1. Small High-Speed Craft and Racing Boats: Extreme Lightweight, Cost Secondary
These boats prioritize weight reduction above all else, typically using carbon/glass hybrid skins + top-tier foam cores.
- Preferred Core: PMI Foam (Polymethacrylimide)
- Density: 50–80 kg/m³
- Rationale: Offers the highest specific strength and modulus among foams, with high heat resistance (180–240°C). It can withstand the elevated curing temperatures of vacuum infusion or prepreg processes, allowing the hull to be as thin as an eggshell while maintaining stiffness.
- Typical Structure: Skins are a hybrid layup of unidirectional carbon fiber and glass fiber, with a core thickness of 15–25 mm.
- Alternative: SAN Foam (Styrene-Acrylonitrile)
- Density: 60–100 kg/m³
- Rationale: Higher stiffness than PVC, with excellent weight and water absorption control. Commonly used in racing shells and sailboats. However, its impact toughness is slightly inferior to PMI, making it unsuitable for areas subject to frequent impacts.
2. Mid-Sized Yachts and Workboats: Balanced Performance, Cost-Effective
This is the most common application range for FRP vessels. The solution focuses on impact resistance, fatigue endurance, and ease of processing.
- Primary Choice: Cross-Linked PVC Foam
- Density Selection: 80–100 kg/m³ for the hull shell, 60–80 kg/m³ for the deck.
- Reference Brands: DIAB Divinycell H series, or Gurit G-PET PVC product line.
- Rationale: Excellent toughness, capable of resisting low-speed dock collisions and wave slamming. Its good elongation allows it to deform compatibly with the FRP skins, reducing the risk of interfacial debonding. It is well-suited for vacuum infusion, with low resin absorption.
- Upgrade Option (Balancing Eco-Friendliness and Heat Resistance): PET Foam
- Density: 100–130 kg/m³
- Rationale: If the hull has dark-colored paint, PET can withstand up to 150°C, outperforming standard PVC. As a thermoplastic, it offers good fatigue resistance and is recyclable. Using it to replace PVC in hull shells and bulkheads is a growing trend.
- Note: To achieve equivalent stiffness, a slightly higher density than PVC may be required.
3. Impact and High-Load Zones: Localized Reinforcement Solutions
In areas like the forward bottom hull, bilge, and engine mounts, foam alone is insufficient. A transition to higher-strength or tougher core materials is necessary.
- Option A: End-Grain Balsa Wood
- Application: Local replacement with 100–150 kg/m³ end-grain balsa in high-impact zones below the waterline at the bow.
- Rationale: Its compressive and shear strength per unit weight far exceeds that of foams of similar density, enabling it to withstand berthing impacts and grounding loads. However, the edges must be meticulously sealed to prevent water ingress and rot.
- Option B: PP Honeycomb (Polypropylene)
- Application: Deckhouses and interior partitions of high-speed craft.
- Rationale: Extremely low water absorption and excellent toughness; it can even be hammer-formed and is impervious to repeated impacts. However, due to its honeycomb surface, an adhesive film is required for bonding; dry fabric cannot be directly vacuum-bagged onto it.
4. Key Design and Process Constraints
The success of an FRP sandwich solution depends on adhering to several hard process conditions:
- Core Thickness and Curvature Compatibility
- For areas with tight curvature, it is recommended to use grid-scored or grooved foam cores (e.g., Divinycell H in grid-scored form). These can flexibly conform to the hull’s curved surfaces and also allow smooth resin flow.
- PMI and SAN foams are relatively brittle and should not be heat-bent excessively. PET and PVC offer better thermoformability.
- Preventing “Hard Spots” and Stress Concentrations
- The stiffness of a sandwich structure far exceeds that of a single skin. Around all penetrations (such as portholes and cleats), local solid FRP reinforcement must be applied, or a gradual transition using high-density foam must be created to prevent hard spots from causing hull cracking.
- Skin Layup Design
- Typical Layup: Outer gelcoat + 300 g/m² CSM (resin-rich layer for waterproofing) + multi-axial glass fabrics (0°/±45°/90° combination) + core + symmetrical inner skin.
- Core Principle: The skins must be symmetrical about the core, with a thickness deviation of no more than 0.2 mm. Otherwise, significant warping will occur after curing.
- Cores for Vacuum Infusion
- Low-resin-uptake cores with a high closed-cell content must be selected. PVC, PET, and PMI all have dense surfaces that keep resin absorption controllable.
- For honeycomb structures, an adhesive film barrier must be used to prevent resin from flooding the cells, which would lead to an uncontrolled weight increase.
