What are they for?
Rings replace blocks. And not just furling lead blocks, but vang cascades, too, backstaies, or jib’s sheet leads. Since their first commercialization, in 2007, under the name of Low Friction Rings, they literaly invaded the sailing market. Sailors went low tech gear, choosing simple, non-moving, unbreakable products over standard sheaves.
How much can it hold?
A lot. More than needed. Load isn't a problem for Rings, friction may rather be. Antal gives a safe working laod for Rings that corresponds to a situation where the aluminum structure holds, but undergoes a slight (slight) deformation. So they don't break. Nor do they ever loose their low friction property.
Do we have to worry about friction?
This is a good question. Let's quickly dig into this. Friction gives higher values on rings than on sheaves, but how much higher? Of course, it depends on two main points: the deflection angle (180° gives higher friction than 90°) and the control system (more rings comes with more friction). It goes without saying it also depends on the Ring quality and the line material. Focusing on the first two elements (angle and number) allows to shed some light on the complicated matter of friction. First: Rings are not suitable for all maneuvers. Second: one chooses rings in cases where a loss in friction is acceptable in favour of some other benefits (weight, cost, size, durability, resistance).
Where can Rings go?
Though it is not easy to determine how much more convenient it is to use blocks over rings, however, there are several cases sailors have thought of, where loads allow the use of rings. This also depends on the fact that some maneuvers on sailboats are trimmed less regularly than others. Or are adjusted before tension kicks in, or maybe load isn't in fact that high. Here is a list: vang cascades, gennakers' tacks, jibs' sheet leads, lazy jacks, backstays, reefing points and any small deflection angle. (Of course this depends also on the size of the boat! but 70' ocean racing boats use rings ..) Why not use rings, then?
Why use rings?
Today factory new boats for cruising are fitted with rings. Big boatbuilders see the advantage of rings in their being low cost. But not only mass production sailboats use them. Racers, too. For 3D adjustment of jibs and gennakers, on the roof and on the sails. They love their being lightweight and resistant. Also, weekend sailors trust them, for being just so strong and flexible in their use. These catchy, simple products seem to have made their way not only on small boats, but anywhere possible on bigger and faster boats. Why so? Why did they raise the interest of so many different kinds of sailors? I guess the reason lies in their multiple benefits. They are light, very light. They are inexpensive - just imagine what a block with roller bearing would cost you, instead of a simple ring. And they are easy to use. Their versatility seems to have no limits on the boat.
Types and uses
Rings are aluminum disks with a central hole and a lateral groove. But since their first appearing, they managed to evolve into different shapes suiting different uses. Antal names Soft Links the group of low friction products that includes the different families: Rings, Rings and Loops, Solid Rings, Hooks, Deck Rings, Multi Ring Organizers. Should one use Rings on one’s boat? All seems to suggest that having some middle sized rings when you set sailing is a good option. Maybe just some spare ones in your pocket, and try and be creative :)