{"id":51663,"date":"2026-04-17T22:51:25","date_gmt":"2026-04-17T21:51:25","guid":{"rendered":"https:\/\/maritimehub.co.uk\/?p=51663"},"modified":"2026-04-17T22:51:25","modified_gmt":"2026-04-17T21:51:25","slug":"slings-strops-shackles-and-blocks","status":"publish","type":"post","link":"https:\/\/maritimehub.co.uk\/slings-strops-shackles-and-blocks\/","title":{"rendered":"Slings, Strops, Shackles and Blocks"},"content":{"rendered":"
\n

ON DECK → Seamanship Fundamentals<\/strong><\/p>\n

Operation Group:<\/strong> Seamanship \/ Lifting Operations<\/p>\n

Primary Role:<\/strong> Safe selection and rigging of lifting gear for deck and cargo operations<\/p>\n

Key Skills:<\/strong> Gear selection, load angle calculation, shackle assembly, block and tackle rigging, certificate and marking interpretation<\/p>\n

Risk Category:<\/strong> Critical<\/p>\n<\/div>\n

The load does not know what you assumed it weighed.<\/em><\/p>\n

Every working deck has a locker full of slings, a bin of shackles, and a rack of blocks. Most people can rig a lift. Fewer people rig it correctly every time, and fewer still can tell you exactly why they made each choice. That gap – between knowing what the gear is and knowing how it behaves – is where accidents live.<\/p>\n

This is not a beginner’s guide to lifting. You already know what a shackle is. What follows is the detail that matters when the load is awkward, the gear is mixed, and someone is watching you to see whether you actually know your business.<\/p>\n

Sling Types and When to Use Each<\/h2>\n

The four types you will encounter on a working vessel are flat woven webbing slings, round slings, wire rope slings, and chain slings. Each has a proper application. Using the wrong one is not a minor procedural issue – it is how you damage cargo, drop loads, or injure people.<\/p>\n

Flat woven webbing slings<\/strong> are your first choice when the load surface matters. Painted steelwork, polished equipment, finished goods – webbing protects the surface and grips well. They are light, easy to store, and easy to inspect visually. What they are not is robust against abrasion, heat, or contamination. A webbing sling with a cut, a chemical stain, or heat damage needs to come out of service immediately. Damage is not always obvious – run the full length through your hands, feel for inconsistency in the weave.<\/p>\n

Round slings<\/strong> – the continuous loop of polyester fibres inside a protective sleeve – offer very high strength-to-weight ratios and handle well around irregular shapes. The sleeve hides the load-bearing core, which is both their advantage and their critical weakness. You cannot inspect what you cannot see. If the sleeve is damaged, cut, abraded, or heat-affected, the core may be compromised and you will not know until the sling fails. Treat any sleeve damage as a reason to retire the sling, full stop.<\/p>\n

Wire rope slings<\/strong> are the workhorses of heavy, repetitive lifting. Resistant to abrasion, tolerant of heat that would destroy synthetic gear, and available in configurations – single leg, multi-leg, grommets – that suit most industrial lifts. Inspect for broken wires, particularly at the ferrule ends where fatigue concentrates. Kinking is fatal to wire slings; a kinked sling loses a substantial proportion of its rated capacity and the damage is permanent.<\/p>\n

Chain slings<\/strong> are the choice where heat, sharp edges, or sustained abrasion would destroy any other type. Foundry work, hot steel, rough castings. Chain handles abuse that would ruin wire or synthetic gear, and it is relatively easy to inspect – deformed links, stretch beyond tolerance, cracks, and corrosion are all visible. The weight is the penalty. Chain slings are heavy to handle and will damage soft or finished surfaces without padding.<\/p>\n

Sling Angle: The Calculation Everyone Skips<\/h2>\n

Ask any rigger whether sling angle affects capacity and they will say yes. Ask them what the capacity of their current rig actually is and most will go quiet.<\/p>\n

The principle is simple. In a two-leg sling configuration, as the angle between the legs increases, the tension in each leg increases beyond the proportion of the load they are sharing. At 60 degrees included angle (30 degrees from vertical), each leg carries roughly 58 percent of the load – already more than half each for a load that is theoretically shared between two. At 90 degrees included angle, each leg is carrying approximately 71 percent of the load. At 120 degrees, you are approaching 100 percent of the total load in each leg.<\/p>\n

