From your test setting, the submerged volume of the sphere is (3.1415926/3)d^2(3*R-d), where d is the draft and R is the radius. It gives 0.041867 m^3. To be balanced in still water, the pre-tension may be 0.041867*1000-4.305=37.583kg. But your pre-tension is 12.2 kg. Please check.
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