Anchor connector testing


The use of the anchor and its warp ranges from a means of spending a period of time in a safe, benign anchorage to the last defence against losing the vessel when all else has failed.  In either case the yachtsman needs to know that the equipment is safe from any likely loads that are imposed upon it.  Initially the chain connecting the anchor to the boat needs to have sufficient strength for all expected conditions.  A connector of some sort is needed to join the chain to the anchor, for which a variety of shackles and proprietary fittings is available.  Many windlass manufacturers advise that a swivel be installed between the anchor and chain to prevent the chain from twisting and possibly jamming in the gypsy.  Should the chain break a link it may be necessary to join the two parts with a link that can pass through the gypsy.

An additional problem that needs to be addressed is that the swivel can jam against the anchor attachment when the direction of pull changes with the tide. The usual means of overcoming this is to insert a larger shackle between the swivel and anchor, although the correct articulation of the assembly needs to be checked before deciding on a final line-up.  Where the bow roller is narrow or partly occupied by forestay or other fittings it may be necessary to fit a shackle that has an Allen or other low-profile screw.

Test programme

Manufacture of chain in all materials and sizes is a closely controlled operation, using well-understood methods.  The product is inspected and tested before dispatch to the point of sale, resulting in a reliable product that rarely gives problems in service.  Likewise, the better modern anchors are engineered to exacting standards and tested to ensure that these are met.  But what of the shackles, links and swivels that connect the two together?  Any yachtsman browsing the pages of the forums or the magazines will find no shortage of questions and discussion about the merits of the various types available, and quite a number of horror stories detailing failure of some of them. Manufacturers have reacted to these well-documented problems by producing new designs that appear to be well engineered.  However, the only way to determine the strength of these fittings in practice is to test them to destruction.

We designed a programme to assess the strength of a wide selection of such fittings, using a standard tensile testing machine. Chain, shackles, swivelling and fixed connectors and various links were purchased in well-known chandleries in the UK. The intention was to use 8 mm as the standard size, increasing this in accordance with recommendations that one size larger may be required in some cases. A few larger sizes were tested for interest.

It is rarely possible to find the materials of construction of fittings on sale in chandleries.  Better quality fittings may have their materials stamped on them, for example 316 stainless steel.  It is usually possible to tell a galvanised fitting from a stainless steel one but otherwise assumptions must be made.  Galvanised chain is normally a hot forged low carbon steel, producing a tough material of adequate strength.  Fittings such as shackles are made by a variety of methods that are cost dependent, cast or stamped being the cheapest and drop forged the most expensive. As might be expected, the most expensive are the strongest, so in general you get what you pay for.  A good guide can be that better equipment will have a safe working load (SWL) value stamped into its surface.  Galvanised shackles intended as components of lifting and hoisting equipment tend to be stronger than the standard chandlery item and are little more expensive. The SWL figure of lifting equipment allows a factor of safety of at least six, whereas that for chain is normally four.

A 50-ton Dennison tensile testing machine was used to determine the breaking load or ultimate tensile strength (UTS) of the fittings shown in the table below.   A maximum strain rate of 1.0 millimetre per minute was used throughout the test programme.    All fittings were pulled using either 12 mm or 10 mm chain links as a mandrel. No attempt was made to measure the extension of fittings.

 What was tested


Description Diameter Source Material
1 Calibrated chain to DIN766A.  Nominal breaking load 3200 kg 8 mm Marine super store Galvanised
2 Calibrated chain to DIN766A.  Nominal breaking load 3200 kg 8mm Aladdin’s cave Galvanised
3 Calibrated chain 8mm Aladdin’s cave Stainless steel

