Then they had to start lightening the ship, presumably because it was taking on water faster than they could bail it out. Under the strain of going over the huge waves, the hull of a ship is stressed, and the caulking works loose between the boards, so the ship begins to leak. This is in addition to the water washing over the decks above from the huge waves. This water pours in thru every possible opening. The only solution, and a temporary one at that, was to jettison the cargo. Everything of importance -- the very reason the ship had set sail in the first place, all the cash paying cargo -- was cast overboard and into the sea, v 18. The captain of the ship was now bankrupt. The sailors would make no money on this voyage.
But that was not enough! They had to lighten the ship even more. They had to throw overboard all the tackle and rigging of the ship, v 19. All the pulleys and lines and sails and masts, the anchor and its heavy chain, everything must go if they were to save themselves.