Vang Installation
Create an attachment point at the mast at or as close to the mast base as possible. The mast step may have built-in connection points (I don't know the specifics of the Mac 21).
Measure the distance from that connection to the underside of the boom = 'A'.
Try to find a location a minimum of 'A' to a maximum of 1.5x'A' along the boom away from the mast, but don't create any weak condition in the boom by installing a tang or bail near any other fastener points in the boom. The boom's aluminum wall thickness isn't typically that great and if something is ever going to fail, it would be where all the 'holes' weaken the boom.
Purchase a minimum 3:1 or 4:1 tackle, perhaps with the becket (adjustable cam cleat end) low at the mast connection point. You need to be able to 'snap' the line out and realease the boom vang easily under load.
Use about 5/16 to 3/8" line to be friendly to your hands.
When in doubt, look at other similar sized boats set up with a vang for racing.
