A GRP hull's strength comes not only from skin thickness but from how the laminate was made and how the internal structure is bonded to the skin. The survey reads the skin, the internal grid and the bond between them separately.
Hull structure
- Single skin
- Common below the waterline; thickness and laminate quality decide.
- Sandwich
- A core between two skins. Light and stiff; when wet, the core separates or rots.
- Internal grid
- Floors and frames bonded to the skin. Loads from keel and engine beds pass into it.
- Build method
- Hand lay-up or vacuum infusion; resin ratio and voids decide laminate quality.
What to look for
- Stress cracks: regular cracking at corners, stiffener ends and around the keel.
- Hard spots: a line or crack visible outside where internal structure meets the skin.
- Grid bond: floors and frames separating from the skin; checked with borescope and torch.
- Repair marks: changes in colour and texture; the area gets extra sounding and moisture readings.
- Flex: unexpected movement of the skin under pressure.
Measurement
The hull is sounded and moisture-read on a grid; dull or wet areas are marked on the sketch. Where repair is needed, the laminate schedule and ply count follow the original.
What we often see
- Star cracks in gelcoat: Hairline cracks radiating from a centre around cleat and winch bases.
- Opening at the keel-hull joint: A clear crack and paint separation along the keel joint, at the leading edge.
Continue with this topic
- Hull-deck joint: Types and signs of leaks.
Build era and method
GRP production has changed over the years. Older boats often have thicker, hand-laid skins; newer boats more often use infusion and sandwich construction, with less weight. That changes what the survey looks for. On a thick single skin, laminate moisture and osmosis come to the fore; on thinner, lighter structures, the bond of the internal grid to the skin and the state of sandwich areas need more attention.
Signs of impact and grounding
Past impacts often leave no mark outside; they may have been repaired and painted. So the bow, around the keel and the internal floor bonds get particular attention on survey. Floors and frames separating from the skin, a crack at the keel's leading edge and differences in colour or texture outside can all point to a past grounding. When these signs come together, a keel drop or a controlled opening may be recommended.
Quality of old repairs
A good repair matches the surrounding laminate in thickness and ply schedule; it sounds the same when tapped and reads within the same moisture range. A poor repair shows voids filled with filler, a short scarf and edges that open over time. If the invoice and the firm that did the repair are known, assessing it on survey is easier.
How it appears in the report
In the hull section, the moisture map and sounding results are given with the sketch; each finding is written with its photo, location and grade. If there is a structural doubt, the report states plainly which further inspection is recommended and how it may affect the purchase or refit decision.
What the owner can watch
Between surveys, cracks appearing in gelcoat around the keel, new stains inside the hull and doors that suddenly stick are worth noting and photographing; they can be early signs of movement in the structure.
Photos taken at the same spots each season make any change easy to see.