The choice of connector depends on a number of things. Essentially, if you don't know you need a swivel, then you probably don't, so just use a shackle.
The need for a swivel
There are several possible scenarios which could demand the use of a swivel.
- Your chain leaves the boat. This means that the orientation of the chain has a 75% chance of not being maintained, especially if you are using an auto rope/chain gypsy and do not manually 'right' the chain upon its return. This means the anchor is likely to come up sideways or upside-down, and must right itself on the roller. Lacking a swivel, the chain will then twist and discourage the anchor's righting.
- You plan on doing lots of 360s in the same direction while anchored (unlikely in most situations). Some tidal anchorages with unique conditions could present this scenario. In this case, you do not want the chain or rope twisting, as it could kink or eventually even begin to un-lay 3-strand ropes. Chain will only endure a few twists before bunching and knotting.
The Rocna should right itself without fuss on the roller, and bring itself home. We suggest in brief that you use a simple shackle at first, and introduce a swivel only if you feel it is required.
Recommended swivel types
If you do use a swivel, use one of a reputable brand. This mostly precludes generic brands and anything of questionable origin. Galvanized swivels are easily available, cheaper than stainless, but are more "agricultural". They tend to bind when new, then rust at the joint as the galvanizing wears. Stainless swivels are expensive, and you get what you pay for. The budget must be a lot higher than for shackles to obtain equivalent quality and security.
The failure mode of swivels is typically when they are subjected to lateral forces. This means it is ideal to install the swivel in such a way that lateral forces cannot be applied across the joint. One way to do this is to use a reputable inline type with a few links of chain between the anchor and the swivel. Generally, installing the swivel directly on the anchor shank is not a great idea.
Ball-and-joint types go some way toward mitigating this issue, but an articulation of only the typical 30 degrees is barely adequate. The safe-working-loads on these swivels is normally not calculated with a force applied outside of this operating range, which is unrealistic.