862 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.37 



0.2 ft. Here the .r-distance involved is slightly 

 less than the mean wetted length, because the 

 propeller is to be forward of the transom. 



Without any more than instinctive or intuitive 

 knowledge of viscous flow, boundary layers, or 

 wake velocities, N. G. Herreshoff buUt many 

 successful high-speed launches, yachts, and tor- 

 pedoboats in the half-century between 1870 and 

 1920 Mith extremely small tip clearances [Herre- 

 shoff, L. Francis, "N. G. Herreshoff and Some of 

 the Yachts He Designed," The Rudder, Mar 

 1950, pp. 33-35; Sep 1950, pp. 26-27, 56-58]. 

 He was fully as conscious as are modern marine 

 architects of the advantages of smooth running 

 and the need for holding vibration to a minimum. 

 There is some question, therefore, as to whether 

 radial or hull tip clearance is an important feature 

 in a small craft [Tomalin, P. G., SNAME, 1953, 

 p. 611]. 



The structural scale effect, which causes a small 

 structure of a given material to be more rigid 

 than a geometrically similar large one oj the same 

 material, may render the hull less susceptible to 

 vibration than on a large vessel. Nevertheless, it 

 seems wise on any small craft, regardless of size, 

 not to reduce the tip clearance below 0.083 ft 

 (1 mch) or, at the most, 0.0625 ft (f inch) [Lake- 

 land Yachting, May 1952, p. 20]. 



On all small boats buUt for pleasure purposes, 

 and on most utihty craft as well, comfortable 

 riding and freedom from vibration acquire an 

 importance comparable to that on large vessels. 

 P. G. Tomalin has discussed this matter at some 

 length [SNAME, 1953, pp. 610-613] so that it is 

 considered necessary only to reference it here. 



77.37 Still-Air Drag and Wind Resistance. 

 It is mentioned in item (6) of Sec. 77.2 that in 

 an ultra-high-speed motorboat the still-air resist- 

 ance may approach the hydrodynamic resistance 

 in magnitude. In any type of motorboat, with 

 by far the greater part of its total volume above 

 the water surface, neither the still-air drag Dsa 

 nor the wind resistance i?wind is ever a negligible 

 or an inconsiderable part of the total resistance. 

 If it is not a major factor in resistance and power- 

 ing at cruising speeds it can still become important 

 for maneuvering. When the drag in a beam wind 

 creates a swinging moment that, for example, 

 always causes the craft to fall off and swing 

 downmnd, the crew may have difficulty holding 

 it in a hove-to position, head to both wind and 

 sea. 



Proper design of the upper works on a motor- 



boat, with the center of its "sail" area in the 

 proper fore-and-aft position, is usually more 

 important than on a large vessel. 



Chap. 54 contains sufficient information for an 

 estimate of the still-air drag of the hull and deck 

 erections of a small craft. Sec. 77.26 contains a 

 computation of this drag for the planing-type of 

 ABC tender. 



77.38 Design of Control Surfaces and Appen- 

 dages. The principal control surfaces and ap- 

 pendages on a motorboat or other self-propelled 

 small craft are: 



(1) Deep keel or skeg, or a combination of the two 



(2) Vertical stabihzing fin on an ultra-high-speed 

 craft, to provide a sort of fulcrum about which the 

 rudder moment can act. Many types of these are 

 shown by J. Baader on pages 115, 119, 322-325, 

 and 341 of the reference listed under (4). 



(3) Propeller shaft(s), usually exposed 



(4) Shaft struts for supporting propeller bearings. 

 These may be either forward of or abaft the pro- 

 peller. On ultra-high-speed craft the propeller 

 bearing is sometimes carried by a swivel fitting 

 on the rudder [Baader, J., "Cruceros y Lanchas 

 Veloces (Cruisers and Fast Launches)," Buenos 

 Aires, 1951, Fig. 104, p. 130]. 



(5) Steering rudder(s) 



(6) Inlet scoops for cooling water to the propelhng 

 machinery; see Fig. 34 on page 42 of the Baader 

 reference just cited. 



The deep keels or skegs act partly as stabilizing 

 fins to give the craft stability of route, they 

 prevent skidding and sideslipping, and they 

 probably add some roll-quenching effect [Fig. 265 

 on p. 328 of the Baader reference listed in (4) 

 preceding]. Practically, they serve as partial 

 housings for centerline propeller shafts and as a 

 protection for the hull when grounding. There 

 appear to be few design notes or rules in the 

 literature other than those given by D. Phillips- 

 Birt ["Motor Yacht and Boat Design," 1953, 

 pp. 65-66]. 



Exposed propeller shafts in a high-speed craft 

 of shallow draft are a constant source of hydro- 

 dynamic difficulty. The drag normally encoun- 

 tered on such an appendage is aggravated by the 

 ditch or hole in the water made by it, where the 

 ambient pressure is too small to develop a pressure 

 gradient which will cause the water to close in 

 abaft the shaft. Further, the ditch, hole, or 

 separation zone extends aft into and through the 

 propeller disc. The only design solution here, 



