This is part of the pile of rigging that came with my new/old Unicorn A-Class. I'm sure I've seen this style before but can't for the life of me remember how it goes together. I dropped the mfgr (Sea Sure UK) a note to ask whether someone could take a peek at an old catalog and give me a hint, but so far I haven't heard back. Anyone?
https://www.thebeachcats.com/pictures?g2_itemId=137838
The aluminum bit is about 9" long.
Edited by jonathan162 on Sep 19, 2023 - 09:40 PM.
It’s a lever vang. Mechanical advantage is gained through the aluminum arm, rather than a series of pulleys, so it’s more efficient than a traditional vang since there is no friction loss from multiple pulleys.
I’m not familiar with the setup, but I suspect that the end of each wire connects to the mast and boom. Then the small purchase attaches somewhere either lower on the mast, or aft on the boom. As you tension the line, the lever will rotate bringing the two wires closer together and tightening the vang.
sm
Steve M.
That's what I figured, but it doesn't make a lot of sense. It's a lot of hardware for a relatively short pull, and who cares about friction loss because the rope's not constantly moving through the blocks. Plus, I can't see where the stupid thing is supposed to be cleated. It'd be nice to see a picture of its intended application.
jonathan162 wrote: ….and who cares about friction loss because the rope's not constantly moving through the blocks.
The friction would be present anytime the blocks are rotating - which includes when you’re trying to tighten the system. So less friction translates to the potential for more mechanical purchase for a given load input.
That may be less relevant today with modern low-friction blocks, a cascading system might be more appropriate. But I suspect that back when your boat was built, blocks were not as efficient or refined as they are now and designers were looking for ways to get a lot of purchase as easily as possible. Something that you would need for a vang to be effective.
I guess the other question is why do you even need a vang at all? A powerful mainsheet system and a well running traveler system would achieve the same function. No modern catamarans use a vang.
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Steve M.
Yeah, I get the "pulleys supply mechanical advantage" thing (I'm an engineer as well as a longtime sailor, so that doesn't really need to be explained). My point is that considering that most of the upward/sideways load that the boom experiences is carried by the mainsheet (out on a much longer lever arm than is the vang), and that the vang is a rather small control element not carrying much of the load, I see little advantage in such a clumsy and space-consuming arrangement vs. a couple of small blocks. Even back then.
As for the "no modern cats" thing, you may have missed that this is a 50-year-old boat that I'm piecing together.
This is what I had among my files in the Moth folder.
Edited by revintage on Sep 20, 2023 - 04:31 PM.
Fantastic - thanks. Exactly the kind of example application I was after.
revintage wrote: This is what I had among my files in the Moth folder.
Lars nailed it.