Sec. r,'i3 



GENERAL PROBLEMS OF DESTGNER 



455 



interpretation or direct discussion with the owner- 

 operator, arrive at a schedule of allowances or 

 performance factors which is to be followed 

 throughout the course of the design. Granting 

 that the designer's estimates are invariably 

 correct and his calculations precise, he is truly a 

 prophet if he can predict the combinations of 

 overload, overspeed, and other operating con- 

 ditions to be encountered in the life of the ship. 

 If the ship takes these in its stride, he is praised 

 for having designed a good one. If not, he is 

 liable to be blamed because the ship can not take 

 a little extra once in a while. Indeed, the mark of 

 a superlative designer may be as much in the 

 margins, allowances, and design factors which, 

 in his knowledge, intelligence, and engineering 

 intuition, he inserts here and there as in the 

 balance which he achieves in the overall design. 



It is difficult to give rules for these allowances. 

 The more performance that is being squeezed out 

 of a design, the smaller the allowances must 

 necessarily be and the greater the knowledge of 

 the forces and other factors involved. In a high- 

 speed racing motorboat, for example, they 

 approach zero. In an icebreaker they can and 

 should be large. 



Emphasis is laid on the primary function of the 

 vessel, and the allowances favor the continued 

 and reliable performance of that function. A 

 ferryboat running on a published timetable, year 

 in and year out, with many people relying on its 

 schedule, is given a generous power and speed 

 allowance, provided economy is not sacrificed. 

 A tug can always use extra power, to meet the 

 increasing demands of progress as the vessel 

 puts in more and more years of service. 



The less accurately some quantity can be 

 determined the greater allowance a designer is 

 generally forced to place upon it. The less real 

 proof he has of the validity of some estimate or 

 prediction, or the less confidence he has in it, the 

 greater must be his allowance. 



The mission of the ABC ship requires, from 

 Table 64. a, adherence to a "rigid, year-round 

 schedule, estabhshed well in advance, regardless 

 of local and seasonal weather conditions." The 

 owner-operator is emphatic in pointing out that 

 this means what it says. The study forming the 

 basis of the speed requirements of Table 64.d 

 indicates a minimum average or sustained speed 

 of 18.7 kt to achieve the mission. How is this best 

 attained when there are so many unknown 

 factors? 



It is well at this point to discuss briefly the 

 expression "sustained speed." This term often is 

 used but seldom defined, perhaps because to 

 sustain a speed in one area or on one run means 

 something quite different from sustaining it on 

 another run. For the ABC ship it means both 

 parts of the one-way run, as well as the voyage 

 as a whole. For the general case, the ability of a 

 ship to make a given sustained speed means that 

 it has, in self-contained fashion, whatever it takes 

 in the way of allowances and margins to average 

 this speed over any run, in spite of: 



(a) Wind, waves, and weather 



(b) Roughness drag due to deterioration of the 

 paint or other coating and that due to fouling of 

 any and all kinds 



(c) Improper trim or attitude for the speed range, 

 due to causes beyond control of the ship personnel 



(d) Temporary slowdowns or stoppages due to 

 inadequacies of or casualties to personnel or 

 materiel 



(e) Loss of capacity, power, or efficiency because 

 of deterioration, delayed overhaul, and general 

 wear and tear 



(f) Low quality of fuel 



(g) Any combination of the foregoing and any 

 other adverse influences. 



To average a given sustained speed on a run, 

 come what may, means that a ship has to possess 

 a reserve capacity or ability of some kind. This 

 not only is to make up for lost time but to keep 

 going regardless of the circumstances, short of 

 hurricanes, typhoons, and the like. No ship can 

 be designed to cope with extreme emergencies. 

 The reserve is designed into and built into the 

 ship, in the form of allowances. The design 

 allowance is based upon 100 per cent functioning 

 of all personnel and materiel. The performance 

 allowance takes care of incomplete, functioning, 

 as set forth in (d), (e), and (f) preceding. 



There are several ways of making these allow- 

 ances. The most logical and undoubtedly the 

 preferred one is for the designer to modify the 

 owner-operator requirements for his own use and 

 then to design as closely as practicable to those 

 modifications. It is thus possible, when the vessel 

 is completed, to check the actual design from the 

 observed performance, and to use the information 

 thus confirmed for future designs. For example, 

 instead of calling for 15 per cent extra shaft 

 power, or some other amount picked from 

 operating data to insure that the sustained speed 



