For the Marstrom type tramp sheeting, the "blocks" are enclosed in a frame that is sewn to the tramp. This is a special unit. There is a picture of the A-Class version attached. Check the Marstrom Website for prices(http://www.marstrom.com/) From the tramp the mainsheet goes through turning blocks on the front cross beam and the boom to a 2:1. The 2:1 pulls a 4 or 5:1 bewteen the boom and the traveler car. This system requires a very stiff boom (about twice as heavy as a standard F18), it's complicated, it's heavy, and it works real well.

A a boom mounted system that has been used by some A-Class sailors is shown: (http://www.layline.com/llf/Boats/505/505_mainsheet.htm)

This mainsheet runs through an eye to a 8 or 10:1 set of blocks between boom and the traveler car. It's cheaper lighter and it's easy for the skipper and the crew to use it.

Be sure you like a mid mainsheet system before you add one. they make me slower.