Sec. 77.11 



PRELIMINARY DESIGN OF A MOTORBOAT 



827 



from too much squat, increases the power un- 

 necessarily and leads to impaired maneuvering 

 and wavegoing. On the other hand, the craft 

 with only the crew on board, but with full fuel, 

 should not trim unduly by the bow. Better still, 

 it should not trim by the bow at all. 



Since the steering is to be entirely by hand, 

 the rudder(s) must trail under all running con- 

 ditions. 



Freedom from pounding and slamming in both 

 designs calls for V-sections in the lower f orebody, 

 with appreciable rise of floor. The speeds in all 

 cases approach those for planing, especially at 

 Hght loads, hence the buttock lines aft should be 

 straight or only very shghtly convex downward. 



77.10 Tentative Selection of the Type and 

 Proportions of the Hull. For the principal design 

 it appears practicable to shape one hull that will 

 run well at both 14 and 18 kt. However, the total 

 weight displacements will be different for these 

 two speeds because of the different useful loads 

 of items (8) (a) and (8)(b) of Table 77.a. The 

 underwater portions of the hull will thus be some- 

 what different in the two conditions, especially 

 if the trim changes between loadings. It is ex- 

 pected, nevertheless, that the 18-kt condition 

 will be the controUing one in the principal design. 

 The 24-kt craft, to run definitely in the planing 

 range, is destined to have an entirely different hull. 



From Sec. 77.9, the T, value for an 18-kt 

 design of 37-ft length is below 3.0. If the whole 

 available length of 40 ft overall is used, the 

 WL length will be about 39 ft and the T, value 

 2.88. A semi-planing type of hull is tentatively 

 indicated. This, combined with the important 

 requirement for excellent wavegoing performance, 

 points to a round-bottom hull as the preferred 

 shape. For the faster 24-kt boat, running at a 

 T, of the order of 4.0, well within the planing 

 range, a hard-chine, V-bottom hull is called for. 

 These will hereafter be called, for convenience, 

 the round-bottom boat and the V-bottom or 

 full-planing boat. 



On a basis of quiet-water resistance, D. De 

 Groot says that "the upper limit for the advan- 

 tageous use of the U-form (round bottom) hes . . . 

 somewhere about F/a/L = 3.25" [Int. Shipbldg. 

 Progr., 1955, Vol. 2, No. 6, p. 70]. Fig. 77.A is 

 adapted from De Groot's diagram, showing the 

 round-bottom and V-bottom regions in graphic 

 form. However, the dividing hne in his plot 

 corresponds to a T, of about 3.13; this was not 

 changed in the adaptation. The 18-kt 37-ft com- 



15 ZO 25 



Speed, kt 



Fig. 77.A Ranges op Size and Speed for 

 Round-Bottom and V-Bottom Craft 

 Adapted from the data of D. De Groot, referenced in 

 the text. 



bination for the ABC tender hes shghtly inside 

 the round-bottom region in the figure. 



Because the design of a planing craft represents 

 what might be termed the general case, embodying 

 practically all the variables to be encountered in 

 lajdng out and selecting parameters for small 

 craft, it is described first. This is not to make the 

 problem more difficult but to avoid introducing 

 entirely new features into the story at points 

 which would break up the reader's line of thought. 

 The round-bottom design procedure then becomes 

 one of simplifying that for the general case. 



77.11 First Space Layout of the 24-Knot Plan- 

 ing Hull. The first actual step in the design of 

 the ABC tender, as for any motorboat, is to 

 determine about how large the boat has to be to 

 meet the design requirements. This is done by 

 making a series of sketches to a convenient scale, 

 usually free-hand on ruled or Ught-lined coordinate 

 paper, showing proposed internal-space layouts. 

 Sec. 77.3 points out that these craft are relatively 

 small in proportion to a human and his accommo- 

 dations afloat. They must, therefore, almost 

 invariably be designed on a volume basis rather 

 than a weight-carrying basis. The size of the 

 boat is determined, not so much from those 

 dimensions which will, with a normal form, give 

 enough volume displacement to float the weight 

 to be carried as from the positions of the boat- 

 shaped boundaries which wiU enclose the various 

 spaces and units needed to meet the specification 

 requirements. Several separate space studies are 



