2 

 Hydrodynamic Design of an S Semisubmevged Ship 



Research and Development Center (NSRDC) (Ref 3) 



DESCRIPTION 



The typical design of an S semisubmerged ship, shown in 

 Figure 1, consists of two parallel torpedo-like hulls which support 

 an above-water platform by means of four well-spaced streamlined 

 vertical struts. Stabilizing fins attached to the aft portions of the 

 hulls provide pitch stability at higher speeds. The water plane area 

 and spacing of the struts provide static stability in both roll and 

 pitch. Small controllable fins called canards may be placed near the 

 hull noses. These canard fins can be used in conjunction with control- 

 lable stabilizing fins at the rear to provide motion control over heave, 

 pitch and roll. If rudders are placed in each of the four struts, mo- 

 tion control over yaw and sway can be obtained, especially when an 

 S travels obliquely to waves. It should be noted that an S design 

 is inherently stable at all speeds, without the use of control surfaces. 



Some of the advantages of the S hull type relative to a mono- 

 hull are : greatly improved seaworthiness, both at rest and underway; 

 reduced wave drag at higher speeds , greater deck area and internal 

 volume ; certain advantages of the unusual hull shape for placement 

 of a central well, mounting sonars, carrying small craft, placement 

 of propulsors, and potential for modular construction ; improved 

 propulsive efficiency and greater cavitation resistance ; greater top- 

 side weight capacity; and the potential for a near-level ride in high 

 sea states. 



These advantages are to be weighed against the disadvantages. 

 The primary disadvantage is the increased structural weight due to 

 its relatively dispersed design form. Other possible disadvantages 

 include the large draft, and the need for ballast control over trim. 



Many variations of the typical design shown in Figure 1 are 

 possible. The strut thickness and chord lengths can vary, the hull 

 lengths and diameters can change, the hull cross -sectional shape can 

 vary, the rudders can be located behind the propellers, the sizes and 

 positions of the stabilizing and control fins can be varied, and the 

 ship can be propelled by means other than propellers, such as pump- 

 jets. Still other S 3 variations from the typical design form are 

 presented in Figure 2; these include a two-strut and a six-strut, 

 twin-hulled design, and several types of single-hulled designs. There 

 is no "best" S hull form, since the form will vary as a function of 

 size, mission, and design constraints. 



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