Why the Kenter Shackle Replaced the D-Type Joining Shackle

If you work with anchor chains, you‘ve probably noticed that Kenter shackles are everywhere, while D-type joining shackles are increasingly rare. Modern vessels overwhelmingly use Kenter shackles as the standard for connecting chain sections. But why did this shift happen? 

The answer comes down to one fundamental issue: the D-type joining shackle cannot pass through the windlass gypsy wheel. 

The Physical Problem: It Doesn't Fit the Gypsy 

According to multiple industry suppliers, the D-type joining shackle is designed to connect two lengths of chain, but it requires studless end links on both sides to function. More critically, it “does not fit the gypsy”. 

Franklin Offshore Australia explains it clearly: “'D' Type Joining Shackles are designed to allow connection between two studless end links of anchor chain. Unlike the Kenter Link, 'D' Type Joining Shackles do not fit the gypsy.” 

This is not a minor inconvenience. The gypsy wheel is the heart of the windlass system – it engages the chain links to hoist and lower the anchor. Any component that cannot pass through it creates a serious operational problem. 

In the words of one industry source, the D-type joining shackle "was therefore made obsolete when the Kenter shackle was introduced".  

The Engineering Solution: Kenter Looks Like a Chain Link 

The Kenter shackle (also known as a lugless joining shackle, detachable link, or connecting link) solved the problem by matching the outer dimensions of the chain it connects. 

The Kenter design consists of two interlocking halves secured by a central taper pin. This design was developed over 100 years ago and has been the standard ever since. 

Because the Kenter shackle‘s outer dimensions match the chain, it “can run through the vessel’s gypsy wheel with no issue”. This is the defining advantage that made D-type joining shackles obsolete for connecting chain sections. 

The Kenter shackle can connect directly to common links without requiring end links or enlarged links – it is a direct, simple connection. The D-type, by contrast, required a stepped transition: joining shackle → end link (E) → enlarged link (EL) → common link (C).  

The Technical Trade-Off: Kenter's Fatigue Life Is Shorter 

The Kenter shackle is not without its own limitation. According to U.S. patent history, Kenter shackles have high stress concentrations at internal section discontinuities, reducing their fatigue life to one-third that of corresponding common links. 

This is why offshore drilling vessels historically used continuous chain lengths of up to 5,000 feet to avoid the known high incidence of fatigue failures of joining shackles in service. 

In 2021, The Crosby Group introduced the HFL Kenter (High Fatigue Life Kenter), made from Grade 6 steel, offering superior fatigue life compared to Grade 4 designs from the 1980s. This new design features DNV-GL type approval and the “Fastlock” system to reduce assembly time and risk. 

Inspection and Replacement: Critical for Safety 

Because Kenter shackles have a fatigue life of only one-third that of common chain links, regular inspection is essential. 

According to industry safety reports, a vessel once lost an anchor and four sections of chain during anchoring operations. The investigation concluded that an un-replaced Kenter shackle – which had been left in place during a previous dry dock – had fractured. 

What to inspect:

- Lead seal condition – ensure the taper pin seal is intact

- Cracks, deformation, or excessive wear

- Rust and corrosion – old Kenter shackles often seize

- Cross pin security 

When to replace:

- When chain is renewed – Kenter shackles are typically worn as well and should be replaced together

- When wear exceeds 10-12% of original chain diameter

- After forced disassembly – if rusted shackles are opened by heating or mechanical force, they may develop micro-cracks and should not be reused 

Some modern Kenter shackle designs now feature interchangeable center parts, making them easier to inspect and maintain.  

 D-Type Still Has a Role – But Not in the Middle of the Chain 

Today, D-type shackles are still used, but in a different role: connecting the chain to the anchor, not connecting two chain sections. The D-type anchor shackle (also called an end shackle) is used to connect the studless end link of the anchor chain to the anchor crown shackle. 

Ocean Force notes that D-type joining shackles “find extensive use in modern vessels for anchoring the chain to the anchor” – a different job than connecting chain sections. 

Historically, D-type shackles served a purpose similar to today‘s Kenter shackles – connecting sections of cable. But when used for that task, the rounded crown must face forward to prevent interference during deployment. With the widespread adoption of Kenter shackles for this purpose, D-type shackles are now limited to their specialized anchor-connection roles.

Need a Kenter shackle for your anchor chain? SMEOCEAN provides certified Kenter shackles in various sizes, with full certification and factory traceability. Tell us your chain diameter and certificate requirement – we’ll reply within 24 hours.