Ultrasonic Thickness Measurement (UTM) is used to measure the thickness of steel and aluminium hulls.
It can be used in prepurchase surveys, or as a periodic monitoring of the hull condition.
Insurers may require a survey of a steel or aluminium hull before insuring it.
Apart from UTM, a hull survey will also encompass Cathodic protection and galvanic corrosion, welds, buckling of plates, through hull pipes and fittings, and bow thruster tunnels.
Most surveyors use relatively simple equipment (read cheap, about € 300,--) to measure hull thickness. But the big drawback is that the coating on the metal needs to be removed. In my surveying business a Multiple Echo equipment is used, the only one of this kind that allows me to measure without removing coatings up to 6 mm. thick.
Also a lot of measurements can be made in a very short time frame, which is impossible if you have to grind.