To make a good ultrasonic survey takes much time. To get a good opinion of the hull I would propose (basic survey pre-purchase type) to divide the underwater hull into 1m x 1m squares and do minimum 9 measurements per square and more in an area where excessive diminution found. For owners consultancy (separate ultrasonic job) I am working to 50cm x 50cm with minimum 9 per square. The ultrasonic work generally takes full day or two for a 20m. I would examine the underwater plating only. Topsides are usually faired and if areas of underwater plating are faired I cannot take measurements there.
The pre-purchase survey will also include bilge access where possible to determine the condition of the inside surface of the plating and therefore the extent of the ultrasonics on the hull external surfaces. Usually the external surfaces are very good and it is the interior surfaces especially where inaccessible where coatings could not be maintained and where there is diminution.
These surveys are performed with the latest cygnus multiple echo equipment for through coating measurement. Hull surfaces need to be clean of marine growth and high pressure washed before the measurements. Couplant used is water based and easy to remove with water.
|