2 

 Hydrodynamic Design of an S Semi submerged Ship 



a function of F . 



Figure 3 is reproduced from Reference 1, and shows the 

 approximate hull efficiency E at maximum speed for a variety of 

 ship types as a function of displacement Froude number Fy in calm 

 water. Hull efficiency is an important parameter since the equation 

 shows that it is directly proportional to range. Note that the hull 

 efficiency of an S 3 is somewhat less than that of a monohull at low 

 Fy , but somewhat greater than that of a monohull at high Fy 

 where monohull wave drag becomes large. The reason for this re- 

 sult is that an S 3 has a greater frictional drag than a monohull due 

 to its increased wetted surface area, but has less wave drag at 

 higher speed due to its unusual hull form. A Cn of 0. 05 and an 

 7? of 0. 80 have been used for the S 3 curve in Figure 3 at Fy = 

 2. 0, with Cp. /v reducing slightly at lower Fy , and increasing 

 slightly at higher F^ to reflect reduced propulsive efficiency. The 

 propulsive efficiency v is somewhat greater for an S 3 than for 

 monohull s since the boundary layer inflow to the propulsors will be 

 more axially symmetric ; therefore, the S 3 propulsors can be more 

 completely wake adapted, as in the case of torpedoes where propul- 

 sive efficiencies of 85% to 90% are not uncommon. The line 

 shown in Figure 3 for monohulls is the locus of the highest measured 

 values of E. In rough water, the value of E for monohulls will 

 reduce considerably, as shown later, while E for an S 3 ship will 

 not change appreciably. 



The dashed lines in Figure 4 show the measured C D from 

 model tests. The model data relate to a small-craft S 3 design. The 

 solid lines are the estimated drag coefficients for several 3 000 -ton 

 ships, including an improved low -wave-drag four-strut S , and the 

 estimated Cp of an improved two-strut design taken from Ref 3. 

 Notice that the values of Cq for the 3000-ton ships are significant- 

 ly lower than those of the small models, primarily due to the 

 Reynold's number effect on frictional drag and the use of thinner 

 struts. The wave drag portion of the estimated value for the ST 3 

 ship was calculated by Dr. R. B. Chapman of NUC using linearized 

 thin ship theory in which all strut- strut, strut-hull, and hull-hull 

 interactions were included. This same theory has provided excellent 

 agreement with a large number of tests conducted on various struts, 

 strut-hull combinations, and complete S 3 models. Reference 4 by 

 Dr. Chapman contains data for estimating the spray drag of surface- 

 piercing struts at high speeds. 



Figure 5 shows the ratio of the drag in waves to the drag in 

 calm water for tests on a 5-foot model of a DE-1006 destroyer 



553 



