Most of the Turkish second-hand fleet is glass-reinforced composite, GRP, or fibreglass in everyday speech. Built and kept properly, GRP lasts for decades; neglected, a boat can need major repair far sooner. What decides the difference is not the material itself but whether water has found its way into the laminate.
This page sets out what is examined on a GRP hull, what each finding means, and which defects can be repaired.
With a moisture meter the information is not in the absolute figure but in the difference between areas. Find a dry reference area on the same boat, then read the suspect area against it.
The instrument reads the electrical behaviour of the laminate; paint thickness, temperature and surface damp all shift the reading. Writing a single number into a report is easy, writing the conditions it was taken under is what makes it useful.
1. How a GRP Hull Is Built Up
| Layer | Function | Why it matters in survey |
|---|---|---|
| Gelcoat | Outer protective resin layer, 0.4-0.8 mm | The only barrier keeping water out of the laminate. The crack pattern describes the movement underneath. |
| Surface tissue | Fine fabric directly under the gelcoat | A resin-rich zone; osmosis usually begins here. |
| Reinforcement | Chopped strand mat, woven roving, biaxial and triaxial fabrics | Structural strength. Thickness and ply sequence define the boat's class. |
| Core (on sandwich hulls) | Balsa, PVC or SAN foam | Gives lightness and stiffness; once wet it becomes the most expensive repair on the boat. |
| Inner skin | Reinforcement on the inner face of the core | The second shell of the sandwich; delamination happens here too. |
2. Resin Types and Why They Matter
- Orthophthalic polyester: most production before 1990. The least water-resistant resin; the majority of osmosis cases sit in this group.
- Isophthalic polyester: widespread from the 1990s. Markedly lower water absorption.
- Vinylester: usually specified in the outer plies as a barrier. High water resistance.
- Epoxy: highest performance, highest cost. The critical rule in repair: epoxy bonds over polyester; polyester does not bond reliably over epoxy. That rule decides whether a repair already carried out will hold.
3. What the Survey Does
1. Moisture map
The hull is scanned on a grid with a Tramex-type meter and the values are read comparatively. An absolute number means little on its own; what matters is the difference against a reference area accepted as dry. For the reading to be valid, the boat must have been ashore and dry for at least a week. On a hull just lifted, everything reads high, and such a reading does not belong in a report.
2. Sounding
The hull is sounded systematically with a phenolic hammer. Sound laminate rings; delaminated laminate answers hollow. Sounding confirms or refutes what the moisture meter indicates. Neither is interpreted without the other.
3. Visual inspection and reading cracks
| Crack pattern | Likely cause | Severity |
|---|---|---|
| Spider web (fine, scattered) | Gelcoat ageing, moulding stress | Usually cosmetic, class C |
| Star pattern | Impact, radiating from one point | Laminate beneath is checked, class B |
| Crack along a straight line | Movement in the structure beneath (bulkhead, stringer) | Structural, A or B |
| Cracks at corners and fittings | Load concentration (chainplate, lifting eye, keel root) | Serious, class A |
| Smile crack at the keel root | Grounding or slack keel bolts | Class A; measurement also required |
4. Core inspection on sandwich construction
A wet core is the most expensive defect on a GRP boat. The signs: flex underfoot on deck, a hollow response on sounding, high readings over a wide area, darkening around deck fittings. Balsa rots when wet; PVC foam does not rot but loses its bond to the skins. Either way the repair means cutting the outer skin and replacing the core.
4. Osmosis: Diagnosis and Facts
Osmosis is water passing through the gelcoat, reacting with soluble matter in the laminate and raising blisters. Three common errors:
- "Moisture is high, so it must be osmosis." No. Moisture alone is not osmosis. Diagnosis needs blistering, the vinegar smell of the fluid inside a blister, and drying behaviour, read together.
- "There are blisters, the boat is finished." No. Osmosis is not structural collapse; hull strength is retained in most cases. It is a problem of cost and time, not danger.
- "We painted over it, it's fixed." It is not. The trapped moisture stays inside and returns within two seasons.
The correct sequence: peel the gelcoat → dry the laminate for weeks → confirm dryness by meter → fill and fair → epoxy barrier coat → new antifouling. Shortening the drying phase means doing the job twice.
5. What to Expect by Age
| Age | Typical agenda | Survey focus |
|---|---|---|
| 0-8 years | Build defects, warranty items, early hardware wear | Structural bonds, chainplate bedding, gelcoat cracking |
| 8-15 years | First major maintenance cycle: rig, shaft seal, electrics | Moisture trend, areas around deck hardware, engine hours and records |
| 15-25 years | Osmosis risk, wet core, systems due for renewal | Full moisture map, sounding of sandwich areas, age of wiring |
| 25+ years | The quality of past repairs decides the boat's future | Method of previous repairs, resin compatibility, modification records |
6. Is It Repairable? The Criteria
- Widespread osmosis: repairable. It is a cost and calendar item, not a structural one.
- Localised delamination: repairable. The area is opened, wet laminate removed, and rebuilt with compatible resin.
- Widespread wet core: repairable but expensive; the decision follows the value of the boat.
- Structural damage at the keel root: specialist work; the quality of the repair affects future insurability.
- Laminate after fire or prolonged submersion: caution is warranted, heat and long immersion can change resin properties irreversibly.
7. Buyer's Checklist
- How long has the boat been ashore? Less than a week and the moisture survey should wait.
- Is there flex underfoot on deck?
- Any darkening, cracking or staining around deck hardware?
- Has osmosis work been done before? If so, by what method and with how much drying time?
- Any crack indication at the keel root, or rust weeping in the bilge?
- Are there invoices and photographs for previous repairs?
Frequently Asked Questions
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Findings we often see on site
- Star cracks in gelcoat: Hairline cracks radiating from a centre around cleat and winch bases.
- Osmosis blisters below the waterline: Blisters from pinhead to lentil size under the waterline; some weep an acidic fluid when opened.
- Wet core in the transom: High moisture readings and a dull sound in the transom around the ladder and exhaust outlet.
Continue with this topic
- Seacocks and through-hulls: Materials and a six-step check.
- Hull, deck and joint: Laminate, sandwich core and the hull-deck joint on their own pages.