Originally Posted by knuterikt
It's an antiquity
These types of contraptions are used with a roller reefing boom - the old type where you furl the sail on the outside of the boom.
Pic of one in use
FWIW, a yoke vang/preventer was not normally lead to the base of the mast but to pad eyes on the rails when the yacht was sailing off the wind. The loading on the yokes needs be in the plane of the yoke. Diagonal loads along the line
of the boom, transverse to the plane of the yoke will tend to drag the bearing surface down the length of the boom which can easily damage the sail, if reefed, or the yoke bearing rollers, if not. In fact, yoke vangs worked pretty well and functioned a lot like many of the more current preventers one sees on the market today.