IIMS Category A, B and C Defects — A Practical Guide for Marine Surveyors

The way a surveyor classifies defects determines the usefulness of their report. An unclassified list of findings tells a buyer and their insurer very little. A structured defect schedule, with each item clearly rated Category A, B, or C, tells them exactly what to prioritise, what to negotiate on, and what to budget for maintenance.

The A/B/C defect classification system is the standard adopted by most professional marine surveyors in the UK, consistent with IIMS guidance and widely recognised by UK insurers and brokers. This guide explains each category with precise definitions, real-world examples from typical boat surveys, and the downstream implications for buyers, insurers, and the surveyor's own professional standing.

A Critical Distinction: IIMS Defect Categories vs MCA Coding Categories

Before discussing defect classification, it's worth clearing up a distinction that trips up newer surveyors: IIMS defect categories (A, B, C) and MCA Coding categories are entirely different systems.

MCA Coding categories refer to a vessel's area of operation — Category 0 (unlimited ocean), Category 1 (offshore), Category 2 (coastal), etc. These determine what safety equipment the vessel must carry and what operational limits apply. They are applied to the vessel as a whole, not to individual defects.

IIMS defect categories — A, B, and C — are applied to individual findings in a survey report. They describe the severity of a specific defect, not the vessel's operating classification.

Conflating the two in a report will confuse clients, brokers, and insurers. Always make clear which classification system you are using and what it refers to.

Why Classification Matters

The defect schedule is the operational heart of any survey report. It is used by:

• Buyers to negotiate price adjustments or require remedial work as a condition of completion
• Brokers to manage vendor expectations and facilitate deals
• Insurers to assess risk, set policy conditions, and issue warranty notices
• Lenders to decide whether to advance finance and on what terms

All four parties need consistent, unambiguous language. "Needs attention" means nothing. "Category A — immediate safety hazard — cracked chainplate, port side cap shroud" means everything.

The Unified Code and Defect Classification

Since the MCA's Unified Code of Practice took effect on 12 December 2025, surveyors conducting coding surveys on small commercial craft (narrowboats, RIBs, small passenger vessels) need to ensure that their defect classification aligns with the updated safety equipment and stability requirements under the new consolidated standard. A defect that was Category B under the previous Blue or Yellow Code requirements may warrant Category A treatment under the tighter Unified Code provisions.

This does not change how defect categories are defined, but it does affect the threshold at which a finding crosses from B to A on MCA-coded vessels.

Category A: Immediate Safety Hazard

Definition: A defect that presents a danger to life or vessel that must be rectified before the vessel is used.

Category A defects are non-negotiable. A surveyor issuing a report with Category A findings will advise that the vessel should not be used until the items are rectified.

Insurance implications: UK leisure marine insurers — including Pantaenius, Markel, and GJW Direct — typically issue a warranty requiring Category A items to be remedied within a defined period (often 30 days, though this varies by insurer and policy). If the buyer proceeds with purchase without rectifying Category A items, their insurer may refuse a claim arising from those defects. Surveyors should make clients aware of this practice explicitly.

Category A Examples

• Cracked or corroded chainplate — Chainplates carry the lateral load of the rig. Any cracking at the weld toe or active crevice corrosion at the below-deck section is Category A — chainplate failures have caused complete rig loss and fatalities.
• Keel attachment — failed or suspect — Cracking at the keel stub/hull interface, weeping at keel bolts, or significantly corroded keel bolts are Category A. Keel detachment results in immediate capsize. This is one of the most consequential findings on a sailing yacht and one of the most commonly missed on surveys where the bilge is inaccessible.
• LPG/gas system defects — A leaking LPG connection, absent or non-operative overboard drain from the gas locker, or missing automatic solenoid isolation is Category A. LPG is heavier than air and pools in the bilge; ignition causes an explosion, not a fire.
• Inoperative bilge pump — A manual bilge pump that is seized or missing its handle is Category A on any vessel intended for coastal or offshore use. For offshore sailing yachts, the electric bilge pump is a backup to the manual under World Sailing OSR requirements. On dayboats or sheltered-water craft, assess in context.
• Seized seacocks — Through-hull fittings below the waterline that will not close under hand pressure. In an emergency, an inoperable seacock cannot be closed.
• Fuel leak at engine bay — Any active or recent fuel leak in an enclosed engine bay presents fire and explosion risk.
• Failed stanchion bases — system failure — Where multiple stanchion bases have failed or the lifeline circuit as a whole provides no effective restraint, this is Category A. An isolated single failed stanchion base with an otherwise intact lifeline circuit on a vessel not intended for offshore use is Category B — significant, but the vessel is not immediately unsafe to use.
• Non-functioning navigation lights — Category A on any vessel used or likely to be used at night or in reduced visibility (COLREGS Rule 20). On a day-only vessel with documented operational restriction to daylight, Category B.

