We are not fans of anchor swivels, having seen several disasters due to their failure. This photo clearly shows the problem.
Your anchoring system needs to be consistently engineered, from the bow roller, to the anchor chain connection, the chain itself, chain stopper, and windlass.
Look at the chain in this photo, and compare it to the swivel clevis pins. The steel in the chain is probably a third (at least) stronger than the stainless in the swivel (depending on chain alloy). The links have two legs, equal in diameter to the swivel clevis pins. So the stronger chain alloy has twice the material to carry the load. If you had to pick a weak spot in this system it would be the swivel, which might carry half the load of the chain before failing.
Do you want to bet your boat, and maybe your body on this set up?
Posted by Steve Dashew (October 26, 2009)
October 27th, 2009 at 11:10 am
Steve
Are you totally againest swivels or just undersized ones. What do you use on the FPB series?
Bo
October 27th, 2009 at 12:10 pm
Bo:
To be safe the a swivel would need to equal the breaking strength (not working strength) of the chain and be proof tested.
The 3/8″ schedule seven chain on the FPB 64 has a break strength of 26,000+ pounds and we do not use swivels.
We have always avoided swivels and have not had problems with chain twist.
October 28th, 2009 at 8:29 am
I believe this example would be best styled as “why this swivel is a bad idea”, as all the issues [correctly] highlighted may be dealt with by way of an appropriate design and material choice. That said, it is unfortunate that many (most?) swivels on the market quote SWL figures which work out to be a wildly different ratio to break as the ‘standard’ 4:1 of chain. Moreover, many are designed to be attached directly to the anchor shank (as shown in the picture), where their failure mode is typically not a straight line pull but rather a lateral load on a fouled anchor under which circumstances their effective strength is greatly reduced further still.
http://www.rocna.com/kb/Swivels
November 11th, 2009 at 7:59 pm
Even though your conclusion is good, your proof is not totally accurate. The pin is working in DOUBLE shearing mode, that is there are 2 surfaces supporting the tension (the cross section on each side), so it matches the two legs of the link. But then, shearing strength is about 80% of tensile strength in steel, so for a same material, same diameter, we are 20% weaker. Then stainless steel being weaker too, we are looking at another lost of strength of about 30%, as you mention.
Great Blog! I truly enjoy reading you!
December 16th, 2009 at 2:17 pm
The other (and in my mind, the major) shortcoming with this arrangement is that the swivel is attached directly to the anchor. There have been some well-documented swivel failures in the last couple of years, leading to some testing by West Marine, as recounted in Latitude 38 earlier in 2009. The testing revealed particular problems with side/lateral loads on the swivel. When attached directly to the anchor, a load on the chain that is not in alignment with the anchor shank is going to put an enormous load on the swivel, potentially deforming it. Interetingly, West Marine suggested that for certain kinds of anchors (like a Delta or Bruce) with rigid one-piece contruction, the swivel should be separated from the anchor by a shackle. I don’t think that information is well known.
–Nelson Lee
April 21st, 2010 at 1:36 pm
EC Directive 2007/87/EC (Technical Requirements for Inland Waterways Vessels (TRIWV)) requires swivels to be 20% stronger than the chain.