Systematic, close-up, suspect, substantial — what is the difference?
Every extent table in this chapter is built out of four words. They sit beside each other and look like four items on one list. They are not: one is a schedule, one is a method, one is a status and one is a number. Almost every mis-scoped gauging plan comes from treating them as the same kind of thing.
The short answer
- Systematic — a schedule. Named steel, fixed by ship age. Known before you sail.
- Close-up — a method of examination. Gauging rides along on whatever it covers.
- Suspect — a status a location carries. Two ways to acquire it, either one enough.
- Substantial — a measured threshold. It does not add a location; it widens the ones you have.
- Only the first is plannable in advance. The other three are decided at the ship.
Four words, four kinds of thing
Open any extent table in this chapter and the four appear together, usually inside one column, often inside one sentence. The table gives no hint that they belong to different categories — and they do.
| Word | What kind of thing it is | Known when? |
|---|---|---|
| Systematic | A schedule of named structure, set by survey number. | Before the ship arrives |
| Close-up | A method of examination, with gauging attached to its members. | Scope before, findings during |
| Suspect | A status attached to a location, carried between surveys. | From the ship's record, plus new ones |
| Substantial | A threshold on a measured value. | Only after reading |
Read the right-hand column and the shape of the job appears. A quotation can be firm on one row and can only be a rate on the other three.
Systematic: a schedule, and the only one you can price
The systematic column names steel and ties it to a survey number. Two transverse sections at the second special survey; the whole deck outside the hatch line at the third; every bottom plate at the fourth. Nothing about it depends on what anybody finds.
That makes it the backbone of a gauging plan and the only part that can be drawn from the ship's particulars alone — age, type, hull configuration, deadweight where a threshold applies. It also makes it the part where a mistake is systematic in the other sense: choose the wrong column and every figure downstream is wrong by the same amount.
One caution that this chapter documents at length. The systematic column of the IACS requirement is a floor, not a description of what will be asked. Several societies add items to it — peak tanks, hatch covers, sea chests, superstructure decks — that the baseline never mentions.
Close-up: a method, not a list of plates
Close-up survey is an examination carried out within reach of the structure. It is a way of looking, not a category of steel — and on its own it produces no readings at all.
The gauging arrives through a separate line that every extent table carries, in nearly identical words across every document: measurement of the members under close-up survey, for general assessment and for recording the corrosion pattern. That phrase is doing real work. The readings are not there to accept or reject a plate — they are there to characterise how the ship is wasting. It is why the close-up gauging is often the largest count in a survey and the least discussed.
The practical consequence is an ordering rule: the close-up table has to be settled before the gauging plan can be drawn, because the gauging line points at it rather than listing anything itself. Two societies do not even restate the members — ABS and LR simply send you to their own close-up table and ask for readings on whatever it covers.
Suspect: a status, and there are two ways to earn it
A suspect area is a location that carries a label. The definition sets two gates and joins them with and/or — which means either one on its own is enough, and this is uniform across the whole IACS set, enhanced survey and not.
- Gate one — the location is showing substantial corrosion. Measured, objective, and a link back to the fourth word on this page.
- Gate two — the Surveyor considers the location prone to rapid wastage. Judgement, and the requirement takes it no further: there is no test, no list of locations, no criterion.
Gate two is the one that surprises people, because it needs no reading at all. A location can become suspect on the strength of what it is and where it sits — a coating breakdown at a weld, a bilge well, a boundary between two service conditions — before any thickness is taken there. UR Z7 · Z10.2 · Z10.1
Once a location has the label it stays with the ship, and it splits into two different obligations at every later survey. Areas recorded as suspect are examined. The parts of them found to have substantial corrosion are gauged. One is an eye survey and one is readings — and the two sentences appear side by side in the same clause, at the special survey, at the annual survey and at the intermediate.