Category B: Significant Defect

Definition: A defect that does not present an immediate safety hazard but must be rectified within a defined timeframe to prevent deterioration, hazard, or non-compliance.

Category B items drive the substantive price negotiation in most transactions. They are real problems — not cosmetic — but the vessel is not immediately unsafe. Many surveyors assign a recommended timeframe: "Within 3 months" or "Before next offshore passage."

Category B Examples

• Osmotic blistering — moderate — Numerous blisters below the waterline with some penetrating to the laminate. Not immediately structurally compromising, but will progress to delamination if untreated.
• Rudder bearing wear — Measurable play beyond manufacturer tolerance. The bearing will eventually fail but is not an immediate safety risk.
• Delaminated deck section — Soft spot indicating core moisture ingress. Walking on deck is not currently dangerous, but water ingress will progress to structural failure.
• Standing rigging at replacement interval — Wire fatigue at swage fittings on a rig at or beyond its inspection interval.
• Corroded battery terminals — Moderate oxidation: Category C. Severe corrosion reducing current capacity: Category B. Where arcing risk is present — significant corrosion with compromised insulation in a battery compartment, or inadequate ventilation — escalate to Category A. Do not conflate cosmetic terminal oxidation with a genuine ignition hazard.
• Degraded fuel tank inspection port seal — Fuel odour from tank cover, suggesting deteriorated seal; fire risk under certain conditions.
• Worn propeller shaft seal — Weeping gland seal with progressive ingress; not an immediate hazard but requires attention before next season.

Category C: Maintenance or Cosmetic

Definition: Items that should be addressed as part of routine maintenance or that are cosmetic in nature. No immediate structural or safety implication.

Category C items are not irrelevant — a long Category C list tells the buyer something about the vessel's maintenance history. But they should not drive price negotiation in the same way Category A and B items do.

Category C Examples

• Antifouling due for renewal — Standard maintenance item.
• Teak deck caulk — surface crazing — Drying out but not cracked through. Will progress to Category B water ingress if unaddressed over several seasons.
• Winch drum wear — Surface scoring on primary winch drums. No functional impact.
• Canvas bimini — UV degradation — Fading and brittle. Maintenance item.
• Navigation instrument displays — fading — Screen sun-faded, reducing readability. Category C unless so severe that safe navigation is impaired.
• Anchor windlass — stiff operation — Requires servicing; operates but sluggishly. Maintenance item.

How the Defect Schedule Looks in Practice

A well-structured defect schedule is presented as a table with each finding listed by reference number, system, location, category, description, and recommended action. Below is an illustrative example — the type of schedule that appears in a Marine Inspect-generated report.

Each row in the schedule should also reference the photograph number where the defect was documented.

The Audit Trail and Reclassification

A surveyor may occasionally need to reconsider a classification after a report has been issued — for instance, if a client has a specialist inspect a Category B item and it turns out to be more serious than initially assessed. When this happens, it is important that the original classification is preserved.

Marine Inspect generates a SHA-256 integrity hash of the complete survey data at the point of report generation. This hash is printed in Appendix B of every report. If a classification is subsequently changed and a new report issued, the original report and its hash remain on record. This provides professional protection: it can be demonstrated exactly what the surveyor recorded at the time of the inspection and when any amendment was made.

Tracking Defects Digitally in the Field

Traditional survey practice involves paper notes or a laptop, with defect classification applied retrospectively at the office. This introduces risk: a surveyor who has inspected 60+ items across a full day may misclassify or miss items when transcribing from memory.

Digital field tools address this directly. With Marine Inspect:

• Each checklist item has an inline defect classification selector active in the field
• Fail items trigger a Category A/B/C picker while the surveyor is physically present at the defect
• Voice notes can be dictated directly to the finding, reducing transcription time
• Photographs are attached to the individual finding — not dropped into a folder for later sorting
• The completed checklist exports directly to a structured defect schedule

Classifications made in context, not from memory, are more consistent and less likely to be challenged.

Summary Table

Consistent use of this classification system is what separates a professional marine survey report from a vague inspection letter. It protects the surveyor, serves the buyer, and satisfies the insurer.

For a worked example of how Category B applies to aged standing rigging — where the rig looks clean but the failure mode is invisible — see The Marine Surveyor's Guide to Rigging Inspection.