452 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 64.4 



added. These are based upon entering the ship 

 head-on, directly from the sea entrance, and upon 

 swinging the ship by a considerable amount, 

 when backing out of the slip, by working on a 

 spring line attached to the outer end of the pier. 



Translating the ship maneuvers of Figs. 64.A 

 and 64. B into specific maneuvering requirements 

 gives the items hsted as (32) and (33) in Table 

 64.e. Because of lack of authentic information on 

 the astern maneuvering characteristics of ships, 

 the requirements in (33) are left rather general 

 in character. The requirements in (34) and (36) 

 are intended to insure that the vessel can be 

 maneuvered handily in an emergency. 



Rather exceptional controllability as regards 

 steering is called for, because of the more than 

 450 miles of restricted-water traveling that is 

 required of the vessel on each voyage. 



Power is not a factor in these restricted waters 

 because the speeds are limited by wave wash on 

 the banks, the presence of other vessels close by, 

 and the excessive sinkage at the stern that would 

 be encountered at the higher speeds. 



64.4 Absolute Size as a Factor in Maneuvering 

 Requirements. Maneuvering requirements, in- 

 volving steering, turning, stopping, backing, and 

 the equivalent, form a sadly neglected part of the 

 specifications for all types and sizes of water craft. 

 Increasing emphasis has been placed, and will 

 continue to be placed, upon the safety of life 

 and of vessels at sea. It has been necessary in 

 past emergencies, and it will be more necessary 

 in future ones, for all types of craft to undertake 

 turning and other maneuvers which will confuse 

 those who are dropping or firing missiles from the 

 air. It may be expected, therefore, that maneuver- 

 ing requirements will appear with increasing 

 frequency in ship specifications. The insertion of 

 specific numbers, making the requirements more 

 definite, will certainly follow. 



When selecting definite numbers for maneuver- 

 ing requirements it is to be remembered that 

 these can not be based, for small craft as well as 

 large ships, entirely upon some linear dimension 

 such as the length. Things happen much faster on 

 a small vessel than on a large one. The rate of 

 motion on a small craft, for geometrically similar 

 maneuvers, increases directly as the square root 

 of its linear ratio to the large craft. As long as 

 human beings with more-or-less fixed reaction 

 times operate the craft, it is necessary to take 

 account of these factors. Thinking and giving 

 piloting orders for a ship transiting a canal is a 



far cry from the same procedure for a self-propelled 

 model in a miniature channel, with a time rate 

 possibly five or six times as fast. 



Consider a modern 25-ft pilot model of a large 

 planing boat. The pilot model might well run at 

 33 kt, with a T, of 6.6, while the 81-ft full-scale 

 craft makes 60 kt at the same T^ . The former 

 covers its own length in 0.45 sec whereas the 

 latter requires 0.80 sec. With a linear scale ratio 

 of 3.24, the time ratio is (3.24)°'' or 1.8 times as 

 fast for the pilot model. Other things being equal, 

 the 25-ft craft turns in a circle over 3 times as 

 tight as the 81-ft one, and so on. 



As the absolute size increases, on the other 

 hand, the proportion between (1) the time for a 

 disturbance to manifest itself and (2) a human 

 perception or an automatic-control time increases 

 rather rapidly. 



On small craft, often with large powers relative 

 to their size, too-rapid maneuvering can incon- 

 venience or injure personnel and result in damage 

 to materiel. On large ships, demands for improved 

 maneuvering performance can run rapidly at 

 times into increased weight, complication, and 

 cost. 



64.5 Tabulation of the Secondary Require- 

 ments. When all the principal requirements have 

 been set down, and it is known what the ship 



TABLE 64.f— DESIRABLE FEATURES 

 INVOLVING HYDRODYNAMICS AND FORM 



Insofar as practicable, consistent with the principal 

 requirements of the design, the ship shall: 



(42) Possess a square moment of area coefficient of the 

 designed waterline, about the longitudinal axis, not 

 less than 0.55 



(43) Take in and discharge water for heat enchangers at 

 points not below the 1-ft waterline, reckoned from 

 the baseplane 



(44) Be prepared to anchor with two bower anchors in 

 the river leading from Port Correo to the sea, in 

 currents up to 2.3 kt 



(45) Discharge the products of combustion, from whatever 

 source, at such a point and in such a manner that 

 they will not form a nuisance to either crew or 

 passengers 



(46) Be free of spray and high-speed local air currents so 

 that in "outdoors" weather, passengers shall not be 

 inconvenienced in their enjoyment of the sun, sky, 

 and sea 



(47) Provide internal heavj'-weather access to all spaces 

 in which personnel are required during cruising at sea 



(48) Have no vulnerable projections outside the main hull 

 within the first 150 ft of the length, from the keel up 

 to the 35-ft waterline, which might be damaged by the 

 old stone pier at Port Bacine; also have no projections 

 below the fair line of the bottom of the main hull. 