Chain links and shackles

Description Diameter Source/maker Material
4 C-link chain joining links, assembled by riveting 8 mm Plastimo Galvanised
5 D-shackle 10 mm Marine superstore Stainless steel
6 D-Shackle 10 mm Jimmy Green Stainless steel
7 D shackle rated to 5200 kg 10 mm Wichard Stainless steel
8 Allen-screw countersunk bow shackle, rated to 4300 kg 10 mm Wichard Stainless steel
9 Bow shackle with yellow coloured pin. Rated to 0.75 tonne SWL 8 mm AP Lifting Galvanised
10 D shackle 8 Jimmy Green Galvanised
11 D shackle 10 Jimmy Green Galvanised
12 Bow shackle 10 Jimmy Green Stainless steel
13 Bow shackle 10 Marine superstore Stainless steel
14 Bow shackle 10 Jimmy Green Galvanised
15 Bow shackle 10 Marine superstore Galvanised
16 D shackle 10 Jimmy Green Galvanised
17 D shackle 10 Gael force Galvanised
19 Bow shackle 10 Gael force Galvanised
20 Bow shackle rated to 0.6 tonne SWL 12 Gael force Galvanised
21 Bow shackle rated to 0.15 tonne. Bow 8 mm, pin 10 mm 8/10 Gael force Galvanised
22 D shackle rated to 0.5 tonne. D 10 mm, pin 12 mm. 10/12 Gael force Galvanised

Fixed and swivel connectors

Description Size Source/maker Material
A Eye/eye swivel. Two eyes connected by axial nut and bolt 8mm Pro-boat Stainless steel
B Swivel connector with countersunk Allen screw attachment pivots For 8mm chain Plastimo Stainless steel
C Fixed connector with countersunk Allen screw attachment pivots For 8mm chain Osculati Stainless steel
D Swivel connector with additional articulated joint. Countersunk Allen screw attachment pivots For 8mm chain Osculati Stainless steel
E Swivel connector with long, cranked arm.. Clamped type with solid attachment pivot and eye in arm For 8mm chain Osculati Stainless steel
F Fixed connector with barrel bolt and screw attachment pivots. 850 kg SWL For 8mm chain Marine Superstore 316 s/s
G Swivel connector with barrel bolt and screw attachment pivots. 850 kg SWL For 8mm chain Marine Superstore 316 s/s
H Fixed connector, clamped type with solid attachment pivots 850 kg SWL 6-8 mm Kong Galvanised
I Swivel connector, clamped type with solid attachment pivots. 850 kg SWL 6-8 mm Kong Stainless 316
J Swivel connector, clamped type with solid attachment pivots. 2000 kg SWL 10-12 mm Kong Stainless 316
K Fixed connector with countersunk Allen screw attachment pivots 8 mm Plastimo Galvanised
23 Mooring swivel, two eyes connected by an axial thread and nut. 8 Gael force 2.3T

Connector drawings

Results and comment

Chain results

Chain fractures

G30 chain fracture
A typical ductile fracture in Grade 30 chain. Note that yielding and plastic deformation have occurred in all links. This demonstrates the gradual and progressive failure of the chain and is desirable.
Stainless chain fracture A more brittle fracture in stainless steel chain, at or near to a weld. There is noticeably less deformation of the links prior to failure. Although the load sustained was well above the nominal strength of the chain, failure in a brittle mode is less desirable as it could occur with a snatch load.
Description Load (tonnes)
1 All links elongated.  Fracture of one occurred by a ductile mechanism 4.40
2 All links elongated.  Fracture of one occurred by a ductile mechanism 4.93
3 Slight link elongation.  A link fractured at the welded joint 4.25

Both of the galvanised chains tested failed at loads considerably higher than the specification value of 3.2 tonnes.  The specified strength of the stainless steel chain was not quoted but a typical figure might be 4.6 tonnes, which it failed to reach.  Comparison of the fractures does give some cause for concern. The galvanised chain failed in a typical ductile manner, whereas the stainless steel deformed less and appears to have failed in a more brittle manner at the joining weld.  This may indicate that the higher nominal strength of the material has been gained at the expense of some loss of ductility.  It would be less able to withstand shock loading and corrosion over a period of time could further reduce its ductility.

