On a metal hull the survey turns on one question: how much plate is left? What moisture and delamination are to a composite hull, corrosion loss is to steel and aluminium. Visible rust is the tip of it; the real information comes from ultrasonic thickness testing.
Scope: steel sailing yachts and motor yachts · aluminium hulls and catamarans · steel gulets and trawlers · classic and older metal craft · acceptance inspection after a yard period. Readings are reported as percentage loss against original scantlings.
1. How Ultrasonic Thickness Testing Works
A probe is coupled to a cleaned surface and the return time of the sound wave from the far face is converted to thickness. A single reading tells you nothing, meaning comes from the measurement grid and the comparison against original plate.
- Surface preparation: paint and scale are removed at each point; heavy coatings give false readings.
- Grid plan: the hull is divided into zones and read at set intervals, typically 80 to 300 points depending on size.
- Denser sampling at critical zones: bilge line, under-tank areas, the waterline band, shaft exit, sounding well, hull-to-deck joint.
- Comparison: each reading is expressed as percentage loss against original scantlings, taken from the build file or an undamaged reference area.
- Mapping: values are plotted on a zone drawing and read like a heat map in the report.
2. What Loss Is Acceptable?
| Loss | Assessment | Action |
|---|---|---|
| 0-10% | Normal ageing | Recorded and monitored |
| 10-20% | Watch zone | Denser sampling; coating system reviewed |
| 20-25% | Threshold | Structural or class opinion; localised renewal plan |
| Over 25% | Structural risk | Plate renewal (cut and weld) |
These are widely used bands; on classed vessels the class society's own criteria govern. The report always carries both the raw value and the percentage loss, interpretation may differ, the numbers do not.
3. Steel-Specific Checkpoints
- Bilge line: water and salt collecting inside start corrosion from within. Plate that looks sound from outside can be thin from inside.
- Under and around tanks: hard to access, critical to measure; verified visually with a borescope.
- Waterline band: the wet-dry cycle plus mechanical contact (fenders, pontoons) wears this band.
- Weld seams: crack search with a 30× loupe; corrosion accelerates in the heat-affected zone.
- Cathodic protection: anode count, position and consumption rate; protection potential measured with a zinc reference electrode.
- Coating system: epoxy thickness and integrity, on a steel hull paint is not cosmetic, it is the corrosion barrier.
4. Aluminium-Specific Risks
Aluminium behaves differently: it does not rust, but it is far more sensitive to galvanic corrosion and pitting.
- Dissimilar metal contact: a stainless bolt, bronze seacock or copper lug in direct contact with aluminium causes rapid loss; isolation is required.
- Stray current: shore power and onboard faults can cause serious damage to an aluminium hull within months. Reference electrode measurement is essential here.
- Pitting: small but deep local craters. Average thickness can read well while a local pit threatens the structure, which is why the grid is tightened.
- Antifouling compatibility: copper-based antifouling must not be used on an aluminium hull; if it has been, that is a finding in its own right.
5. What the Report Contains
- Zone drawing with every measurement point mapped
- Raw thickness in mm and percentage loss against original for each point
- The ten lowest readings and their locations
- Cathodic protection potential and anode assessment
- Weld and heat-affected zone observations with photographs
- Approximate area (m²) and cost range where localised renewal is required
If you are buying: on a metal hull, "the hull is sound" without ultrasonic readings means nothing. This is the single most expensive omission a pre-purchase survey can have.
If you are selling: a current thickness map removes the buyer's largest uncertainty before it becomes a negotiation, and that shows up in the price.
If you are selling: a current thickness map removes the buyer's largest uncertainty before it becomes a negotiation, and that shows up in the price.
Frequently Asked Questions
A probe measures plate thickness point by point and the result is compared with the original scantlings to calculate corrosion loss. On steel and aluminium hulls this is the only way to know the true condition; visible rust does not show thinning that progresses from inside.
Typically 80 to 300 depending on the size and structure of the vessel. The grid is tightened at the bilge line, under tanks, along the waterline, at the shaft exit and around welds. A single reading is meaningless, value comes from the grid and the comparison.
As widely applied bands: up to 10% is normal ageing, 10-20% a watch zone, 20-25% the threshold, and over 25% a structural risk requiring plate renewal. On classed vessels the class society's criteria govern. The report gives both raw values and percentage loss.
Aluminium does not rust but is far more sensitive to galvanic corrosion and pitting. Dissimilar metal contact, shore-power stray current and copper-based antifouling all cause rapid damage. Cathodic protection measurement with a zinc reference electrode is therefore mandatory and the measurement grid is tightened.
Yes. Paint and scale must be removed at each measurement point, heavy coatings give false readings, and the underwater body, cathodic protection and weld seams can only be assessed ashore.
It is prepared in the structure underwriters and class societies expect: zone drawing, raw values, percentage loss, the ten lowest readings and cathodic protection measurement. If your institution requires a specific format, tell us before work starts.
6. Get a thickness map of your hull
Ultrasonic readings, zone drawing and cathodic protection assessment in one report.
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Related Pages
Our StandardsMethod, equipment, ethics, liabilitySurvey TypesWhich survey do you need?Survey Report ContentsWhat the report containsCommon Boat DefectsThe 20 findings we record most
Prepared by SurveyYat, ABYC Professional Member, 149 GT Yacht Captain. Last updated: September 2026.
Scope: The survey is non-destructive and nothing is dismantled; details on the survey types page.
Continue with this topic
- Weld inspection: Procedure, welder and methods.
- Building in steel and aluminium: Critical points by material.