822 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.3 



appearance. He must like to look at her as well as 

 to sail her. 



Many modern motorboats, and small sailing 

 yachts as well, are of the V-bottom, hard-chine 

 type, as contrasted with the round-bottom craft 

 discussed in earlier chapters of Part 4. The fast, 

 hard-chine, full-planing craft may have stepless 

 hulls or hulls with double or multiple steps. In 

 the former, there is a single bottom surface 

 generating dynamic lift, terminating in a single 

 transverse edge at the stern. In the latter there 

 are two or more separate lift-generating surfaces, 

 each with its own sharp downstream edge. In 

 some cases the sharp, deep edges of steps may 

 run in diagonal directions, both fore-and-aft and 

 transversely. 



77.3 Special Design Features for Small-Craft 

 Hulls. The design of small-craft hulls under 

 about 110 ft (or 33.5 m) in length requires par- 

 ticular emphasis on certain features of lesser 

 relative importance on large vessels. Among these 

 are: 



(1) The normal size of a human adult and the 

 more-or-less fixed deck heights, headroom, berth 

 and bunk sizes, messing spaces, seating room, 

 passages, access areas, and stowage facilities 

 resulting therefrom. A skiff, for example, must 

 have enough stability so that a man can stand 

 up in it without risk of capsizing. A motorboat 

 can not have bunks smaller than a given minimum 

 size. 



(2) The diminution in transverse metacentric 

 stabUity with scale as the vessel becomes smaller, 

 especially when certain factors or parameters 

 remain constant. This is the reason why, when 

 sailing in the same wind, a model yacht has to 

 have a much larger and heavier ballast keel in 

 proportion to its hull than the full-scale prototype. 



(3) The increased space required for handling 

 and stowing relatively larger items of equipment 

 on deck. A dinghy carried for safety purposes is 

 much larger in proportion than a lifeboat on a 

 larger vessel. 



(4) The proportionately larger area of huU and 

 upper works on a small craft, because the fixed 

 deck height is large with respect to the hull size 



(5) The increased inconvenience from spray with 

 everything closer to the water, with higher 

 relative speeds, and with heavier impact loads 

 from slamming at those speeds 



(6) The relatively larger T, = V/VL or F„ 

 values at which most small craft run. Anything 



less than 6 kt is likely to be unacceptable in any 

 pure power boat and more is generally needed. 

 For this reason the values of T^ for non-planing 

 craft rise from a minimum of 1.3 to a maximum 

 of 1.5 or more, F„ in excess of 0.45 [Phillips-Birt, 

 D., The Motor Boat and Yachting, Apr 1953, 

 p. 158]. 



(7) The larger change of trim for small craft at 

 their designed speed. Vision ahead, and fairly 

 close aboard, must be maintained at all trim 

 angles to be encountered in the speed range. 



(8) The increased importance of aerodynamic 

 loads with high absolute motorboat speeds, and 

 the effects of natural Avinds on high abovewater 

 structures 



(9) The types of hull construction and building 

 procedures are of considerably greater variety 

 than for large vessels. This is of importance to 

 the hydrodynamic problem because of the possi- 

 biUty that the finished weight may exceed the 

 estimated weight. If so, the calculated or pre- 

 dicted dynamic Uft is not sufficient to raise or to 

 trim the boat to the position or attitude where its 

 hydrodynamic resistance is matched by the 

 power and thrust available. 



The considerations of Sec. 77.2 combine with 

 the special features mentioned in this section to 

 render the preliminary design of a motorboat of 

 the semi-planing or full-planing type exceedingly 

 complex as compared with that of a large dis- 

 placement-type vessel. W. P. Walker describes 

 the situation admirably by saying that: 



". . . (he) considers it one of the parado.xes of his profession 

 that the smaller the ship the greater are the problems 

 associated with its design, . . ." [lESS, 1948-1949, Vol. 92, 

 p. 304]. 



The large number of successful displacement- 

 type, semi-planing, and planing boats in service 

 proves that they can be designed. However, the 

 embarrassingly frequent poor performers and 

 downright failures leave most designers with a 

 distinct sense of uncertainty in the behavior of 

 their next product if it is different from what has 

 already been built and run. 



The pubUshed design comments and notes 

 suffer to some extent from this uncertainty, as 

 well as from the lack of a straightforward pro- 

 cedure or sequence of operations by which one can 

 actually design a boat. However, they can be 

 greatly improved if experienced designers are 

 able to find time and are willing to write down, 

 systematically and in detail, the most reliable 



