3 



Hudvodynamic Design of an S Semisubmerged Ship 



The metacentric height in pitch is calculated from the same 

 equation as for roll, except I now refers to the longitudinal area 

 moment of inertia. Tests to date on S 3 models have shown that 

 motion in waves reduces as the pitch GM increases. In other words, 

 the struts should be well- spaced in the longitudinal direction. This 

 is one of several reasons why the four-strut configuration was 

 selected as a typical (but not the only) design form for an S 3 . 



Figure 8 shows typical waterplane areas for a monohull, a 

 catamaran ship, a two- strut low waterplane ship, and a four- strut 

 S 3 . Note that the S 3 has the greatest static stability in both roll 

 and pitch per unit waterplane area because the waterplane area is 

 concentrated in the four corners of the ship where it is most effec- 

 tive. Another advantage of the four -strut configuration is that it has 

 less virtual mass in the transverse direction than the two strut 

 design, and therefore will have less motion and hydrodynamic load- 

 ing in beam seas. 



One of the first questions explored in a series of S 3 model 

 tests conducted in 1969 concerned the dynamic stability of an S 3 . 

 Figure 9 shows pitch data obtained on several 5 -foot model confi- 

 gurations tested in calm water in the General Dynamics Aeromarine 

 Test Facility model towing basin in San Diego, California. The hull 

 diameters were 4 inches. Figure 9 shows that all models were stable 

 at all test speeds except the non-S 3 model designated C + N which 

 had no stabilizing fins. Thus, these tests showed that the S sta- 

 bilizing fins were necessary for dynamic stability at Fy greater than 

 about 0. 9. This result was in good agreement with S 3 design theory 

 which shows that the dynamic instability of bare hulls will overcome 

 the static stability provided by the struts above some critical speed 

 unless stabilizing fins are incorporated. 



A very useful device to further investigate the dynamic 

 stability of an S is the 5-foot radio-controlled model shown in 

 Figure 10, which was tested in 1970. This model was stable under 

 all test conditions and controlled well. All motions were well damped 

 at rest and highly damped when underway. It operated well in waves 

 and wind at all angles, although the greatest motion occurred in large 

 following waves. Figure 11 shows an 1 1 -f oot model built and tested 

 at the Naval Ship Research and Development Center in 1971. This 

 model performed similar to the 5 -foot model suggesting that model 

 tests and the known scaling relationships are valid. 



555 



