Use these blocks to create a 3:1 hoisting system to raise overboard crew out of the water and back on to the boat. Included carabiner hooks on to COBs harness or West Marine Lifesling attachment point. LIne should be of sufficient length to put the upper block 10' above the upper-most life line or hand rail. Aided by a winch, this arrangement enables a single crewmember to recover a COB. Line sold separately.