A holding device with a chain net was developed for automation and labor reduction in lifting a heavy object. The device was a simple structure composed of a chain net which hangs from the support frame and forms four meshes. It is lightweight without a power unit and can be applied to object with various shapes. The device has been successfully applied to the removal of wave-dissipating blocks up to 25t. Four-leg blocks, six-ten leg blocks, and broken blocks were removed.
　This device can adjust to deep sea only by extending the length of the hanging wire. The device applied to recovery of a Hydrofoil sank to the seabed. The horizontal position control seemed to be difficult due to the swing of device in water. However, it became to control the device position by a simple operation method of putting a part of the net chain on the sea bottom.
　Although its operation is simple, safety checks on lifting must be rigorous, particularly when the object to be lifted is heavy. Therefore, models of the lifting situation would be highly desirable.
　The holding of a four-leg block with the device was modeled, showing that its geometrical and static constraint conditions are expressed as a set of nonlinear simultaneous equations having the same number of unknowns. By solving the simultaneous equations, the holding configuration of the block and the chain net, and the tensions acting on the chains are obtained for various ensnaring cases. This is useful for determining whether the device is capable of holding this type of block. The proposed modeling method can also be applied to a broken four-leg block missing a leg.
　The generalization of the proposed modeling method for application to general objects and the effect of friction between the chains and the object to be lifted was discussed.