CHAPTER 65 



General Problems of the Ship Designer 



65 . 1 Interpretation of Ready-Made Design Re- 



quirements 454 



65.2 Departures from the Letter of the Specifica- 



tions 454 



65 . 3 Design and Performance Allowances .... 464 



65 . 4 Basis for the Selection of Ship Dimensions . 457 



65 . 5 Determination of the General Hull Features . 457 



65.6 Limits for Wavegoing Conditions to be En- 



countered 458 



65.7 The Bracketing Design Technique 458 



65.8 Adherence to Design Details in Construction . 459 



65 . 9 Guaranteeing the Performance of a New Ship 



Design 459 



65.1 Interpretation of Ready-Made Design 

 Requirements. It is possible for the hapless ship 

 designer to be confronted with a set of pre- 

 posterous requirements and specifications, not of 

 his own formulation. On first reading these may 

 seem to call for achieving the impossible. After an 

 anxious time the designer may be relieved, but 

 nevertheless bewildered, to find that he is not 

 expected to meet them, but only to be awed by 

 their severity. He may, on the contrary, find that 

 they have both claws and teeth, and that he is 

 expected to accomplish what no one before him 

 has done. It is most important, therefore, that 

 when a set of requirements and specifications is 

 handed to him, the designer assess them carefully 

 and that he determine in advance how they are 

 to be interpreted. 



However, it is assumed here that the ship 

 requirements are laid down in all sincerity and 

 that they are expected to be observed in the same 

 fashion, both as to their spirit and their letter. If 

 there is in them any semblance of reaching for 

 the moon, the reaching is clearly indicated and 

 there is good reason for it. A case in point is the 

 superlative deep-water and open-sea performance 

 required of the ABC design, which in many 

 respects is a Umited-draft river vessel. If parts of 

 the requirements are intended primarily for 

 information and guidance rather than strict 

 compliance, they in turn are clearly so marked. 



65.2 Departures from the Letter of the Specifi- 

 cations. Notwithstanding the most conscientious 

 effort, it is rarely possible to comply with every 

 letter of the complete design requirements and 

 performance specifications for a ship. Some com- 

 promises must always be made to produce the 

 design and some relaxation of the specifications 

 be accepted. The burden of accepting these 



compromises rests with the owner or operator for 

 whom the ship is being designed. A decision 

 concerning a modification or relaxation of the 

 specifications can only be made intelligentl}'' on 

 the basis of reasonably accurate and reliable 

 information concerning the price which the owner 

 or operator must pay for this change, either in 

 money or in speed, power, endurance, and 

 performance. 



It is a duty of the ship designer to offer con- 

 structive suggestions, propose compromises, or 

 put forth alternative solutions in all cases of 

 conflict between the original requirements and 

 specifications. This procedure applies as well to 

 conflicts between these requirements and the 

 capabilities of construction materials, machinery, 

 and equipment in the present state of the art. 

 For example, vertical or wall sides are indicated 

 for the bow sections of a fast or liigh-speed ship 

 in way of the bow-wave crest. To accomplish this 

 may require a rather sharp reentrant curve in the 

 bow section lines above the top of this wave 

 crest. This spoils, in a way, the gently flaring 

 V-sections which may have been drawn in forward 

 above the water fine for good wavegoing per- 

 formance. The designer therefore prepares to 

 estimate the increase in smooth-Avater resistance 

 and power due to the use of fair V-sections and 

 to predict the possible adverse effects of somewhat 

 hollow V-sections when pitching or plowing into 

 head seas. Similarly, he is prepared to give figures 

 relating to the change in resistance and power, 

 and the modification in roll-quenching charac- 

 teristics, for proposed variations in the bilge-keel 

 length. 



65.3 Design and Performance Allowances. 

 Whether or not they are written specifically into 

 the requirements, a ship designer must, by 



454 