The practical rule: keep the included angle below 90 degrees wherever possible. Below 60 degrees is better. Above 120 degrees, you are creating forces that will exceed the rated capacity of correctly sized gear, and you are putting lateral compressive force on the load itself, which may be unwelcome for some cargo.<\/p>\n

Do not eyeball it and assume it looks about right. If you cannot close the angle with a spreader bar or a longer sling, recalculate your SWL for the angle you have actually got. The reduction factors are on every rigging chart. Use them.<\/p>\n

Shackles: Dee, Bow, and Wide-Body<\/h2>\n

The shape of a shackle body is not cosmetic – it determines how and where load can be applied.<\/p>\n

Dee shackles<\/strong> (anchor shackles in some references) are designed for in-line loading. The narrow bow means the load should be applied along the axis of the pin. They are strong in that direction and significantly weaker under side-loading or when the load can rock. Use dee shackles where the pull is clean, consistent, and in-line.<\/p>\n

Bow shackles<\/strong> (omega shackles) have the wider, rounded bow that distributes load across the curve. They are designed to accept loads from multiple directions – multiple sling legs, a block, a ring. When you have more than one leg landing in the same shackle, or when the load direction may vary, the bow shackle is the correct choice. The wider body means the pin is less likely to be cross-loaded in a way that damages it.<\/p>\n

Wide-body shackles<\/strong> are a heavy-duty variant of the bow pattern, with a proportionally wider bow to accept large-diameter wire or chain without the geometry forcing the load to one side of the pin. When you are landing a thick wire strop into a shackle and the wire diameter is significant relative to the bow width, a standard bow may pinch the wire against the pin. The wide-body resolves that.<\/p>\n

Screw Pin vs Safety Pin: Which Goes Where<\/h2>\n

Screw pin shackles are quick to rig and fine for static, supervised lifts where the shackle is checked before every use. They are not appropriate where the shackle will rotate under load, where it will be in service for an extended period unattended, or where vibration may back the pin out. Rotation under load is the particular hazard – a sling leg rotating in a screw pin shackle will unscrew the pin as reliably as if you turned it by hand.<\/p>\n

Safety pin shackles – bolt-type pins secured with a nut and split pin (or cotter pin) – are the correct choice for any permanent or semi-permanent installation, any application where rotation is possible, and any lift where the consequences of an inadvertent opening are severe. They are slower to rig and de-rig, which is exactly the point.<\/p>\n

Mouse the screw pin when it matters. A single turn of seizing wire through the pin hole and around the shackle body will not prevent rotation under heavy dynamic load, but it will stop a pin that has backed off slightly from dropping out during the lift. It also tells you if the pin has moved. If the mousing is broken when you come back to the lift, the pin has moved – investigate before you proceed.<\/p>\n

Never replace a safety pin bolt with a screw pin because you cannot find the right hardware. Find the right hardware.<\/strong><\/p>\n

Pin Orientation<\/h2>\n

The pin of a shackle should be oriented so that if the pin were to come out – for whatever reason – the load would hold the shackle closed rather than force it open.<\/p>\n

In practice: when a shackle is used with a sling or strop passing through the bow, the pin should be uppermost (or on the fixed anchor side) so that the weight of the load bears on the bow. If the pin were to unscrew partially, the load keeps the bow engaged. If you invert this – pin at the bottom, load pulling upward on the bow and pushing out through the pin end – a partially loosened pin will be pushed clear by the load.<\/p>\n

It sounds like a small thing. It is not a small thing.<\/p>\n

Shackle Markings, SWL, and Certificates<\/h2>\n

Every shackle in use for lifting should be marked with its working load limit (WLL) or safe working load (SWL) and should be traceable to a test certificate. Shackles in the EU\/UK supply chain are CE or UKCA marked and come with documentation showing proof load test and minimum breaking load.<\/p>\n

Proof load is typically twice the WLL for most standard shackles. It is the load the shackle has been tested to without permanent deformation – not the load at which it fails. Working load is the load you are permitted to apply in service. These are not the same number and they are not interchangeable in conversation.<\/p>\n

