Ground tackle is a chain of components and it holds to the strength of the weakest. That is almost never the anchor or the chain — both are rated conservatively and both are large. It is the small hardware between them.
Sizing a shackle
Two requirements, and they conflict.
Strength. The shackle's working load limit should meet or exceed the chain's. Because shackle steel is shaped differently, this usually means a shackle whose nominal size is one step larger than the chain — a 5/16 in chain typically wants a 3/8 in shackle.
Fit. That larger shackle still has to pass through the hole in the anchor shank, which is sized by the anchor maker.
Where the two conflict, use a shackle designed for the job — anchor shackles and high-strength alloy shackles carry more load for the same pin diameter than a general-purpose galvanised shackle.
Never chain two shackles together to bridge a size gap. Each is a failure point and the pair is weaker than either alone.
Mouse the pin
A shackle pin backs out. Not usually, but often enough, and it does it slowly over weeks of the boat swinging.
Seizing wire through the hole in the pin, twisted off, is the standard fix. Monel or stainless wire, and check it each season — the wire fatigues where it is twisted.
Some pins take a split pin instead. Either is fine. Nothing is not.
This is the single cheapest failure to prevent in the entire ground tackle system, and losing an anchor to a backed-out pin is a well-documented way to lose one.
Swivels: useful, and commonly fitted wrong
A swivel lets the chain untwist. Chain twists as the anchor is recovered and as the boat swings, and a badly twisted chain jams in the gypsy.
The problem is where people put it: directly between anchor and chain.
There the swivel becomes a rigid connection at the anchor shank, and when the boat pulls from an angle it gets side-loaded. Swivels are designed for tension along their axis, not bending across it, and side-loaded swivels have failed.
Fit the swivel a metre or so up the chain, with a short length of chain between it and the anchor. That length lets the anchor articulate and keeps the swivel in pure tension.
Or leave the swivel out entirely. Many experienced cruisers do, on the grounds that it is one more component that can fail in a system where failure means losing the boat's position.
Replace on a shorter cycle than the chain
Shackles and swivels wear faster than chain, because all the movement concentrates at those points and the galvanising wears through first at the pin and the bearing surfaces.
Inspect each season. Look for wear at the pin, elongation of the shackle's eye, and rust where the coating has gone. Replace them well before they look alarming — they are inexpensive and they are the parts holding everything else together.
Chain sizing and grade, which set what the shackle has to match, are in anchor chain size and grade.
FAQ
What size shackle for anchor chain?
Usually one nominal size larger than the chain, so a 3/8 inch shackle for 5/16 inch chain, provided it fits the anchor shank hole.
Do I need to wire the shackle pin?
Yes. Seizing wire or a split pin through the pin. Pins back out over weeks of swinging, and losing an anchor that way is common.
Should I use an anchor swivel?
Optional. If fitted, place it about a metre up the chain rather than directly at the anchor, so it cannot be side-loaded.
Why do swivels fail?
They are designed for tension along their axis. Fitted directly at the anchor shank they get bent sideways when the boat pulls from an angle.
How often should shackles be replaced?
Inspect every season and replace well before they look worn. They wear faster than chain and cost very little.