boom vang for 20ft boat
While the precise answer depneds on the geometry of the boom vang, 3:1 sounds a little on the light side. It would be cheap and easy to add a wire cascade which would bring the vang up to 6:1 which should proove more than adequate. To make a wire cascade, you would buy a small bullet block made for wire and a length of 1/8" 7x19 wire with thimbles in each end. One end gets shackled to the vang bale and the other end to the upper block of your existing 3:1 vang. You are then pulling with a 3:1 mechanical advantage on a 2:1 tackle resulting in a very low friction 6:1 system.