784 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.19 



which is intended purely for racing, certain general 

 and detail requirements apply to all yachts: 



(i) Appearance is usually a major item, if not the 

 principal one. To be a true yacht, a craft must 

 look like a pleasure rather than a utihty craft. 

 Styles, tastes, and fashions vary with individuals 

 and change with time. No one but the owner can 

 say whether he wants a 3-kt houseboat to look 

 like a Roman chariot, or whether he wants a 

 40-kt speedboat to look Uke an airplane, 

 (ii) Accommodations for engaging in manifold 

 activities, even on a small craft. At first sight, 

 this and the preceding item appear far removed 

 from hydrodynamics but they involve freeboard, 

 sheer, air and wind resistance of the hull and 

 upper works, and other features having to do 

 with maneuveiing and wavegoing. 

 (iii) Speed in smooth water, coupled with endur- 

 ance and fuel capacity at some specified speed 

 (iv) Abihty to travel safely, and not too uncom- 

 fortably in waves, to ride out storms when hove 

 to, and to negotiate passages through inlets and 

 rivers with currents, tide rips, and choppy water 

 (v) Useful load-carrying capacity, generally on a 

 usable-volume rather than a weight-carrying 

 basis. This item is closely related to that of ac- 

 commodations. 



(vi) Maneuverability, like other items to follow, 

 is usually not specified by the prospective owner 

 but can never be overlooked by the naval archi- 

 tect. This includes the ability to handle the 

 craft, usually with a very large part of its volume 

 out of water, in high winds blowing from un- 

 favorable directions. 



(vii) Practically any yacht which expects to run 

 in open water should have adequate metacentric 

 height for a large range of stabihty. For the sailing 

 yacht there should be a positive righting moment 

 at 80 deg angle of heel. This means a reasonable 

 width of deck on the lee side, high watertight 

 coamings, self-baihng cockpits, and no internal 

 ballast which can shift when rolling or when 

 heeled to the extreme angle, 

 (viii) A distribution of weights as nearly amid- 

 ships as possible, to keep down the polar moment 

 of inertia of the whole craft about the pitching axis 

 (ix) For the saihng yacht, the maximum practic- 

 able performance as to speed, freedom from lee- 

 way, and steering with the craft running at an 

 angle of heel, when the underwater body is 

 definitely and drastically asymmetric about any 

 longitudinal axis 



(x) As is customary for all other sets of require- 

 ments in the book, items not related to hydro- 

 djmamics are omitted. 



The renowned yacht designer and builder 

 Nat G. Herreshoff laid great emphasis on quiet 

 smooth running in all his craft, and devoted much 

 time and attention to achieving this end. WhUe 

 most of the noise made by mechanically propelled 

 craft comes from the main and auxiliary machin- 

 ery, much of the vibration and rough running 

 may well have a hydrodynamic origin. No specific 

 design rules are given here for insuring quiet 

 smooth operation in a yacht. It is pointed out 

 only that these features are by no means negUgible 

 in a craft intended for pleasure, rest, and relaxa- 

 tion. 



A sailing yacht usually receives its greatest 

 driving force and makes its highest speed when 

 running at a large angle of heel. It is the aim of 

 the designer, and it is often possible, so to shape 

 the hull as greatly to increase its effective length 

 in the heeled position. This in turn, acts to decrease 

 the speed-length or Taylor quotient or the Froude 

 number and to make the vessel drive more easily 

 as the propelling thrust is increased. Most sailing 

 yachts, when driven hard, reach Taylor quotients 

 of 1.2 to 1.3, correspondmg to a Velox wave 

 length (from bow-wave crest to first crest follow- 

 ing) somewhat greater than the waterline length 

 at rest, mth zero heel. 



To illustrate some of these features. Fig. 76. M 

 is a body plan of a one-design saihng yacht, 

 having a waterline length of 32.0 ft, kindly fur- 

 nished by Olin J. Stephens, II, of the firm of 



Actually, when He 

 ot 30 de(), the Yocht is 

 Trovelinq So Post that 

 the Wo»e Profile Alters 

 the Surface Woterplone 

 Apprecioblij 



Inclined Woterlii 

 for Heel of 

 30 deg. 

 ot which the DisplQ 

 Volume Equals That 1 

 Upriijht Conditic 



Constructic 

 Sections 



Fig. 76.M Body Plan op Sailing Yacht with Inclined 

 Waterline 



