It’s all well and good having a solid anchor, but having ground tackle that is going to keep you secure is equally important, says Vyv Cox
The equipment used to anchor our boats is constantly developing as new materials and designs appear, equipment is adapted from other technologies or existing items are improved.
Old ideas are constantly questioned by rigorous testing and field experience.
Secure anchoring isn’t just about the anchor itself, however.
The whole rode that connects the anchor to the boat, made up of a number of different parts, is arguably at least as important, if not more so, than the specifics of the anchor.
If you get your ground tackle set up correctly, with an understanding of its abilities and limitations, you can be confident that the much-maligned ‘weakest link’, is not going to let you down in challenging conditions.
First, some clarification, what is meant by a rode?
A rode (more archaically called ‘cable’) is whatever comprises the connection between the anchor shank and the fixed point at the other end on the boat.
It is customary to refer to an all-chain rode or a mixed rode, meaning chain plus rope, but realistically the term also includes any component used to join any part of it together.
SHACKLE OR SWIVEL?
Many people will argue that there is no need for a swivel at all.
In many cases where there is no problem with the chain twisting, this is true and my own adage is to fit one if you find you need it but not otherwise.
My choice is to fit one because it makes turning the anchor after recovery so much easier, when inevitably it comes up the ‘wrong’ way around, and this may even be essential for some self-launching and recovering anchor systems.
Some chains twist naturally, perhaps due to uneven wear on adjacent links and some shapes of anchor rotate quite violently as they are being recovered.
If you find that your chain is regularly twisted on recovery, or becomes twisted in the locker, it may be that a swivel will help.
There are other solutions to anchors emerging the ‘wrong’ way, to be discussed shortly.
If opting to use only a shackle it’s wise to select one a size bigger than the chain.
The pin of a 10mm shackle will fit through an 8mm chain link and most modern anchors are slotted to allow the eye of a shackle to pass through it.
The same applies to a 12/10mm combination.
Shackles come in two basic shapes,:a ‘D’ and a bow.
A ‘D’ would appear to offer better straight line strength but the bow would seem more able to cope with changes in pull direction.
The reality is that when I carried out destructive testing on both types there was no significant difference between either shape.
Chandlery-bought stainless steel shackles were generally stronger than their galvanised equivalents as shown in Table 1 below.
If we look at galvanised alloy steel shackles for the lifting and hoisting industries, however, we can see that those in the Crosby G209 A range, in Table 2 for example, are considerably stronger than any of the ‘marine’ offerings tested.
CMP, makers of the Rocna anchor, market another high strength shackle, the Titan Black Pin.
This make is marketed by some UK mail order chandleries.
Again, the strength offered by heat-treated alloy steel greatly exceeds the figures obtained from chandlery bought items, Table 3.
An answer to the problem of anchors recovered the ‘wrong’ way has been developed in Australia.
This is a simple asymmetric device called a Boomerang.
Gravity simply causes the anchor to rotate on the bow roller.
It is shackled to the anchor chain at one end and to a short length of chain between it and the anchor.
It therefore requires two shackles and an adequate distance between the bow roller and windlass.
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I tested a cranked swivel by Osculati working by the same principle but in my experience found it to inhibit setting of the anchor.
This may be due to the greater angular displacement offered by this device.
The marketplace offers a bewildering variety of swivels, ranging from roughly galvanised designs costing under £10 to beautifully engineered works of art in exotic materials costing well into three figures.
Connectors that are built down to a price will be fairly lightly built and will rely on two metal loops bolted together, as seen bottom right.
This design is widely sold in chandleries and mail order outlets but any design that relies upon bolted components to carry the load of the chain or anchor is likely to have poor loading capacity and is best avoided.
In the destructive tests that I carried out the only swivels that had higher strength than the chain to which they were intended to connect were those in which the bolt simply held two forged parts together, the Osculati and the Kong.
In these the strength is provided by the forged construction, inherently strong and tough, as in the image below.
The only potential weakness would be if the connecting bolt were to undo, so I always use some thread lock on the swivel bolt.
This has proved to be totally reliable over 10 years plus.
The type illustrated has the disadvantage that any angled loads to the end of the anchor tend to bend the parallel arms of the swivel, although the designs normally offer lateral loading capacity equivalent to the SWL of the chain.
I devised a simple way to avoid the problem in 2007, reported in YM, and now widely used in anchoring advice.
This is the addition of two or three links of chain between the swivel and anchor, allowing total articulation.
More recently several manufacturers including Mantus and Ultra have introduced compact, expensive designs that allow articulation by eliminating side arms.
The top swivel shown above is by Mantus, using an incorporated bow shackle and forged pin to carry chain loads, while below it, the Ultra flip swivel uses two forged pins and utilises a ball joint that gives better articulation than parallel side arms up to about 45o-degrees of lateral displacement. Wasi make a similar swivel.
Were the anchor to be wedged in rock and the tide direction reversed, it could be imagined that high bending loads might be imposed upon the rather narrow necks, though the manufacturers claim breaking loads above that of chain.
As a rough guide to the right size chain for your boat, in 8mm Grade 30 chain is sufficient for boats up to about 37ft, 10mm up to 45ft and 12mm above that, but the displacement of the boat is an additional factor.
There is also clearly a difference in the chain needed for weekend pottering and extended high-latitude cruising.
A good way to decide on chain size is to consult chandlery websites, where good information is available.
Chain failure is very rare, thanks to a typical safety factor of 4:1.
The length of chain needed is also very much location dependent.
Cruising the Irish Sea I carried little more than 50 metres but for more sustained cruising I have extended that and now carry 65 metres.
