Lewmar Synchro single block. Synchro Blocks are engineered for speed, efficiency and superior handling at a more affordable price. Lewmar uses a combination of scientifically optimized block geometry, a Free-Spin bearing, and a self-aligning head to reduce friction and increase efficiency up to 40% over conventional mainsheet systems. Each component is perfectly synchronized with the movement of the rope, providing an easier, smoother transfer of power from deck to sail and less wear on your rope.