Shackles with no markings, or markings that are worn to illegibility, come out of service. A shackle you cannot identify has no SWL you can stand behind.<\/p>\n

Blocks, Sheaves, and the Diameter Ratio<\/h2>\n

A block is only as good as the match between the sheave diameter and the rope it carries. The relevant figure is the D:d ratio – the diameter of the sheave (D) to the diameter of the rope (d). For most fibre and synthetic rope, a D:d ratio below 5:1 starts to impose significant bending stress on the rope fibres as they pass around the sheave. Below 3:1 and you are damaging the rope with every load cycle, regardless of whether you can see it.<\/p>\n

Wire rope is more demanding – minimum D:d ratios of 18:1 are common for running rigging wire, higher for standing applications. Running wire over an undersized sheave work-hardens the wires at the contact point, promotes fatigue cracking, and will produce broken wires far ahead of the rope’s nominal service life.<\/p>\n

An oversized rope in an undersized sheave groove will also ride up out of the groove under load – jamming, surging, and wearing unevenly. Match the rope to the block it is designed for.<\/p>\n

Snatch Blocks: Useful and Genuinely Dangerous<\/h2>\n

Snatch blocks are rigged and de-rigged without threading the line – the side plate opens, the rope drops in. That convenience is also the risk.<\/p>\n

The opening side plate must be fully and correctly closed and secured before any load is applied. This sounds obvious. Snatch blocks are often rigged quickly, under time pressure, in confined spaces, by people who have done it a hundred times. The latch gets a cursory check and the job moves on. When a snatch block opens under load, it does not fail gradually.<\/p>\n

Additionally, a snatch block redirecting a line changes the direction of the resultant force on the attachment point. If a line makes a 90-degree turn over a snatch block, the load on the block’s attachment point is not equal to the line tension – it is up to 1.4 times the line tension, depending on the angle. The attachment point – pad eye, deck fitting, post – must be rated for that resultant load, not just the line load.<\/p>\n

In Practice<\/h2>\n
    \n
  • Read the sling angle – do not assume it is acceptable because the legs look roughly even.<\/li>\n
  • Check slings over their full length before every lift. Webbing end to end, wire especially at the ferrules, chain link by link for any deformation.<\/li>\n
  • Match shackle type to the load direction. Dee shackles for in-line. Bow shackles for multi-directional or multi-leg.<\/li>\n
  • Safety pin shackles on any lift where you cannot watch the gear continuously, where rotation is possible, or where the consequences of opening are high.<\/li>\n
  • Orient every pin so that load holds the shackle closed, not open.<\/li>\n
  • Mouse screw pins on any lift that warrants it – which is most of them.<\/li>\n
  • Check block sheave diameter against the rope you are running. If it is wrong, fix it before the lift, not after the rope is damaged.<\/li>\n
  • Inspect snatch block latch closure before every use. Check the attachment point can take the resultant load at your rigging angle.<\/li>\n
  • Certificates and markings are not paperwork formalities – they are your evidence that the gear was tested to what it claims. No marking, no certificate, no lift.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"

    From sling angle calculations to shackle pin orientation – the rigging fundamentals that separate a competent hand from an experienced rigger.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"fifu_image_url":"","fifu_image_alt":"","c2c-post-author-ip":"2a02:c7c:2ef8:2400:931:afb1:9971:4a62","footnotes":""},"categories":[1,14],"tags":[7315,9256,9257,9252,4086,2943,9254,9255],"class_list":["post-51663","post","type-post","status-publish","format-standard","hentry","category-latest","category-on-deck","tag-blocks","tag-deck-seamanship","tag-lifting-gear","tag-lifting-operations","tag-rigging","tag-seamanship","tag-shackles","tag-slings"],"acf":[],"_links":{"self":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51663","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fcomments&post=51663"}],"version-history":[{"count":1,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51663\/revisions"}],"predecessor-version":[{"id":51674,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51663\/revisions\/51674"}],"wp:attachment":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fmedia&parent=51663"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fcategories&post=51663"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Ftags&post=51663"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}