PURPOSE: To prevent the breakdown of a crosspiece and a hull due to the overload, by providing a load detector and a load limiter for releasing the oil in a cylinder when the oil pressure exceeds over the hydraulic cylinder to the hydraulic crosspiece.

CONSTITUTION: In a crosspiece 1 arranged at the bottom of a girder 2, several number of crosspieces in the range of 5W10m from the rear end are constructed hydraulically while movably up and down comprising a cylinder 3, piston 4 and crosspieces 5 on the piston 4. Said crosspiece 1 is equipped with a load detector 6 for detecting the load on the crosspiece 5 by the oil pressure in the cylinder 3 and a load limiter for releasing the oil in the cylinder 3 to lower the crosspiece 5 on the basis of the data detected by said detector 6 when exceeding over the predetermined level, thus to prevent the breakdown of the crosspiece or the hull due to the overload caused by the inclination of the hull at the advancing.