Sec. 77.39 



PRELIMINARY DESIGN OF A MOTORBOAT 



863 



short of eliminating the shaft altogether or raising 

 it out of the water at speed, is to make it as small 

 as possible in diameter. 



Because the shaft struts of motorboats are 

 usually short, and because they gain in relative 

 rigidity with decrease in size, due to the structural 

 scale effect, it is frequently possible to make them 

 of the single-arm type [Phillips-Birt, D., "Motor 

 Yacht and Boat Design," 1953, p. 124; Baader, J., 

 "Cruceros y Lanehas Veloces (Cruisers and Fast 

 Launches)," Buenos Aires, 1951, Fig. 47, p. 60]. 



Design notes for motorboat rudders are dis- 

 cussed in Sec. 74.11. Additional notes are given 

 by D. Phillips-Birt on pages 67 and 68 of the 

 reference just cited. 



77.39 Third Weight Estimate. Having set- 

 tled on the general arrangement and equipment 

 and the principal details of hull, machinery, and 

 appendages of a motorboat design, it is now 

 possible to make a more reliable estimate of the 

 weights, using smaller parts and groups than 

 those listed in Sees. 77.13 and 77.29. Moreover, 

 when this stage of the preliminary hydrodynamic 

 design is reached, detailed arrangement layouts 

 and sketches of the framing and structure will 

 have been started. These will include rough 

 drawings of the revised internal arrangements, 

 drawn to a scale considerably larger than those 

 of Figs. 77.B and 77. Q, as well as framing layouts 

 showing the tentative scantlings, sizes, positions, 

 and characteristics of many of the principal parts 

 of the hull and some of the details. 



It is now time to determine, for example, 

 whether the hull proper of the round-bottom 

 ABC tender is hable to weigh more than the 34 

 per cent of the total weight allowed for it in 

 Sec. 77.29. The sUngs for hoisting the ABC 

 tenders, if carried partly rigged in the boats, 

 involve weights not normally included under hull 

 fittings. Indeed, the hulls themselves may require 

 strengthening to take the constant wear and tear 

 of hoisting them in and out of the parent ship. 



The third weight estimate involves a more-or- 

 less detailed listing of all the principal parts in 

 each group, and a calculation of the weight of each. 

 Instead of the 8 items of Sees. 77.13 and 77.29, 

 there should be more nearly 80. Even 180 items 

 are not too many at this stage. As an excellent 

 example, the weight calculations for a U. S. 

 Navy 52-ft rescue boat, with direct drive, em- 

 bodied 24 separate weight groups for the light-load 

 condition, 30 groups for the hoisting-load con- 

 ditions, and 31 groups for the full-load condition. 



Prepared by the Chris-Craft Corporation for the 

 Bureau of Ships of the U. S. Navy Department, 

 the entries for the light-load condition alone 

 occupied 56 tabulated sheets. 



Of the large weight groups, the machinery is 

 usually the heaviest. Indeed, the fuel weight 

 itself may be appreciable. Only rarely is a motor- 

 boat designed unless there is a propelling plant 

 ready to put into it. The engine and its attached 

 components usually have been built, tested, and 

 weighed, so that the designer knows just what 

 figure to set down for the units furnished by the 

 machinery manufacturer. The fuel has a weight 

 density that is known within close limits, so that 

 when the fuel capacity is fixed, the designer can 

 set down a second fixed figure for the fuel weight. 



The largest single remaining weight group is 

 that of the hull structure, including fastenings 

 but excluding hull fittings. As the largest unknown 

 item it deserves the most attention in the weight 

 estimates. Lacking reliable information concern- 

 ing previous construction or faced with a novel 

 design for which weight data probably do not 

 exist, the designer can: 



(a) Calculate the area of the hull boundaries, 

 both below and above water, then multiply this 

 area by an average weight of hull planking or 

 plating, framing, and reinforcing 



(b) Follow the same procedure for the weather 

 deck, if of appreciable area, and for internal bulk- 

 heads, flats, and platforms 



(c) Calculate the weight of the deckhouses and 

 deck erections 



(d) Estimate the weight of what might be termed 

 hull trim, such as guard rails, fenders, rubbing 

 rails, spray strips, chafing pieces, doublers, and 

 foundations for main and auxihary machinery. 



By a somewhat different procedure, the designer 

 may: 



(e) Rough out the structure by drawing the usual 

 midship section, \vith its scantlings, supplemented 

 by several other typical sections, sho-wing the 

 structure at those points in some detail 



(f) Calculate the weight of all the parts in each 

 typical section and draw a weight curve on a 

 basis of length. The area under this curve repre- 

 sents the hull weight. 



Certain features are of great importance in the 

 final weight estimate; first, that they be included 

 and second, that the estimated weights assigned 

 to them be adequate. To enumerate and explain: 



