Guide 06 Marine engineering and ship repair
Hull plate renewal and thickness gauging basics
Ultrasonic thickness measurement (UTM) is how a hull's remaining steel is checked without cutting into it: a calibrated probe reads plate thickness from one side, and the result is compared with the ship's original scantling and its classification society's allowable diminution to decide whether renewal is needed.
- Topic
- Marine engineering and ship repair
- Checked
- 23 September 2026
- Sources
- 4
- Questions
- 6
01
What ultrasonic thickness measurement is
Ultrasonic thickness measurement, also called UTM or UTG, is a non-destructive test: a calibrated ultrasonic probe reads the remaining thickness of a plate, frame or stiffener from one side, with no need to expose the far face of the steel.
Surveyors compare the readings with the original scantling and the classification society's allowable diminution for that structural member to decide whether renewal is required. The firms that carry out the gauging must themselves be approved by the classification society before their readings can be used in a survey report.
02
Substantial corrosion, in survey terms
In IACS survey terminology, substantial corrosion describes wastage that has gone beyond 75 percent of the classification society's allowable corrosion margin, but is still within the acceptable limit overall. An area found to have substantial corrosion is not renewed immediately, but it is recorded and is normally re-gauged at each subsequent survey.
Wastage that goes beyond the allowable limit itself, rather than just beyond the 75 percent substantial-corrosion threshold, requires the plate, frame or stiffener to be renewed rather than simply monitored.
This two-step approach, record and re-gauge beyond 75 percent, renew beyond the full allowable limit, is what lets a survey report show a trend across successive dockings rather than a single pass or fail reading taken in isolation.
Two thresholds in a hull survey report
- Substantial corrosion
- Wastage beyond 75% of the allowable margin; recorded and re-gauged at later surveys
- Beyond the allowable limit
- Wastage exceeds the class society's minimum; steel renewal is required
03
How the renewal decision is made
The renewal call is made against tables set by the ship's own classification society, differing by ship type and by structural member, rather than against one number that applies everywhere. That is also why a docking budget cannot be set from a single percentage figure quoted out of context; it depends on the gauging results against that society's own tables.
The Indian Register of Shipping, as a full member of IACS, applies this same class-survey framework, thickness gauging compared against allowable diminution, to the ships it classes, and its surveyors carry out the same bottom and structural checks at each dry docking.
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Hull plate renewal and thickness gauging basics
What is ultrasonic thickness measurement (UTM) on a ship?
Ultrasonic thickness measurement is a non-destructive test that measures remaining plate thickness from one side using a calibrated ultrasonic probe. Surveyors compare readings with the original scantling and the classification society's allowable diminution to decide whether plating, frames or stiffeners need renewal. Gauging firms must be approved by class.
Who is allowed to carry out hull thickness gauging?
Thickness gauging used in a classification survey report has to be carried out by a firm approved by the ship's classification society. An unapproved reading, however accurate, cannot be accepted as the basis for a survey decision on renewal.
What does 'substantial corrosion' mean in a ship survey report?
Substantial corrosion is wastage beyond 75 percent of the classification society's allowable corrosion margin, while still within the overall acceptable limit. Areas with substantial corrosion are not renewed immediately, but they are recorded and are normally re-gauged at each subsequent survey to track the trend.
What happens once hull wastage goes beyond the allowable limit?
Once gauged wastage exceeds the classification society's allowable limit for that plate, frame or stiffener, rather than only the lower substantial-corrosion threshold, steel renewal is required. The area cannot simply be recorded and monitored at the next survey.
Does every classification society use the same renewal thresholds?
No. Allowable diminution and renewal criteria are set in each classification society's own rules and differ by ship type and structural member, so the renewal decision is made against that specific society's tables rather than a single figure that applies across all class societies.
Why is a plate not renewed as soon as it shows some corrosion?
Corrosion has to reach a defined point before renewal is required. Wastage beyond 75 percent of the allowable margin is recorded and re-gauged at later surveys as substantial corrosion, but it is only wastage beyond the full allowable limit itself that forces renewal, so a plate can be monitored across several dockings before that stage.
Checked against
Sources
- 01UTM for metallic vessels, Australian Maritime Safety Authority
- 02Ultrasonic thickness measurements on ships, Virtue Marine (excerpt)
- 03UR Z10.2, hull survey requirements, ClassNK, publishing IACS Unified Requirements
- 04About IRClass, Indian Register of Shipping
This guide explains how the system works in general, as the documents above describe it, and was last checked on 23 September 2026. Rules and thresholds change: the governing document is the authority, not this page.
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