826 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.9 



TABLE 77.a— (Continued) 



SAFETY — The following requirements apply: 



(10) The fuel used in the power plant shall be diesel fuel 

 or its equivalent; no type of gasoline or equally volatile 

 fuel is permitted 



(11) The tender shall remain afloat and upright if com- 

 pletely swamped, with crew, passengers, and cargo on 

 board. The cargo may be considered as 50 per cent per- 

 meable, and the special flotation volume as fully intact. 



(12) Drinking water and emergency rations for sixteen 

 persons for two days are to be carried 



(13) Safety and operational equipment as required by 

 regulations of the U. S. Coast Guard for operation in 

 semi-protected ocean waters. 



OTHER REQUIREMENTS 



(14) It shall be possible to warm up and run the power 

 plant for a short period before the boat is hoisted out 



(15) The boat shall preferably bank inward on turns in 

 normal steering 



(16) The craft shall possess adequate steering control in 

 either head or following seas 



(17) It shall not pound or slam excessively, so as to risk 

 injury to the boat structure or to the crew and passengers, 

 when running in rough water at speeds in excess of 10 kt 



(18) The design and construction shall be rugged and 

 substantial, suitable for hard service and reliable operation 

 in salt water. 



by requirements for weight, carrying capacity, 

 speed, endurance, safety, and other features, 

 embodied in items (3) through (18). Still other 

 items may develop as the design proceeds. 



The prospective operators wish an alternative 

 design prepared for a craft to meet substantially 

 the same general requirements as in Table 77. a 

 but to run at a higher maximum speed with less 

 carrying capacity. The detail requirements are 

 set down in Table 77. b. 



It is noted that complete freedom is left to the 

 designer with respect to the following, for both 

 the principal and the alternative designs: 



(a) Type of hull, whether displacement, semi- 

 planing, or full-planing. However, a conference 

 with the prospective operators of the ABC ship 

 brings out that reUabUity of operation, all- 

 weather behavior, and load-carrying performance 

 of the craft are considered of paramount import- 

 ance. Speed can be sacrificed if need be. 



(b) Hull proportions, principally the L/B ratio 

 but more important still, for actual stowage on 

 the ship, the maximum beam 



(c) Hull draft or hmiting draft 



(d) General shape and appearance of hull 



(e) Power plant and power 



(f) Number of propellers 



(g) Materials of construction 



(h) Method of attaching the single-ring hoisting 



sling. 



77.9 Analysis of the Principal Requirements. 



Before starting the design proper it is well to 

 analyze the requirements, in terms of the back- 

 ground already gained in the preliminary design 

 of the ABC ship. The overall length of 40 ft is 

 fixed for both the principal and the alternative 

 designs. To achieve the load- and volume-carrying 

 capacities required, it is almost a certainty 

 that the stern will have to be of the transom type. 

 From an examination of the pubUshed data for 

 other motorboats an average length for the water- 

 hne at rest may be estimated as 37 or 38 ft; the 

 lower value is on the safe side. Its square root 

 is 6.083. Based upon the speeds in item (8) of 

 Table 77.a and item (b) of Table 77.b, the T, 

 and F„ values are: 



Speed 14 kt 



r« = 14/6.083 = 2.30 

 Fn = 2.30(0.2978) = 0.685 



Speed 18 kt 



T, = 18/6.083 = 2.96 

 Fn = 2.96(0.2978) = 0.881 



Speed 24 kt 



T, = 24/6.083 = 3.95 

 F„ = 3.95(0.2978) = 1 . 176. 



Nothing is said in the specifications about 

 running trim; that is, change of trim when 

 underway. Excessive trim by the stern, resulting 

 either from placing the useful load too far aft, or 



TABLE 77.b— Motor Tender for ABC Ship; 

 Requirements for Alternative High-Speed Design 



An alternative design of motor tender is to be prepared 

 having the general requirements of items (1) through (14) 

 plus (18) of Table 77.a, modified by the following: 



(a) Carrying capacity of six passengers plus a crew of 

 two, all having an average weight of 170 lb per person, 

 plus a cargo of packages or luggage weighing not in excess 

 of 900 lb and having a volume not in excess of 50 ft', plus 

 a two-thirds load of fuel 



(b) Speed, 24 kt minimum, in quiet water, with the load 

 of item (a) 



(c) Run at full power for two hr without refueUng 



(d) The boat shall bank inward on turns when running in 

 the planing range 



(e) The craft shall possess adequate steering control in 

 either head or following seas 



(f) It shall not pound or slam excessively, so as to risk 

 injury to the boat structure or to the crew and passengers, 

 when running in rough water in the planing range 



(g) The craft shall not porpoise at any operating speed. 



