830 



HYDRODYNAMICS IN SHIP DESIGN 



/ 



::i 



FUEL- 



ob 



:=!: 



::=^=1 



Froude Number Fn'WT 



USEFUL LOAD 



FUEi:' 



MACHINERY and ELECTRICAL 



■ 1—4— 



IPMEifr, ,5T0RE5, OUTFIT 



Toylor Quotient Jcf '^j^ 



/ 



M 



-1=- 



Sec. 77.12 



1 



20 ' 2Z ' 24 ' 26 ' 2.8 3.0 3.E 3.4 3.6 5B 4.0 4.2 4.4 46 4.8 5.6 5,2 54 i,b 



1.0 12 1.4 1.6 I, 

 Fig. 77.D Percentage Disteibution op Principal Weights for Seven Moderate- to High-Speed Craft 



quirement in item (c) of Table 77.b calls for a 

 2-hr supply at full power. Under these conditions 

 the fuel rate for the two 215-horse engines pro- 

 posed is about 0.5 lb per brake horse per hr, so 

 that 430 lb of fuel is required for both engines for 

 a 2-hr run. Since the owner has had no previous 

 service experience with a tender of this kind, it 

 seems mse at this stage to allow for a considerably- 

 enlarged fuel capacity, say about tmce that 

 actually required. With an assumed weight of 

 6.2 lb per gal, 860 lb of fuel amounts to 860/6.2 = 

 138.7, or say 140 gal. A further study of the prob- 

 able service required of this tender reveals that, 

 despite the increased capacity, it may be neces- 

 sary under some circumstances to return to the 

 parent ship for refuehng before the day's work 

 is done. There may be times when this will be 

 extremely inconvenient, so another 50 gal is 

 added to the fuel capacity for good measure, 

 bringing the total capacity to 190 gal, or 1,178 lb. 

 The weights reasonably well known at this 

 stage are: 



(1) Useful load, from owner's requirements, six 

 passengers at 170 lb plus 900 lb of packages, 

 1,920 lb 



(2) Weight of engines, ready to run, 5,300 lb 



(3) Fuel; although the owner requires only two- 

 thirds capacity on board for the designed speed, 

 the full capacity is hsted here, 1,178 lb 



(4) Crew, two persons at 170 lb, 340 lb. 



The known weights total 8,738 lb, leaving 

 18,000 - 8,738 = 9,262 lb for the weight groups 

 still to be considered. 



One method of making a preliminary weight 

 estimate of the remainder, with the craft not yet 

 roughed out in the form of lines or structural 

 drawings from which weights can be calculated, 

 is to use a weight-percentage diagram such as 

 that of Fig. 77.D. Here the percentages of seven 

 weight groups, making up a total of 100 per cent 

 for the complete boat, are plotted on a basis of 

 Taylor quotient T^ (and f„) for seven different 

 planing craft for which reasonably reliable data 

 are available. Only a general pattern of weight 

 distribution is apparent for any of the boat types, 

 because of great differences in the operating 

 functions of the various craft, hsted in Table 77. c. 

 For the ABC planing-form tender, with an 

 assumed WL length of 35 ft, the Taylor quotient 

 T, = 24/\/35 = 4.05. The only reference craft 

 of Fig. 77. D with a T, of about this value is the 

 24-ft personnel boat, designation D of the table 

 and the figure. Here the combined total of useful 

 load, fuel, and machinery (including electrical 

 items) is 52.5 per cent, working down from the 

 top in Fig. 77.D. For the craft being designed the 

 corresponding percentage is 8,738/18,000 = 48.5, 

 leaving 51.5 per cent for the hull, hull fittings, 

 equipment and Stores, electrical and electronics 

 gear, and margin. 



What appears at this stage to be a reasonable 

 subdivision of the 51.5 per cent is: 



(a) Hull, 28 per cent or 5,040 lb 



(b) Hull fittings, 9 per cent or 1,620 lb. This must 

 include the hoisting shngs if they are built in and 

 carried as part of the boat structure. 



