Sec. 77.12 



PRELIMINARY DESIGN OF A MOTORBOAT 



829 



This may be handled in several ways. All of them, 

 like that used for the ABC ship in Sec. 66.4, 

 involve weight estimates for a craft which is still 

 in a distinctly nebulous state. To make this 

 situation more difficult, reliable pubhshed informa- 

 tion as to the weight percentages of the various 

 groups and detailed information as to the actual 

 scale weights for motorboats are much scarcer 

 than corresponding data for large vessels. A 

 start in this direction has recently been made by 

 E. Monk ["Weight and the Motor Boat," Yacht- 

 ing, Jan 1955, pp. 118-120]. The graphic data 

 presented by Monk, comprising a relationship 

 between waterline length at rest and total weight 

 for wooden pleasure boats, are embodied in the 

 lower broken-line curve of Fig. 77. C. The full-fine 



50 



80 SO 100 



"0 10 20 ?£i 40 



Fig. 77.C Relation Between Weight and 

 Waterline Length of Two Types of Small Craft 



portion of the middle graph, with a tentative 

 extension in long dashes, is based upon data from 

 about a dozen motorboats of the moderately 

 loaded type for which the total weights are 

 known. Because of the spread in some of these 

 data, indicated by the small circles in the figure, 

 a third tentative meanline for boats of the 

 heavily loaded type is shown by short dashes. It 

 is almost certain that any graph of this type will 

 consist of a family of curves, representing boats 

 designed for widely different missions. 



When total weights of motorboats are given in 

 the technical literature, they are rarely accom- 



panied l)y a statement of the corresponding 

 loading condition (s). Lacking accurate and ade- 

 quate sources, the designer is usually forced to 

 create his own or to obtain access, by one means 

 or another, to data that remain unpublished. 



For the ABC tender, assuming that it is to be 

 built of wood (or of a material having an equiv- 

 alent weight), and that it has a waterline length 

 as short as 35 ft. Monk's broken-line curve of 

 Fig. 77. C indicates that it will weigh at least 

 17,000 lb. It win certainly be heavier than a pure 

 pleasure craft, and probably not much lighter than 

 a moderately loaded utility boat. To be on the 

 safe side, the designer should assume a weight 

 not less than 18,000 lb. Incidentally, the two lower 

 graphs of this figure reveal that, for the permis- 

 sible maximum waterline length of about 39 ft, 

 the total weight should be less than the 25,000-lb 

 limit of Table 77. a. 



It is emphasized here, and this emphasis will 

 be repeated several times in the sections to follow, 

 that the preliminary weight estimate, and the 

 revisions to it, can make or break a small-craft 

 design. If the total weight is estimated correctly, 

 the boat should perform as predicted. If the weight 

 is underestimated, the boat almost certainly will 

 fail to meet expectations. 



A rule-of-thumb check on the weight estimate 

 serves as a sort of maximum limit for a boat that 

 is expected to give good planing performance. 

 This says that the displacement-length quotient 

 A/(0.010L)^ should not exceed 200, and that it 

 should be lower if possible. For a total weight of 

 18,000 lb, or 8.04 t, and a waterline length as 

 short as 35 ft, the displacement-length quotient 

 is 8.04/(0.35)' = 187.5. For the moment this 

 requirement is therefore met. 



With what appears to be a reasonable total 

 weight the next step is to make a first approxi- 

 mation of the power i-equired to drive the ABC 

 planing-type tender at 24 kt, so that the machinery 

 and fuel weights can be predicted. From the 

 calculations described subsequently in Sec. 77.14 

 the brake power is of the order of 430 horses. On 

 the basis of using two diesel engines, giving a 

 better arrangement of machinery space and 

 facifitating engine removal and overhaul, the 

 total estimated machinery weight, including all 

 fiquids in the systems, is 5,300 lb. The details of 

 this estimate are taken from returned weights of 

 actual installations, as furnished by the engine 

 manufacturers. 



Considering the fuel weight, the owner's re- 