At the intermediate survey of a bulk carrier the requirement changes character again: suspect areas are put through overall and close-up survey, not gauged alone. The status pulls a method with it. UR Z10.2 §4.2.2
Substantial: a number, and a small circle
Substantial corrosion is the only one of the four that is a measured quantity. In the IACS requirements it is wastage that has run past three quarters of the allowable margin while the plate is still inside acceptable limits — a band, not a rejection. A member in that band is still fit; it has simply used most of what it was given. UR Z10.1 §1.2.7
It behaves differently from the other three in one important way: it does not add locations, it widens the ones already in scope. Systematic, close-up and suspect each bring steel into the survey. Substantial corrosion takes steel that is already there and multiplies the readings on it — the suspect plate plus the plates around it, three longitudinals in way, the suspect frame and each frame beside it.
And there is a small circularity between it and the third word, which is worth naming rather than tripping over. A suspect area is partly defined as one showing substantial corrosion; the widened pattern for substantial corrosion is then written in terms of the suspect member — suspect plate, suspect frame, suspect area. The two definitions lean on each other. In practice the loop closes fine: gate two of the suspect definition needs no measurement, so there is always a way in.
Where the words sit on the page, and why it is worth noticing
The same requirement is typeset differently across three IACS documents that are otherwise near-identical, and the difference changes how a reader weighs it.
- UR Z7 opens every one of the four survey columns with suspect areas, and adds throughout the vessel — no boundary. Z7 Table 1
- UR Z7.1 also opens with it, but without those three words. Z7.1 Table II
- UR Z7.2 prints it as the last row of every column — below the systematic items and below the close-up measurements, in all four surveys. Z7.2 Table II
The obligation is the same in all three. But a requirement printed first reads as the frame around the column, and the same requirement printed last reads as an afterthought — and a gas carrier survey scoped from the top of the column downward can reach the bottom of the page with the budget already spent.
One more difference between those two documents, and this one has teeth. UR Z7 carries a printed carve-out: its escalation clause does not reach the cargo tanks of an oil tanker, a chemical tanker or a double-hull oil tanker, each of which is covered by its own enhanced survey requirement instead. UR Z7.1 carries no such note — the exclusion is simply not repeated there. When two sibling documents differ by a whole paragraph, the safe assumption is that the difference is deliberate. Z7 §3.2.4 · Z7.1 §3.2.4
And UR Z7.2 adds two open-ended qualifiers of its own in a table note: the Surveyor may extend the measurements as deemed necessary, and where substantial corrosion is found the extent is increased to the Surveyor's satisfaction. On a gas carrier, therefore, the ceiling of the survey is not in the table. Z7.2 Table II Note 3
How the four combine on one survey
In the order they actually arrive:
- Read the systematic column for the ship's age, type and society. This is the plan.
- Settle the close-up table, because the gauging line points at it. Add those members to the plan.
- Pull the suspect areas off the ship's record, and expect new ones to be named during the survey. These are locations, not quantities.
- Hold capacity in reserve for substantial corrosion. It will not add a location, but it can multiply the readings on any location already in scope — and the extra work is finished before the survey is credited.
Steps one and two are arithmetic. Steps three and four are not, and no amount of preparation converts them into arithmetic — which is the honest answer to why a gauging quotation has a fixed part and a variable part, and where the line between them falls.
Sources
- UR Z7 Rev.29IACS — Hull classification surveys, ships other than ESPofficial text ↗
- UR Z7.1 Rev.15IACS — General dry cargo ships, hull surveysofficial text ↗
- UR Z7.2 Rev.8IACS — Liquefied gas carriers, hull surveysofficial text ↗
- UR Z10.1 Rev.25IACS — Oil tankers on enhanced surveyofficial text ↗
- UR Z10.2 Rev.37IACS — Hull surveys of bulk carriers, Feb 2023index entry →
- UR Z10.3 Rev.21IACS — Chemical tankers on enhanced surveyofficial text ↗
Class-society and IMO documents are indexed and linked, never re-hosted. Where a class rule is stricter than the IACS baseline, the class rule governs.