Anchor Chain Accessories: A Complete Guide
What are anchor chain accessories? Complete guide to anchor shackles, Kenter shackles, swivel groups & more – brought to you by SMEOCEAN.
In marine engineering, we often focus on the strength and corrosion resistance of anchor chains. But few people know: how are anchor chain sections connected to each other? Why does a several-hundred-meter-long anchor chain, when deployed into deep water, not twist up like a telephone cord?
Today, let's get to know the other important members of the mooring system – anchor chain accessories.
I. Shackles: The Most Critical Connectors
Shackles are the most basic and most critical accessory in any anchor chain system. They connect the anchor to the chain, chain to chain, and chain to the vessel.
Shape: flat, like the letter D, with a straight side and compact structure. Suitable for straight-line connections where the load direction does not change- for example, fixing the end of the anchor chain to the hull's eye plate. Note that D-shackles have weak side-load capacity. If the vessel swings side to side in waves, changing the load direction, a D-shackle can easily deform.
2. End Shackle
One end connects to the anchor chain; the other end is specifically designed to connect to the anchor shank. This is the final connection point between the anchor and the anchor chain. If it fails, the anchor will be lost forever on the seabed. Therefore, the strength grade of an end shackle should typically be higher than that of the anchor chain itself, and it must be fitted with safety wire.
3. Bow Shackle
Large, rounded body, shaped like a horseshoe or the Greek letter Ω. Its curved profile adapts well to multi-directional loads. Commonly used to connect swivels, buoys, or equipment with large eyes. For connections that require flexibility and changing load angles, the bow shackle is the safer choice.
II. Swivel Groups: The "Twist Prevention Experts" in Deep Water
When an anchor chain is deployed in deep water or strong currents, water flow and vessel motion impose torque on the chain. Without a swivel, this torque accumulates, eventually causing the chain to kink, twist, or even become unusable.
1. Anchor Swivel
Installed between the anchor and the first link of chain. When the anchor flips or adjusts its angle on the seabed, this swivel releases the torque, preventing it from transmitting up the entire chain.
In large offshore engineering and deepwater mooring, there is a specialized assembly for transitioning between a swivel and the anchor chain, known as the Forerunner. It is not a single component, but an assembly consisting of three links or five links combined with the swivel body.
- Three-Link Forerunner: Relatively simple structure. Suitable for lighter-load or short-term mooring applications.
- Five-Link Forerunner: More widely used. It consists of the swivel body plus a total of five links (e.g., two above and three below, or other distributions). The five links provide smoother load transmission and better bending flexibility, reducing rigid stress concentration at the connection between the swivel and the chain. In deepwater semi-submersible platforms and FPSO mooring systems, the five-link Forerunner is almost standard.
III. Kenter Shackle: The "Hidden Hero" Connecting Chain Sections
Many buyers know that anchor chain is sold by the "shot" (section), but few understand how one shot is connected to the next.
A standard shackle has protruding pin and body - it cannot pass through the windlass chain wheel; it would jam. The Kenter shackle is a specialized connector designed specifically to solve this problem.
The Kenter shackle has a unique structure: it consists of two halves that fit together, secured by a tapered pin. Once assembled, the Kenter shackle has a smooth surface almost identical to a regular chain link, allowing it to pass through the windlass wheel without obstruction. Wherever two sections of anchor chain need to be connected, a Kenter shackle must be used. When purchasing a long chain, be sure to order enough Kenter shackles for the number of shots.
IV. Chain Stopper: The "Safety Lock" That Protects Your Windlass
The chain stopper is mounted on deck, between the windlass and the hawsepipe. Its importance is often underestimated by buyers, but it is actually critical:
- After anchoring, the chain stopper locks the anchor chain and transfers the load of the anchor and chain directly to the hull, rather than leaving the windlass brake under continuous tension.
- Prevents the chain from creeping out due to vessel motion.
Without a chain stopper, the windlass braking system will quickly fatigue and fail. When selecting a chain stopper, it must precisely match the anchor chain diameter – otherwise it either won't lock, or won't engage properly.
V. Materials, Processes & Selection Principles
1. Grade Matching Is Mandatory
The strength grades of the anchor chain and its accessories must match. This is a non-negotiable principle.
