Delamination in short is a state where the layers of GRP material held in a resin matrix become separated from one another. Delamination can also apply to joinery where plywood normally due to moisture will separate about the bonded sheets. Delamination may also apply to where GRP layers loose bonding with underlying core materials such as foam or balsa.
When localized the presence of delamination does not necessarily need to be of immediate concern, however if for instance present around the keel, or the rudder or deck in way of a chain plate, then the presence of this defect can be serious.
Most surveyors use a sounding hammer in the first instance, but also moisture measurement and Infra-Red cameras can be used.
Delamination is rarely seen until at least the presence is confirmed and the areas ground back or opened up for inspection and repair. Below are some interesting images which illustrate delamination about the hull in way of a keel bolt hole due to grounding and on a rudder blade due to moisture ingress into the blade and due to poor manufacture/quality control.
In most cases the laminate will need to be ground back or the affected area cropped to reveal the damaged material. Re-laminating will be necessary. If delamination is noted around a keel the keel may need to be removed to permit the hull to be fully inspected.