alignment with the slot c in the aft end of the hull the drilling slot c has its open end at the periphery of the barge hull and is of such area as to substantially match the size of the derrick floor this assures that any type of usual operation may be carried out through the slot the drive motors 15 which control the movement of the cantilever structure are disposed in a position inwardly of the end of the drilling slot c whereby such motors will not interfere with drilling operations when such operations are being carried out through the drilling slot c it is also preferable to locate the hold down clamps 20 inwardly of the motors to assure that said clamps will be engaged with the inboard end of the cantilever structure when such structure is in its fully extended position as shown in fig4 thus the hold down clamp will assure that there will be no overbalancing of the cantilever structure when it is working in its extended position from the foregoing the advantages and operation of the combination drilling rig heretofore described are obvious when the barge hull is under tow the legs b are of course raised and at this time the cantilever structure d is in its retracted position fig1 if it is desired to drill through the slot which would be determined primarily by whether or not severe weather conditions may.