An exit block was used in the J Class Shamrock project. You can see this at www.deltamyc.com
. This was mounted on the deck support beam to reduce friction in the sheetline.
Our class site here no longer supports the past building that used the swing arm sheetline system. However, jib trim was used in this system to a block that was mounted to the side of the centerline of the keel on the hull with the line running flat from the swing arm then to the fairlead. The reason for the off center was to prevent the jib sheetline from tangling in drifting condition and the jib listlessly swing back and forth. Sometimes the line would pool in the bilge. The block being off to the side places it out of harms way.