Hull materials and hull repair: fiberglass, wood and aluminum

The hull is the boat's basic protection against water, and as a boat mechanic you encounter three main materials: fiberglass (GRP), wood and metal — typically aluminum or steel. Each material has its strengths, its weaknesses and its own unique damage, and much of the profession involves being able to recognize and correct them before they become expensive or dangerous.

Most leisure boats are built in glass-fiber-reinforced polyester (GRP — glass reinforced plastic). The hull is laminated from fiberglass mats and polyester resin, and the outer glossy surface is gelcoat, a thick coloured layer that protects the laminate from water and UV. GRP is easy to shape, relatively inexpensive to repair and tolerates salt water far better than untreated wood, which is the main reason the material essentially took over the leisure boat market from the mid-1900s.

One of the most known problems on fibreglass boats is osmosis popularly called fiberglass blight. Gelcoat and polyester are not completely watertight and over time water molecules can penetrate through the gelcoat and into the laminate. Here the water dissolves water-soluble substances in the resin and reacts chemically with them — a process technically called hydrolysis — and forms an acidic viscous fluid. The fluid cannot penetrate back through the gelcoat so osmotic pressure builds up pulling even more water in and the result is the characteristic blisters under the waterline.

• 01Small, soft blisters below the waterline, often with a sour smell when punctured
• 02Soft or brittle laminate in severe cases
• 03Symptoms typically appear only after several years in the water
• 04Treatment requires that the hull is completely dried before new gelcoat and barrier layer is applied

For centuries wood was the only hull material, and many older and classic boats are still built from wood. A wooden hull requires constant care: the planks must be kept tight (caulked), the wood must be protected against rot and fungus, and the paint or varnish must be maintained so water does not penetrate the wood itself. Rot damage is typically found at joints, fittings and other places where moisture can collect, and repair often requires replacing entire planks or frames, not just surface treatment.

Aluminum is used especially for work boats, smaller commercial vessels, and robust recreational boats because it is light and strong. On the other hand, aluminum is sensitive to galvanic corrosion if it comes into electrical contact with a more noble metal in saltwater – something you as a boat mechanic must take into account when installing fittings and equipment. Steel is mostly used for larger commercial vessels where strength outweighs weight and where continuous corrosion protection is a permanent part of maintenance.

A fibreglass repair typically consists of grinding or cutting away the damaged laminate laying up new fibreglass and resin or epoxy in layers and finally post-treating the surface with new gelcoat that is ground and polished so the repair cannot be seen. With osmosis the hull must first be allowed to dry for weeks or months before treatment makes sense — if you repair while the laminate is still wet the problem will return.