Steel Hull Ultrasonic Thickness Survey: What the Readings Really Mean
Ultrasonic thickness testing is used on steel vessels to measure the remaining thickness of plating without cutting into the hull. It is particularly valuable on older steel yachts, motor vessels, widebeams and other craft where corrosion may not be obvious from appearance alone.
The reading itself is only part of the survey. The surveyor must understand the vessel's original construction, likely plate specification, local corrosion pattern and whether individual low readings represent isolated pitting or general wastage.
What I am looking at
Before taking measurements I inspect the hull visually for pitting, scale, coating breakdown, weld condition, distortion, previous doubling plates, repairs and areas where water or debris may have remained internally. Particular attention is given to the waterline, bilge regions, chines, keel, stern gear area and plating adjacent to fittings or welds.
Internal inspection is equally important where access is available. Corrosion can progress from the inside even where the external coating remains presentable.
How ultrasonic thickness testing is carried out
The test instrument sends an ultrasonic pulse through the steel and measures the time taken for the echo to return from the back surface of the plate. The instrument must be correctly calibrated and coupled to the steel surface using a suitable couplant.
Readings are taken at representative positions and increased in areas where visual inspection suggests corrosion or where readings become inconsistent. Coatings, rough surfaces and heavy scale can affect results, so local preparation may be required to obtain reliable readings.
How I interpret the readings
A reading should be compared with the expected original plate thickness and surrounding measurements. One isolated low figure may indicate a pit, local defect or poor coupling, whereas a consistent reduction across a wider area is more indicative of general wastage.
The percentage loss is important, but there is no universal single thickness at which every steel vessel becomes unacceptable. The structural role of the plate, vessel type, original scantlings, local reinforcement and intended service all matter.
Problems commonly identified
Findings can include local pitting beneath failed coatings, corrosion at the waterline, internal bilge corrosion, wastage around through-hull fittings, poor historic repairs and plating that has been doubled rather than properly cropped and renewed.
Where corrosion is significant, the surrounding structure and welds should also be assessed because replacing a plate insert into weakened framing or poor adjacent steel does not provide a satisfactory long-term repair.
Recommendation
Areas of materially reduced thickness should be marked and assessed by a competent steel boatyard or coded welder. Where necessary, defective plating should be cropped back to sound steel and renewed with material of suitable grade and thickness, using appropriate marine welding procedures.
After repair, the steel should be properly prepared and protected with a suitable coating system. Bilges and inaccessible internal areas should also be addressed because external repair alone will not prevent continued corrosion from inside the vessel.
