is a good one for craft of this type but 

 unfortunately it does not extend far enough for 

 either the 18-kt or the 14-kt designs considered 

 here. 



From the foregoing it is manifest that the 18-kt 

 light-load condition is the one which controls the 

 amount of engine power required. Of the empirical 

 estimates, that of the Crouch-Werback formula 

 is only 187 horses, and is decidedly low. Those of 

 the Skene graph and of the Tomalin nomogram 

 are identical and low, but to a lesser degree. The 

 identical predictions of the K. C. Barnaby and 

 Philhps-Birt formulas, 251 horses, are higher than 

 the average by a greater amount. Despite these 

 variations, it appears safe at this stage to make 

 use of one HN-10 engine or its equivalent, with 

 a rated brake power Pu of 225 horses. 



All the foregoing estimates are based upon the 

 brake power Pb at the engine coupUng, and upon 

 the designed (maximum) speed V and weight 

 W of the complete boat. 



77.31 Selecting the 18-Knot Hull Shape and 



Characteristics. The round-bottom design dis- 

 cus.sed in these sections is intended to run at 

 two speeds, at different displacements. The high- 

 speed light-load condition is found to control the 

 engine power to be installed but for the selection 

 of hull features the heavy-load condition is used. 

 For the ABC ship itself, where the hull at its 

 designed full-load displacement could have been 

 shaped to run well at either the sustained .speed 

 or the maximum speed, it was considered better 

 design to fashion it for the higher speed. Although 

 it was known that the ship would run for much 

 of the time at reduced load and draft, the design 

 was laid out for full load and full draft. 



Here, however, it is not possible to install 

 enough power to reach the higher speed at the 

 heavier load condition. This limitation would 

 apply, at least with modern (1955) reciprocating, 

 internal-combustion power plants, to all semi- 

 planing as well as full-planing craft. It is con- 

 sidered good boat design, therefore, to shape the 

 hull for the heavier load condition and the deeper 

 draft. 



Although the alternative design of the ABC 

 tender, running in the lower range of speeds, is 

 to have a round bottom it is nevertheless of the 

 semi-planing type. For this reason, as well as to 

 give it adequate metacentric stability and to 

 provide a better internal arrangement, a wide- 

 beam hull is again favored. 



As a first guess, a maximum waterline beam of 

 10 ft is selected. It is placed at Sta. 6 or at O.QQLwl 

 from the FP. At the 14- to 18-kt speeds at which 

 this craft will run, a narrower stern is favored 

 than for the 24-kt full-planing hull. Following 

 seas are more of a problem in steering the slower 

 craft and the liability of broaching is greater. The 

 transom width is tentatively selected as only 

 0.75j5.y or 7.5 ft. A waterline is then sketched in, 

 avoiding any hollow in the entrance portion. 



D. Phillips-Birt recommends certain optimum 

 prismatic coefficients for small craft in the range 

 of T, from 1.3 to 1.8 ["The Design of Small Power 

 Craft," The Motor Boat and Yachting, Apr 1953, 

 p. 160]. While the lowest T^ value for this version 

 of the ABC tender is 2.366 for 14 kt, Phillips- 

 Birt's value of Cp = 0.69 is used as a starter. 

 For an Lwl of 35 ft and a weight displacement of 

 7.187 t, the underwater volume V is 251.4 ft^ 

 and the maximum-section area is 



Ax = 



251.4 

 Cp{L,r,) ~ (0.69)(35) 



= 10.4 ft' 