What loading we can expect?

The American Boat and Yacht Council publish anchor rode forces for a variety of wind conditions:

Boat Dimensions                                Horizontal Load (kgf)
Length  Beam (Power) Beam (Sail ) Lunch Hook Working Anchor Storm Anchor
10’ 5’ 4’ 18 72 144
15’ 6’ 5’ 27 108 216
20’ 8’ 7’ 41 164 328
25’ 9’ 8’ 57 228 456
30′ 11′ 9′ 80 320 640
35′ 13′ 10′ 102 408 816
40′ 14′ 11′ 136 544 1088
50′ 16′ 13′ 182 727 1454
60′ 18′ 15′ 227 909 1818

Source: American Boat and Yacht Council (ABYC).

Note: 1000 kgf = 1 tonne

It can be seen that the ‘Storm Anchor’ figures presented in this table roughly equate to the chain SWL of 800 kg, assuming a factor of four on the specification UTS of 3200 kg. Bear in mind that these ABYC values do not take wave forces into account.

Alain Fraysse has drawn up a spreadsheet that can be downloaded from the Internet, ( which suggests that a 35 ft boat in a 60 knot wind with all-chain rode will suffer maximum dynamic loadings of about 2.7 tonnes. Again, these are wind forces alone, taking yawing into account.  Once we add in the effect of waves and particularly heavy snubbing forces, we are entering the unknown.  Impact forces are notoriously difficult to predict, but a rule of thumb is that the effect on chain and fittings from heavy snubbing might be up to 10 times the static loading, so maybe 8 tonnes for our 35 foot boat based on the ABYC table.

Connector results

Failure description

Load, tonnes
A Failed by stripping of the thread on the interconnecting stud-bolt 2.76
B Single screw sheared at head and thread ends. 1.87
C Single screw sheared across Allen drive base 3.27
D Single screw sheared at head and thread ends. 3.50
E Jaw bent, releasing fixed pivot end 4.39
F Jaw bent, barrel bolt sheared at base of thread for locating screw 3.82
G Jaw bent, barrel bolt sheared at base of thread for locating screw 2.72
H Body fractured, releasing fixed pivot end 4.11
I Fixed pivot bent, fracturing fixed end and releasing free end 4.03
J Fixed pivot bent, fracturing fixed end and releasing free end 6.12
K Sheared pivot at thread end 4.324
23 Not failed, larger link deformed to trap bolt head > 3.67

Every connector failed at a chain/anchor attachment point.  The type and location of the attachment was the major factor in defining the UTS. Chain attachment by a barrel bolt and screw in connectors F and G led to failure of the smaller screw at a lower load. A single, larger screw at the chain attachment gave generally higher values although some failures were initiated at the base of the Allen screw recess, where the metal is very thin.  Swivels B and C failed in this manner.  The highest strengths were given by designs in which the chain is attached by clamping together the two swivel halves with a separate screw, as in E, H, I, and J.   Connector K, a galvanised, fixed connector did particularly well in the test.

Connector types
Connector types  The design shown on the left relies on concentric screws to carry chain loads. This is the weakest link in the component: always the failure point. The design on the right uses forged lugs to carry chain load. The screw only clamps the components together. This design was found to be stronger.

Shackle results

Description Load, tonnes
4 C-link rivets sheared, C-links deformed to release chain 1.75 and 1.95
5 Used for chain testing. Deformed, not fractured More than 4.40
6 Used for chain testing. Deformed, not fractured More than 4.40
7 Pin fractured at the first thread. 6.67
8 Shackle deformed allowing pin to strip threads in bow 5.36
9 Fractured the bow.  Only example of failure at this location 6.14
10 Threads stripped from the ‘D’ 2.38
11 Unthreaded loop of the ‘D’ fractured 4.24
12 Pin fractured at the first thread 5.70
13 Deformation allowed threaded parts to strip 4.85
14 Unthreaded loop of the ‘D’ fractured 2.61
15 Both legs of the unthreaded loop fractured.  Galvanising spalled off 3.62
16 Unthreaded loop fractured 3.24
17 Thread stripped from ‘D’. Crack in unthreaded loop. 3.53
19 Thread stripped from ‘D’. Crack in unthreaded loop. 3.10
20 Heavy deformation, pin and bow threads stripped 3.67
21 Bow fractured adjacent to unthreaded loop 2.85
22 Unthreaded loop detached from ‘D’ 7.35

