Every hawser or rope by which any ocean-going vessel is made fast to any dock, dolphin, or shore must be equipped with at least one metal disc or rat guard of size or position as may be approved by the Harbor Master and in no instance may a guard be allowed with less than 18 inches of metal from the center of the guard to the outside rim. Every metal disc must, if not affixed to the hawser or rope to the satisfaction of the Harbor Master, be removed to a position on the hawser or rope as pointed out by the Harbor Master. Vessels while berthed at a dock must be fended off from the dock a sufficient distance so that rats cannot jump from the vessel to the dock provided that, when river conditions prevent the loading or discharging of any vessel when fended off the proper distance, the Harbor Master may grant permission for the removal of the fending off floats to facilitate the loading or discharging of the vessel. When the vessel is not loading or discharging cargo, the vessel must be immediately fended off the required distance if conditions so permit.