This is the first entry in our Defect of the Week series — practical guides to the findings that matter most in professional marine survey practice.
Chainplate failure is responsible for some of the most catastrophic dismastings in UK sailing. When a chainplate fails at sea, the sequence is rapid: the cap shroud detaches, the mast loads sideways, and the rig comes down. In offshore conditions, the consequences can be fatal. This makes chainplates one of the most professionally significant findings in a pre-purchase or insurance survey — and one of the most commonly underclassified.
What Chainplates Do and Why They Fail
Chainplates are the metal fittings that connect the standing rigging to the vessel's structure. They carry enormous dynamic loads — tensile forces from the cap shrouds, lowers, and intermediates, multiplied by sea conditions and sail loads. A typical sailing yacht's chainplates may experience loads of several tonnes in heavy weather.
Most chainplates on GRP yachts built between the 1970s and 2000s are stainless steel plates bolted through the deck or hull-deck joint into internal structure. Plate thickness varies considerably with vessel size, designer specification, and rigging configuration — offshore and ocean-going yachts frequently use 10mm or heavier plate. Comparison against the designer's original specification is always preferable to relying on a general range. The most common failure modes are:
Crevice corrosion — stainless steel corrodes most aggressively in low-oxygen environments. A chainplate that passes through the deck into a sealed locker is in exactly that environment. The corrosion is invisible from above and below until the metal has been significantly thinned.
Weld cracking — many chainplates are welded to backing plates or tangs. Fatigue cracking typically initiates at the weld toe (the visible surface transition between weld bead and parent metal) under cyclic loading from sea conditions. Weld root cracking — at the base of the weld seam — is subsurface and cannot be detected by visual inspection alone; dye penetrant or other non-destructive testing is required to confirm it. In field practice, the weld toe is what the surveyor can inspect visually. Any cracking or discontinuity at the weld transition should be treated as Category A regardless.
Deck delamination at the through-hole — where water ingress has occurred around the chainplate seal, the deck core becomes saturated and structurally compromised. The chainplate no longer has solid structure to bear against under load.
Inspection Methodology
Above-deck inspection
Begin at the deck plate. Look for:
- Rust staining or streaking from the chainplate slot or cover plate — this indicates water has been running down the plate, which means water has been entering and creating conditions for crevice corrosion
- Movement under hand pressure — grip the rigging and apply a deliberate lateral push. Any detectable movement in the chainplate or deck fitting indicates structural compromise
- Cover plate fixings — many chainplate covers are screwed down with a sealant bed. Lift the cover if possible; if the surveyor cannot access the chainplate directly, state this in the report
Below-deck inspection
Access to the chainplate from below is essential. This is where the critical finding is most often made:
- Inspect the length of the plate visible below deck. Look for discolouration, pitting, weeping rust, or white crystalline deposits (the latter indicates moisture cycling through cracks)
- Check the backing plate and bolts. Bolts should show no corrosion, and the backing plate should be solidly bedded against the hull or structural member
- Tap the area of deck around the chainplate with a mallet or coin. A dull thud rather than a firm tap indicates delamination
The concealment problem
Many production yachts route chainplates through the interior joinery — headlining, upholstery panels, or fixed furniture. When a chainplate is concealed by interior structure the surveyor cannot practically remove, the report must explicitly state this:
"The chainplate for the starboard cap shroud was not accessible below deck due to fixed joinery. The chainplate could not be inspected below the deck level. Further investigation is recommended, including removal of the starboard aft locker panel."
This is both accurate and professionally protective. Do not assume a concealed chainplate is sound, and do not classify it as passed when it hasn't been inspected.
Classification: When Does It Become Category A?
This is where surveyors most frequently err — by classifying a questionable chainplate as Category B when it warrants Category A.
The relevant test is: does this defect present a danger to life or vessel if the vessel is used?