C-link results were particularly disappointing, although it must be emphasised that Plastimo do not recommend these fittings for anything other than temporary service. Other manufacturers state their product to be stronger than similar sized chain but these were not tested as they are not obtainable in a UK chandlery. The two tested would have been impossible to separate on board most boats as the galvanising had soldered the two parts together.  We suggest they be separated before taking them on board.

The shackles failed almost entirely according to material, with most of the galvanised ones in the bottom half of the range and all of the stainless ones in the top.  The exceptions are numbers 22 and 9, both marked up with SWL figures and intended for lifting gear applications. The 12 mm version of this type, number 20, was expected to give the highest UTS but was inexplicably average. Even so, its UTS was within specification. Number 9 was an exceptionally good performer, exceeding its specified UTS of 3 – 4.5 tonnes by a considerable margin.  Number 22 was even better, with a strength more than double its specified figure.

There appears to be no significant difference in the strengths of bow and D shackles.  There is a natural tendency for the load to be applied at either one end of the pin or the other but there is no evidence that this affects UTS significantly. Perhaps the loudest message that emerges from the test programme is that fitting a shackle one size larger than the chain is the best way to increase the strength of the complete warp. The second message has to be that a good quality stainless steel shackle will almost always be better than a galvanised one, although you might be lucky with one that was rated for lifting gear.

Shackle failure modes

shackle types
shackle types
 Shackles failed in a variety of ways, dependent upon many factors. Coloured shackles intended for hoisting applications were no stronger than other types and variable. Stainless steel shackles were consistently stronger than galvanised steel ones.


Galvanised carbon steel chain was stronger than the stainless steel chain tested.

The stainless steel fracture suggested it to be less ductile than the carbon steel and therefore more prone to fracture on impact.

C-links for joining lengths of chain were less than half as strong as the chain.

Five of the connectors tested had strength equivalent to the chain. Four of these were the clamped type in which the connecting screw was not load-bearing. These were all made in stainless steel.  The fifth was a fixed type made in carbon steel.

Galvanised steel shackles that are specified for lifting and hoisting duties had the highest strength but their results were variable.

Stainless steel shackles were consistently stronger than equivalent galvanised steel ones.


Many swivels on sale in chandleries are unsafe and should not be used. The best ones, e.g. Kong, Osculati, and others, are as strong as the chain they are used with. Others are not.

In general, swivels that rely on a bolt, or nut and bolt, are liable to fail at low load and are unreliable. Many of these are sold at chandleries but are actually designed for the fishing industry. In addition to their inherently low strength, threaded fasteners are more susceptible to fatigue and to stress corrosion cracking.

Swivel connector tests. Information provided by Grant Macduff, of Chains Ropes and Anchors, New Zealand
Pulled unknown connector  This example is of Far East manufacture, intended for 16mm chain with a listed break load of 10800kg. This is what it looked like after only 4000kg have been applied. I’m willing to bet a large number of cold beers it wouldn’t make it to 4500kg.

The real spooky bit, if the dodgy break load was not enough, was that the 2 parts of the above 16mm swivel are actually held together with a 12mm bolt.

NZ connector This one, made in New Zealand, is also intended for use with 16mm chain and has a SWL of 5000kg. This is what it looked like after having 23000kg applied during a test for a big boat classified under BV.

FYI- this is held together with a 24mm bolt.