GRP Core repair

It used to be the news that any owner of a boat with a Glass Re-enforced Plastic hull would dread, 'the survey has found moisture in the GRP core'. Although it is possible to dry out small sections using hot-vac treatment, invariably when the rot has set in it spreads rapidly throughout the hull; condemning the boat to a death sentence.

Osmotech, however, have perfected the delicate art of removing rotten or water damaged cores, and using more advanced materials, replacing them to not only save the boat but actually to extend its life too. Moisture in the GRP core is usually due to holes in the outer plastic either caused by hitting debris and flotsam when out on the water, or often by poor workmanship when installing fittings that pierce the hull (such as bow-thrusters or exit valves).

The hull of a GRP boat is made by sandwiching a core, usually balsa wood or high density foam, between an inner and outer 'skin' of fibreglass, made by layering fine mesh fabric and polyester resin to build a tough and rigid structure. The surfaces are then sanded smooth, painted and anti-foul added. Motorboats with angles on the hull are also built with the same method, although solid GRP is used in the angles. This means that although the repair process is essentially the same for both types of hull-shape, light-weight sailing yachts are often much more delicate.

The first stage is to ascertain how widespread the damp is and to locate the areas that will need to be re-cored. By using a combination of moisture sensors and boring small core samples out of the GRP, an external work area map can be drawn onto the hull. The second stage is to then carefully cut away the outer layers of fibreglass skin to reveal the core, where a more accurate assessment can be made. Again, a work area is designated and the wet core removed by hand, so as not to damage the inner layer of fibreglass skin. With a sailing yacht, much of the boat's hull strength is based on the integrity of the GRP, so large sections cannot be cut away at once without the risk of the boat bending or bowing on its supports Osmotech attach extra thick fibreglass wraparound pads for the stands directly on to the hull, giving much needed rigidity whilst the damaged sections are cut away.

Once the rotten balsa wood is removed, the inner skin is carefully checked for perforations, patched if necessary and then re-sealed before the new core is laid. The third step is to add the new core to the exposed areas and Osmotech are now increasingly recommending that their clients use high density foam instead of balsa. The densely packed foam is just as light and strong as balsa, but its 'closed-cell' manufacture means that the even if water does penetrate the outer skin again, only a tiny area will be affected and it will not spread to neighbouring cells. New fibreglass pads for the supports are then laid on the repaired areas; new supports installed; the old ones removed and the process starts again on the adjoining section. On a large motorboat with a strong internal structure this may mean only two or three phases, but on a more fragile sailing yacht this may mean four or five.

When the re-coring is complete down the entire length of the boat, from the bows to the transom, the whole boat is hand sanded with 'long-board' sanders to achieve a perfect surface. The boat is then taken to the paint shed for painting, gelling and antifouling.

Prevention

We are very willing and able to do re-coring work, and whilst accidental damage is often unavoidable, most water damage can be avoided. Whenever fitting any aftermarket system that requires the hull to be breached, it is essential to ensure that the hole is sealed with a strong layer of solid GRP (1). When attaching anything to the interior of the hull - either above or below the waterline, it is vital that the screws do not pierce the skin, but that they are fixed to an additional block of GRP (2) that does not compromise the hull's integrity.