We have never seen the point of anchor swivels. In our opinion all they do is add a potential point of failure to the anchoring system and provide no benefits in return.
Sure, very occasionally our SPADE anchor comes up with the flukes facing forward so it is a bit difficult to stow, perhaps one time in 20. But all we have to do to solve the problem is surge a little chain and bring it in again. And we have been anchored for as much as a month on as little as 80 feet of chain without having any twisting problems that would require a swivel.
Our concern about swivels has always centered on the possibility of one of the two screws backing out, as happened to a friend nearly costing him his boat. And the fact that the swivel shaft—made of stainless and therefore subject to work hardening each time it is torqued over the bow roller every time the anchor is hauled.
All of that was enough to make us eschew anchor swivels, but now we have seen the ultimate reason:
A visiting cruising friend showed us this massive swivel—it is just under 4″ inches (10 cm) long—that failed when his boat was subjected to some nasty katabatic gusting that caused her to surge back and forth and then come up hard on the anchor.
A close examination shows a disturbing weakness in the design that caused the threads holding the pin to strip when a shock load was applied off axis to the anchor shank.
You can see exactly how this failure happened by looking at another anchor swivel, this time in situ, in the photograph below. When the hard jerk from off to the side (as the boat came up short) was combined with the swivel’s inherent inability to align to the load and the lever-arm imposed by its own length, something had to break.
In comparison, an ordinary bow shackle would have simply aligned to the new load without problems.
If you have an anchor swivel, we strongly recommend that you remove it and replace it with the appropriately sized bow shackle.