Percussion & Moisture Testing A Mallard 23 Narrowboat

Narrowboats have a tough life and get more than their fair share of bumps and knocks using the canal locks and coming alongside. As part of a pre-purchase survey of a 23’ 1989 Mallard fibreglass narrowboat, marine surveyor David Pestridge explains how percussion and moisture testing help develop a picture of the condition of the hull laminate and how a marine surveyor might write their findings for their client.
Percussion testing in marine surveying is the black art of striking a surface (hull, deck, bulkhead, propeller etc) with a hammer with the aim of identifying defects in the structure. It is a non-destructive testing technique that takes skill, practice and experience to execute well. The core principle requires choosing an appropriate hammer, striking the structure in the right places, with the right amount of force and interpreting the results gained. Percussion testing is equally applicable to wood, metal, composite and laminate structures and the procedure varies according to the nature and thickness of the structure being tested and the defects that are likely to be present.
Moisture testing is used to determine how much moisture is within the laminate as high moisture levels may lead to the formation of blisters and other moisture related defects. Used together, these two testing techniques can offer in-depth analysis of the condition of the hull’s laminate structure and are an essential part of any FRP survey.
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