Chains and fittings are manufactured in different strengths for specific applications, and the strongest component is not necessarily the best for the job in hand. It could be unsuitable for galvanizing, or made from steel which corrodes more rapidly, and it may be disproportionately expensive. As will be seen later, weight is as important as strength in moorings and the lowest and cheapest strength grade usually offers the best combination of properties. Internationally, there are standards covering chains and most fittings and these recognize the following grades:
Grade 30 (3 or L)
This is the lowest grade with components made from mild steel without heat treatment and strength approximating to that of the now virtually unobtainable wrought iron.
Grade 40 (4 or M)
Normally manufactured from plain carbon steels and heat treated, this is the lowest grade recommended for lifting applications.
Grade 60 (6 or S)
This grade has been largely superseded by grade 80 and is likely to be withdrawn from the International Standards series.
Grade 80 (8 or T)
The highest strength in normal use; components are usually made from alloy steels and hardened and tempered.
Chapter 8 provides more detailed information on these various grades and their suitability for marine applications.
With the exception of stud link chain cable, chains are generally supplied in the grades detailed above. Stud link chain is normally supplied in grades mild steel U1, special quality U2 and extra special quality U3 as specified by Lloyd’s Register of Shipping.
Stud Link Anchor Chain
Originally Studs were added to chain to stiffen the links, preventing them from severely deforming when overloaded. They have an added advantage in that they prevent chain from knotting up when twisted, thus avoiding problems when the chain is retrieved with a windlass.
Most chain of this type is used on larger vessels and is rarely available in the smaller sizes.
Occasionally, good second-hand chain can be found and this makes an excellent ground chain in mooring where the studs give extra weight. As all grades are designed specifically for marine use they are perfectly suitable for mooring applications.
Short Link Chain
Internationally this is accepted as chain with a link of outside dimensions not exceeding 5 times the material diameter in length and 3.5 times in width – e.g. 10mm chain cannot have links larger than 50mm x 35mm outside dimensions. It should be noted that these are maximum dimensions only and if chain is needed to fit a windlass gipsy wheel it is unlikely that short link will be suitable. Excluding studlink, short link is the strongest, heaviest and most flexible of chains, and as such is the best choice for mooring and anchoring. Unfortunately the links are too short to accept shackles of reasonable size, necessitating the fitting of large end links. In effect short link chain needs to be assembled by the manufacturer into a ‘bespoke’ mooring which, although ideal, may not always be practicable.
There is a wide range of other components which could be used in a mooring system and it is important to ensure that those used are of equivalent strength to the chain. Using components which will fit a chain, with no regard for relative strengths, is all too common. As an example, given similar materials, a conventional shackle which will fit directly into short link chain can at best be only about half the strength of the chain. Compatibility of materials is vital, too – the problems of dissimilar materials and electrolysis are only too well known.
There are three main types of shackle in common use and a wide range of pin designs. The Kenter shackle is really a mechanical connecting link, and is designed for use with stud link chain. By far the most popular shackle types are the dees and bows, sometimes known as harps. Dee shackles are usual where two components are to be connected together, whereas bow shackles are more suitable for use as three-way connectors. For moorings the screwed collared pin is the most common, although forelock pins are sometimes preferred.
There are various designs of swivel available and the user should ensure that the one chosen will accept the correct size shackle or shackle pin. A chain swivel for example, is too small to be effective unless fitted with a large link at each end – or, of course, unless incorporated into an all-welded assembly by a chain manufacturer. Mooring swivels will take a shackle eye at one end and a shackle pin at the other. Oval eye and long bow swivels will accept shackle eyes at both ends.
Rings, links and other fittings
The range and variety of fittings which may be used in a mooring and anchoring system is far too wide to cover comprehensively in this booklet. Rings, egg links, bridle plates, slip hooks, stoppers, eyebolts, ringbolts – all have their places. Suffice to say that compatibility in materials, strengths and dimensions is essential. Somewhat more must be said of mechanical connecting links, which appear at first sight to offer an attractive solution to the problems posed by shackles and short link chain. In practice, however, the majority of designs either have short life in salt water or need to be welded after assembly to ensure reasonable strength. They are invaluable for emergency repairs or to use as a stop-gap measure, but are in the main unsuitable for permanent installations.