For chainplates, the escalation from B to A is triggered by any of the following:
Category A:
- Cracking or discontinuity at the weld toe or plate surface — any visible crack in a loaded stainless fitting is Category A
- Active crevice corrosion — visible pitting, significant material loss, or flaking metal at the below-deck section
- Movement detectable under hand load with the rig standing
- Deck structure delaminated at the chainplate, providing no solid bearing surface
Category B:
- Rust staining indicating past water ingress, but plate metal is sound on visual inspection with no pitting, discolouration, or weeping at the below-deck section; backing plate and bolts show no corrosion; no movement detectable under hand load with the rig standing
- Chainplate at or beyond its inspection interval in saltwater service (MAIB investigations have consistently recommended age-based replacement — most practitioners use 15 years in saltwater, shorter for blue-water use — but there is no single published standard; state the vessel's known chainplate history and recommend specialist rig inspection if history is unknown)
Not assessable (state in report):
- Chainplate concealed by fixed joinery with no access
The error surveyors make is rating a "rusty but apparently solid" chainplate as Category B. If the plate shows active crevice corrosion — particularly at the below-deck section where the low-oxygen environment is most aggressive — the remaining section thickness is unknown. You cannot assess load-bearing capacity by visual inspection alone. When in doubt, Category A is the correct conservative classification. The professional risk of underclassifying a failing chainplate is orders of magnitude greater than the client inconvenience of a Category A finding.
Documenting Chainplate Findings
Precise language is the surveyor's professional protection. Compare these two versions:
Inadequate: "Chainplates showing some surface rust. Recommend inspection."
Professional: "Category A — Starboard cap shroud chainplate. Active crevice corrosion observed at the below-deck section immediately beneath the deck fitting, approximately 80mm of plate length. Brown crystalline deposits and surface pitting noted. Material thickness at corroded section cannot be assessed by visual inspection. Plate should be regarded as potentially structurally compromised. Do not use vessel under rig load until chainplates are professionally assessed and replaced if necessary. Photographs: P047–P052."
The second version specifies location, nature of defect, reason for Category A classification, recommended action, and photo reference. It is defensible. The first is not.
When using Marine Inspect's voice dictation feature in the field, you can dictate this level of detail directly to the finding while you're standing at the chainplate, before moving on. The structured output ensures the photo reference is automatically linked.
Scope: Beyond the Shroud Chainplates
The inspection scope should include all standing rigging attachment points, not only the cap shroud chainplates:
Forestay and backstay fittings — the stemhead fitting is subject to forward and upward loads and frequently suffers from casting fatigue or through-bolt pull-through in the hull skin. The backstay anchor point is similarly exposed. Inspect both independently; they fail by different mechanisms to the shroud chainplates.
Toggles and clevis pins — seized clevis pins and cracked toggles at the chainplate-to-turnbuckle connection are among the most common causes of standing rigging failure and are entirely separate from the chainplate itself. Inspect every toggle and pin for cracks, corrosion, and free articulation. A seized toggle that cannot articulate in two planes transmits bending load directly to the fitting below — a significant fatigue risk.
Rigging age and condition — a sound chainplate attached to rigging wire well beyond its service life does not make the vessel safe. Note the rig's known age and condition alongside the chainplate assessment, particularly the condition of swage fittings at the lower ends of the cap shrouds.
Field Notes
A few practical observations for surveyors:
- Always carry a good torch and a small mirror on an extension — many chainplate below-deck sections are in awkward corners
- Press the chainplate with your thumb while the rig is standing; even a slightly loose chainplate will show micro-movement
- On older alloy or bronze chainplates, look for green-white crystalline deposits (verdigris or dezincification) — these indicate corrosion that may have significantly reduced section
- If you cannot inspect a chainplate below deck, never write "chainplate OK" — write exactly what you could and could not inspect
The peak survey season is the time when rigging failures happen — vessels are being used hard. A chainplate classified as Category A at survey is a vessel that doesn't go sailing until it's fixed. That is the correct outcome.
For the full standing rigging inspection methodology — covering swage fittings, turnbuckles, meat hooks, going aloft, and how age-based classification holds up under professional challenge — see The Marine Surveyor's Guide to Rigging Inspection.
Read next in the series: Osmotic Blistering — The Survey Finding That Needs Precise Language