For the two objects tested, the parameter D was obtained empirically 

 from the accelerometer data corresponding to the free-fall phase of 

 the penetration experiments. The technique used may be seen by 

 considering the complete equation of motion for an object falling 

 through water. 



(m + c) a = F 



D V 



(12) 



where 



c = added mass (included to account for F ) 



F = driving force = buoyant weight of object = \-l 



If v^ is ploted versus a for the free-fall phase, it is seen 

 that the slope of the curve will be D/(m + c) and the intercept will 

 be W|j/(m + c) . Therefore, both c and D for the free-fall phase may be 

 evaluated empirically. For the cone penetrator and the corer assemblies 

 used in these tests the values of D and c obtained are as indicated 

 in Table 9. 



Table 9 



Object 



c 



D 

 s 



cone 

 core 



1.0 

 1.0 



0.39 

 0.25 



(ft, lb, sec units) 



It was assumed that both D and c remained constant after the 

 object entered the soil. This assumption ignores several features 

 of penetration. For example, as an object penetrates the soil, less 

 of the object is in contact with water, and it would be expected that 

 the drag coefficient, D, would decrease in some complex manner. 

 Likewise, since the unit mass of soil is greater than that of water 

 and the soil flow mechanism is different from the water flow mechanism, 

 it would be expected that the added mass, c, would vary in some 

 undetermined manner as the object penetrates. For low velocity 

 penetration such as that considered in these investigations, these 

 aspects are probably relatively unimportant, and the assumption made 

 is not unrealistic. For high speed penetration such an assumption 

 would be totally unrealistic. Additional research is needed to 

 evaluate these force terms for seafloor penetration. 



13 



