GRP boats are the great majority of recreational craft built today. A moulded hull needs maintenance but does not corrode, and suits series production. Quality depends largely on how the laminate is made.
Production method
- Hand lay-up
- Fabric is laid in the mould and wetted out with rollers. Flexible, but resin ratio and void control depend on the laminator.
- Vacuum infusion
- Dry fabric is filled with resin under vacuum; resin content is low and consistent, and weight falls.
- Series and one-off
- In series production the mould cost is spread across boats; one-offs use a temporary mould or plug.
Critical points in GRP
- Osmosis resistance: water-resistant resin in the outer layers and a good cure.
- Core: core below the waterline, and protecting core with fill at hardware points.
- Internal structure bond: floors and frames bonded to the skin; the most common production weakness is here.
- Shop conditions: temperature and humidity directly affect cure.
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Why the most common material?
GRP, glass-reinforced plastic, suits moulded production, so it is used in the great majority of series-built boats. It is relatively easy to maintain and does not rot or rust. It can be used for one-off builds too; then the mould cost falls on one boat, or mould-less methods are chosen.
Production methods
Hand lay-up is the commonest and most flexible method; its quality depends heavily on workmanship. In vacuum infusion, fibre is laid dry in the mould and resin drawn through by vacuum; fibre content is higher and the laminate lighter. In sandwich construction, a core goes between two skins; weight drops and stiffness rises. Each method has its own check points.
Against osmosis
To reduce water uptake below the waterline, a suitable layer and resin system are chosen behind the gelcoat. A barrier coat below the waterline on a new boat is also an option. These choices directly affect the osmosis risk the boat will meet over many years.
Secondary bonding
Bulkheads, floors and internal structure are laminated to the skin afterwards. Surface preparation and sufficient width of these bonds decide the structure's life. A weak secondary bond can separate over the years and start movement in the internal structure.
Evidence of quality
On a GRP boat, the evidence of quality is the lamination record, thickness readings and check-point photos; they stay in the handover file.
For used boats
On survey, the same points are read in reverse: moisture below the waterline, sounding for voids and a close look at secondary bonds inside.
Knowing the production method tells the surveyor where to look hardest: around fittings on a sandwich deck, or the underwater laminate on a single-skin hull. Maintenance is relatively easy, but gelcoat cracks, deck hardware bedding and the underwater coating need regular checks.
Lifespan
With these checks made regularly, a GRP boat can be used for decades; neglected, a small leak can grow into a costly repair that spreads into core and internal structure. That is why a maintenance record is part of a boat's value, and why buyers ask to see it.
A well-kept record strengthens the seller's hand in negotiation, because it answers questions before they are asked.
Keep it current, keep the receipts, and keep photos of every repair.
Buyers notice the difference immediately.