Definition
GRP, glass-reinforced plastic (also known as fibreglass), is a composite material made by combining woven or chopped glass fibre with a polyester, vinylester, or epoxy resin. It is the dominant hull material across the production tender market, valued for its low cost, repairability, and consistent serial-build behaviour.
Background and use
Most tenders below the custom carbon segment are built in GRP. The base process is to lay the gel coat into a polished mould, build up successive layers of glass fabric impregnated with resin, and cure the laminate. Production yards (Williams, Castoldi, Zar, Brig at the smaller end; Pascoe and Wajer for parts of their range) use GRP because it scales: a tooled mould produces hundreds of identical hulls with predictable cost and weight.
Within GRP there is a wide quality range. Hand-laid solid laminates with chopped strand mat are the cheapest and heaviest. Vacuum-infused multi-axial fabric with a closed-cell foam core gives a much higher strength-to-weight ratio at higher cost; this is the modern production standard for premium GRP tenders. Resin choice matters too: vinylester resists osmosis and water absorption better than polyester, epoxy better still, and most premium builders use vinylester at a minimum for the outer skin.
Compared to carbon composite, GRP is heavier (often 25-40%), more flexible, and cheaper to build. Compared to aluminium, it is quieter, easier to mould into complex shapes, and avoids electrolytic corrosion concerns. The repair story is the headline practical advantage: every reasonable boatyard can patch GRP. Carbon repair requires specialist skills and access to vacuum equipment.
Related considerations
- Confirm core material on infused builds; PVC foam, end-grain balsa, and Nomex have very different water-ingress and impact characteristics.
- Osmotic blistering is the GRP-specific aging issue; treatment is straightforward but expensive at refit.
- Gel-coat condition is the visual marker; chalking and crazing indicate UV degradation and may signal deeper laminate issues.
- Weight tracking matters; GRP tenders gain mass through ownership as water absorbs into the laminate.
- Surveyor reports should always include moisture-meter readings on used GRP hulls.