- If the anchor chain is Grade U2, then shackles, swivels, and Kenter shackles must also be Grade U2.
- If the anchor chain is Grade U3, the accessories must also be Grade U3.
- For mooring chain grades such as R3, R4, etc., the accessories must likewise be the corresponding Grade R material.
If the accessory grade is lower than the chain grade, the entire mooring system's strength is determined by the lower-grade accessory. This is the most common and most dangerous mistake in procurement.
2. Forging Processes: Die Forging vs. Open-Die Forging
All legitimate anchor chain accessories are forged, not cast. Forging is divided into two types:
- Die Forging: Heated metal is placed into a die and shaped under pressure. Die forging offers high production efficiency, good dimensional accuracy, and a smooth surface finish. Suitable for mass-produced standard shackles, Kenter shackles, etc.
- Open-Die Forging: The metal is shaped between upper and lower anvils using impact or pressure. Open-die forging is highly flexible and suitable for very large or non-standard sizes – for example, buoy shackles.
3. Surface Treatment Methods
Anchor chain accessories are long exposed to seawater or salt spray. Surface treatment directly affects service life.
- Hot-Dip Galvanized: The most common method. Accessories are immersed in molten zinc, forming a dense zinc-iron alloy layer. Hot-dip galvanizing offers strong adhesion and good corrosion resistance, suitable for the vast majority of marine environments.
- Black Tar: Only coated with black anti-rust paint. Corrosion resistance is limited. Generally only for temporary use or indoor applications -not recommended for long-term marine mooring.
- Bare/Plain Finish: No anti-corrosion treatment; the metal surface is left raw. Typically used when the customer will apply their own special coating (e.g., Teflon, epoxy), or for internal components in enclosed lubrication systems on offshore platforms.
4. Classification Society Certificates
For ocean-going vessels, offshore engineering, and mooring systems requiring survey, accessories must carry certificates from recognized classification societies.
Common certification bodies include: CCS , ABS, DNV, LR, BV, NK, and others.
VI. Frequently Asked Questions
Q1: What is the real difference between a Kenter shackle and a regular shackle?
A: A regular shackle has a protruding pin and body. When passing through a windlass chain wheel, it will jam, potentially damaging the windlass or preventing normal anchoring / unanchoring. A Kenter shackle has a smooth surface and passes through the chain wheel just like a regular chain link. It is the only shackle approved for connecting anchor chain shots.
Q2: Should I choose a three-link or five-link Forerunner?
A: The five-link Forerunner is more widely used. The five links provide better flexibility and stress distribution, especially for deepwater, heavy-load, long-term mooring. The three-link design is simpler and suitable for short-term or lighter loads. When in doubt, choose the five-link.
Q3: If I have a Grade U2 chain, is it safer to use Grade U3 shackles?
A: No. Grade U3 shackles have higher hardness but slightly lower ductility. With a Grade U2 chain, under extreme load the chain will elongate first (giving a warning), but a Grade U3 shackle may fail suddenly due to reduced ductility. Matching grades is the correct principle.
Q4: Is thicker galvanizing always better?
A: No. Excessively thick galvanizing can reduce adhesion and toughness, making it prone to flaking under impact. Reputable manufacturers follow standards such as ISO 1461 or ASTM A153 to control the coating thickness within a specified range- typically around 70-90 microns. Thicker is not necessarily better.
Q5: Why must class-approved anchor chain accessories have stamped markings?
A: Stamped markings are the key link between the physical product and its certificate. Accessories without stamped markings, or with illegible markings, cannot be traced if a quality dispute occurs or during a survey inspection. It is recommended to require suppliers to provide products with clear, legible stamping.
The anchor chain is the line, and the accessories are the connection points. The reliability of the entire mooring system depends not on the strongest link, but on the weakest accessory. We hope this illustrated guide helps you truly understand anchor chain accessories- the "small members" that quietly connect giant vessels to the open sea.
Need anchor chain accessories or not sure how to match grades and components correctly? Contact us for expert advice and a complete package tailored to your vessel.
More Links:
What are Anchor Chains Used for ?
How is the Surface of the Anchor Chain Treated?