The single pulley block, which is situated towards the forward end of the boom, feeds the mainsheet from the boom to the ratchet in the boat. This is the boom/traveller block made by Harken with ball bearings and reduced running friction.
The mainsheet is the most frequently used control system in the ILCA, and it is crucial to be able to trim the sheet accurately for optimum speed. Having smooth running mainsheet blocks is one of the best ways to ensure that you get the most feedback from your mainsheet. Good feedback from your mainsheet gives you the hints necessary to appropriately trim your ILCA sail.