Causes of GRP Hull Defects:
(a) Causes of Defects:
(i) De-lamination:
- Improper resin mixing: Incorrect resin-to-hardener ratio can lead to incomplete curing, weakening the bond between layers.
- Insufficient resin application: Not enough resin between layers can create air pockets and weak bonds.
- Contamination: Dirt, grease, or moisture trapped between layers can prevent proper adhesion.
- Uneven pressure during layup: Inconsistent pressure can lead to voids and weak areas.
- Excessive sanding: Removing too much material can thin layers and reduce overall strength.
(ii) Osmotic Blisters:
- Water permeation: Diffusion of water molecules through the GRP laminate.
- Presence of salts: Dissolved salts in the water attract more water, creating osmotic pressure.
- Imperfect laminate: Micro-cracks, voids, or poor adhesion points allow water ingress.
- Lack of protective barrier: Inadequate gelcoat or anti-osmosis coating allows water contact.
- Temperature cycling: Expansion and contraction due to temperature changes can stress the laminate, exacerbating existing weaknesses.
(iii) Stress Cracking:
- Mechanical overload: Excessive loads exceeding the design limits of the hull.
- Impact damage: Punctures, dents, or other forceful impacts can initiate cracks.
- Poor design: Stress concentrations due to sharp corners, inadequate reinforcement, or improper load distribution.
- Fatigue: Repeated loading and unloading cycles can weaken the material and lead to crack propagation.
- Environmental factors: UV exposure, chemical attack, or temperature extremes can degrade the mechanical properties of GRP.
(b) Osmotic Blister Location:
Osmotic blisters most commonly occur on the underwater sections of the hull exposed to prolonged water contact. This includes the keel, bilge areas, garboards, and lower portions of the transom. These areas experience constant immersion and higher salt concentrations, making them more susceptible to water permeation and osmotic pressure buildup.