Vang system for 'roller-boom' reefing .... from the '60s
Its a prehistorical and very obsolete vang system used on roller furling booms that quickly went out of 'vogue' in the 1960/70s ..... as it didnt work very well at all. "Slab reefing" or "jiffy reefing" techniques replaced this 'abomination'.
There is probably a 'gear' (or remnants of same) on the boom near the gooseneck onto which one puts a handcrank to 'supposedly' roll up the mainsail onto the boom as it rotates when 'cranked'. The 4 rollers on that 'torque collar' rolled over the sail as the boom rotated. Problem was that most mainsails at that time were 'shelf footed' (extra cloth panel installed along the foot/bottom of the sail) so that the foot of the sail would take a more curved shape on the boom. The shelf foot didnt roll up very well and caused a lot of 'bunches' on the boom when the sail was 'rolled' leaving the unrolled portions of the mainsail to become a 'god-awful' shape. You probably also have the mainsheet system attached to the very aft end of the boom and is able to allow the boom to rotate on this connection of the mainsheet system.
This equipment was from a long-bygone day when sailors actually 'shaped' their sails to match the wind/sea conditions for 'best performance' and safety.
With the current 'vogue' of loose-footed mainsails (foot rope doesnt fit into a groove on the boom), there is a 'resurgence' of roller boom reefing ... and its still as 'abominable' (just a teeny bit less than) as the original 'roller boom reefing' that was totally abandoned 30-40 years ago. Those who experienced and who 'remember' such god-awful reefing systems will not have the modern so-called in-mast or roller-boom reefing ... as we already lived-through it and supposedly 'know better'.
