Sec. 77.5 



PRELIMINARY DESIGN OF A MOTORBOA E 



823 



and useful information in their possession. These 

 data, when made available to the profession at 

 large, will surely lead to improvement and prog- 

 ress. 



The present effort is to be looked upon, there- 

 fore, as only a beginning in the application of 

 h3'drodynamics to the design of small craft. 



77.4 Design Notes for Displacement-Type 

 Motorboats. There has been a major change in 

 what may be termed the "normal" form of the 

 motorboat in the course of a half-century. The 

 fine, slender launches of the 1900's and the 1910's 

 have become the much fatter transom-stern 

 forms of the 1930's, 1940's, and 1950's. This 

 major change in shape has left the marine archi- 

 tect with httle save his own resources in the 

 design of a displacement-type motorboat. W. F. 

 Durand's design procedure of 1907, embodied in 

 the book referenced in the first part of Sec. 77.2, 

 differed but little from that of a Uner of about the 

 same proportions. Indeed, the analysis undertaken 

 for the preparation of Chaps. 66, 67, and 68 still 

 leaves the marine architect with unfinished spaces 

 and missing design lanes on the following: 



(a) Fig. 66.A; the design lane for fatness ratios 

 should have an upward branch, not indicated on 

 that plot, for relatively high values of ¥/{0.lQLY 

 in the range of T, above 1.0 



(b) Fig. 66.D shows the upper portion only of a 

 lower branch into which the value of the maxi- 

 mum-section coefficient Cx drops rapidly for 

 yachts, tugs, fishing craft, and other small 

 vessels. This is because of the need for large deck 

 space, for wide beam to give metacentric stabihty, 

 for a deep keel with which to resist drifting, or 

 for other reasons. 



(c) Fig. 66.E illustrates the manner in which the 

 beam Bx diminishes to a limiting or minimum 

 average value of about 2 or 3 ft as the vessel 

 length diminishes to zero 



(d) Fig. 66.1; because of the wider beam of small 

 vessels the waterUne slopes at the entrance branch 

 off into a lane of their own, for the present not 

 too well defined. 



A graph given by D. PhiUips-Birt ["The Design 

 of Small Power Craft," The Motor Boat and 

 Yachting, London, Apr 1953, pp. 158-162] for 

 the selection of prismatic coefficient Cp on a basis 

 of a given T, = V/ "s/L Ues well above the design 

 lane of Fig. 66.A for large vessels. This is un- 

 doubtedly due to the need in small craft of finding 

 space for all that must be carried, including the 



people on board. The ends have to be filled out 

 more than on a large ship, where spaces for man 

 and his requirements are more readily provided. 



Some additional design information which 

 apphes to certain kinds of motorboats and small 

 craft, together with appUcable references, is given 

 in Sees. 76.2 and 7G.3. 



77.5 Semi-Planing and Planing Small Craft. 

 It is difficult to define a semi-planing craft, or to 

 limit, by a description in words, the range of 

 variables within which a small craft of this type 

 is best located, as it were. Until something more 

 appropriate is devised, a semi-planing craft is 

 assumed to be one mtermediate between a dis- 

 placement type and a true- or full-planing type, 

 without specifying the speed range too closely. 

 Put ia another way, the craft is of the semi- 

 planing type, when: 



(1) The dynamic lift becomes appreciable with 

 respect to the weight of the boat, say more than 

 5 per cent of the latter. It may, in fact, become 

 the controlling factor in its design and perform- 

 ance. 



(2) The total resistance Rt becomes more than 

 10 or 15 per cent of the total weight W 



(3) The craft trims by the stern at its designed 

 speed and the center of gravity CG at this speed 

 is at least as high with reference to the undis- 

 turbed water surface as when the boat is at rest 



(4) Roughly, the speed range Hes between T, 

 values of 2.0 and 3.0, F„ between 0.60 and 0.89. 

 This corresponds to the upper part of the inter- 

 mediate range of Fig. 29.D. 



A somewhat more practical way of defining a 

 semi-planing craft is to say that it is the round- 

 bottom version of a high-speed planing craft, more 

 suitable than the latter for moderate weights 

 carrying purposes and for continuous operation 

 in heavy weather. 



The length used in semi-planuig craft design is 

 the waterMne length of the vessel when at rest, 

 with the loads on board specified for the designed 

 condition, the same as for a displacement-type 

 vessel. The change of level or of trim that occurs 

 when running at designed speed or below is not 

 taken into account here. 'Un 



The craft is of the fuU-planing type when: 



(a) The dynamic lift is the controlling factor in 

 its design and performance 



(b) The total resistance Rt rises above 10 to 15 

 per cent of the total weight TI^ to a range of from 

 25 to 75 per cent or more of that weight 



