Sec. 16.19 



DESIGN OF SPECIAL-PURPOSE CRAFT 



783 



Fig. 76.L Temporary Bow Fitted to U. S. S. Selfridge 



(DD 357) 



Official U. S. Navy photograph. The waterhne slopes of 



this particular bow are very large, much larger than 



are recommended for a craft required to make a 



transocean crossing under its own power. 



large fixed weights over water, in the manner of 

 pontoons for temporary bridges. In general these 

 pontoons must carry their loads in swift streams 

 or rivers, on the surface of which wind waves of 

 considerable height and magnitude are often 

 formed. The problem here is not exactly com- 

 parable to that of a towed craft carrying the same 

 weight at the same speed, equal to the velocity 

 of the river current, because the pontoon may 

 not be entirely free to trim. Furthermore, the 

 pontoon is usually anchored by a cable or chain 

 which leads downward at a considerable angle 

 and exerts an appreciable downward component 

 of force at the bow. 



For almost any river, the current velocity at 

 the center exceeds the mean current speed, 

 because of the retarded flow in the boundary 

 layers along the sides and over the bed. In a 

 swift river the wavemaking drag may exceed the 

 friction drag. Since the downstream pull on each 

 pontoon varies as the second power at least of 

 the current velocity, and possibly as some higher 

 power, the increase of current velocity from any 

 source whatever is not to be regarded hghtly. 

 The overall drag is an important factor because 

 it affects the type, strength, and arrangement 



of the mooring gear. In the case of rivers at flood 

 stage, the pontoon may lie at an angle to the 

 current even though when moored at a normal 

 stage it may have been in line with the current. 



For floats or pontoons which are open and 

 undecked, adapted for nesting inside each other 

 in transit, adequate freeboard possesses an im- 

 portance only slightly less than that of low drag. 

 If the floats are moored in relatively shallow 

 water, the bow-wave crests are augmented. 

 Furthermore, when floats are placed close to each 

 other along the bridge, the water flows at aug- 

 mented speed between them, with a consequent 

 dropping of the water level there. Even though 

 the wave crests at bow and stern may be high, 

 each float drops bodily in the water, as does the 

 simple ship in diagram 1 of Fig. 29. B. 



76.19 Yacht-Design Requirements; Some 

 Aspects of Sailing- Yacht Design. Yachts, de- 

 fined as craft intended for pleasure, recreational, 

 competitive, or ceremonial purposes only, may 

 be classified first by size: 



(1) Large, exceeding 100 or 125 ft in overall 

 length. For hydrodynamic design purposes these 

 are essentially displacement-type ships. 



(2) Medium, over 50 ft in overall length of hull 

 proper but not exceeding 125 ft 



(3) Small, under 50 ft m overall length of huh. 



By type and method of propulsion they may 

 be classified as: 



(a) Displacement-type, mechanical propulsion 

 only, mcluding water jets. These are usually de- 

 signed by the rules applying to larger vessels. 



(b) Semi-planing type, mechanical propulsion 

 only, including airscrews and water jets 



(c) Planing type, mechanical propulsion only, 

 including airscrews, water jets, gas jets, rockets, 

 and the hke 



(d) Sailing yachts with auxiliary mechanical 

 propulsion 



(e) Sailing yachts without mechanical power. 



The design notes in Chaps. 66 through 75 of 

 this part should suffice for the hull and propdev 

 design of the large and medium yachts with 

 mechanical propulsion. The semi-planing and 

 planing craft of groups (b) and (c) preceding are 

 classed as motorboats, for which design procedures 

 and rules are set down in Chap. 77. Some features 

 of sailing-yacht design are discussed subsequently 

 in this section. 



Leaving aside for the moment the sailing yacht 