Some areas further afield are blessed with deeper water anchorages for which lengths of up to 100 metres may be desirable.
Chain is heavy
A yacht intending to cruise extensively might well carry 100 metres, amounting to 140kg for 8mm, 230kg for 10mm, stowed well forward where sailing performance would benefit least.
By substituting a smaller size but a higher grade some useful weight-saving is gained.
As an example, by reference to Table 4, carrying 100 metres of 8mm, Grade 70 instead of the same length in 10mm Grade 30 would save 90kg in the anchor locker and almost double the strength of the rode, from 4,800 to 8,400kg.
Marine chain in sizes up to 12mm is dominated by Chinese production, although one or two European manufacturers continue to produce.
The main UK importers of galvanised chain are probably Bainbridge and William Hackett.
The chain is nominally Grade 30 but testing has shown UTS figures to approach or even exceed the values required of Grade 40.
Many manufacturers reduce the thickness of zinc on production chain, with the result that many purchasers see rust after only two or three seasons.
AISI 316 stainless steel chain is normally to Grade 30 specification.
It is almost rust-free and its smooth finish does not heap in the locker, but it costs roughly four times that of galvanised chain.
Higher grades of stainless steel, 318L in Grades 50 and 60, are known as Cromox.
They have double the strength of 316 chain and considerably better corrosion resistance.
Chain in this material does not come cheap.
The chief advantage of a mixed rode is weight saving, desirable in smaller or lighter yachts and particularly in catamarans.
Rope for mixed rodes may be three-strand or octoplait, either of which can be spliced to the chain if it is required to pass through a windlass.
Instructions for doing this are widely available on the internet but it will be necessary to consult the windlass manual to determine the precise type of splice that will pass through the gypsy.
Nylon is probably the most widely used material for this duty but polyester is also employed, nylon having rather more elasticity, especially in three-strand form, although nylon three-strand goes quite hard and inflexible after some time, not a desirable property in an anchor rode.
Octoplait seems to retain its flexibility to a greater extent.
Elasticity is very desirable in the rode, provided by a snubber in an all-chain rode but inherent in a mixed rode.
A medium-term problem with splices is that the rope remains wet for long periods, causing premature corrosion of the chain.
Regular inspection and fresh water washing is advisable.
For boats without a windlass, or for kedge use, it may be more convenient to splice a thimble into the end of the rope for attachment to the chain with a shackle.
A typical mixed rode will have about 30 metres of chain and maybe 50 metres of rope.
For most anchoring in moderate tidal ranges only the chain will be used, avoiding the difficulties that sometimes occur feeding rope into the chain locker, or worse, down a spurling pipe.
It is sometimes necessary to join two or more lengths of chain that are required to pass through a windlass.
This might be because it has been decided to carry a longer chain due to changing cruising grounds, or just because some corroded links need to be cut out.
The only way to do this is with a C-link.
This clever little device comprises two halves of a chain link that can be riveted together to form a single link.
When made up, and in the same material as the chain, a C-link has about half the strength of the mild steel chain that it is intended to join.
For this reason a top quality C-link is made from heat-treated alloy steel that is about twice as strong as mild steel.
The result is a link that is as strong, or more usually stronger, than the chain.
It is an unfortunate truth that the vast majority of C-links sold in chandleries are made from mild steel, or possibly stainless steel.
These will have 50 to 60% of the chain strength when well made up.
Once again we turn to the lifting and hoisting industry where we find alloy steel C-links that will not compromise the strength of our chain.
Crosby also supply these, known as the G335 ‘Missing Link’, in Table 5.
Due to the fact that they are hardened and tempered, it takes some serious effort to peen the rivets.
A large hammer and drift on an anvil is the recommended technique.
ATTACHMENT TO THE BOAT
It would be remiss not to mention attachment of your anchor rode’s bitter end to the boat.
Should you pay out too much chain or should the windlass fail without doing so could easily result in the loss of your ground tackle.
Do not, however, shackle your chain to the boat.
If the anchor becomes fouled, or you need to let go the anchor in an emergency, you need to be able to let it go under load, and the only reliable way of doing this is by lashing the end of the chain to a dead-eye in the anchor locker, so that it can be cut in a hurry, or untied and attached to a large fender, should you need to let go the chain.
It’s also worth inspecting the dead-eye itself: is it big enough fitting?
Is the bulkhead it is bolted into in good condition, and does it have something to spread the load on the other side?
Modern anchors offer great improvements in holding power compared to those of yesteryear.
The rode that attaches them to the boat should also be of good quality.
Nominal Grade 30 chain is probably the most widely used and is generally totally reliable but, if the boat size is marginal for the recommended size, increasing the grade provides greater strength without the expense of a windlass gypsy change.
Grades 40 and 70 offer strength increases while Cromox gives the additional corrosion resistance.
Swivels should be types that do not rely on bolts to carry anchoring loads, whether on the anchor or the chain attachment.
Only use a swivel if you find it useful as they are not essential and can introduce weakness to the rode.
Galvanised alloy steel shackles from the lifting industry provide the greatest strength.
Wichard HR shackles (17/4 PH) were the strongest stainless steel shackles in testing.
Nylon rope has greater elasticity than polyester and three-strand construction has more elasticity than octoplait.
Nylon octoplait is a good compromise.
Alloy steel C-links from the lifting industry are as strong as Grade 30 chain but not advised for higher grades.
Mild steel and 316 stainless steel C-links have about half the strength of Grade 30 chain.
About the author
Vyv Cox is a retired metallurgist and engineer who normally spends six months of the year on board his Sadler 34 in the Mediterranean