The through-hulls are one of the critical points of boats; a break or copious seepage from one of these holes can even lead to sinking. On the other hand, in order to allow an exchange of water between the interior of the boat and the sea, and with this also the correct functioning of much of the equipment on board, we have no choice but to install through-hulls.

As the seasons and owners change, the needs and equipment on board change, so we can end up with fairleads that are no longer in use: what to do? In theory, the wisest thing to do would be to remove everything and plug the hole the first time we wing the boat - after all, it is a hole in the hull that could cause us problems one day.

If our hull is made of fibreglass and we decide to get it out of the way, we have to plug the hole by laminating from scratch. What we commonly call fibreglass is a laminate of glass fibres and resin. The fibres give rigidity and strength, the resin makes the whole thing stick together. Various types of fibres and various types of resin are used in shipbuilding. Without going into too much detail, the most commonly used fibres are glass fibres, but there are many hulls on the nautical scene that have aramid fibres (Kevlar) inside them, which give greater rigidity and, above all, greater resistance to cutting and abrasion of the laminate. Furthermore, in recent decades, carbon fibres have made their way in, with which we can create laminates that are extremely light and rigid at the same time.

To facilitate the lamination phase, the fibres are arranged into fabrics. On the market, the most common are mat (categorised as non-woven fabrics) and multi-axial fabrics. In mats, the fibres are short and oriented in each direction. In multi-axial fabrics, the fibres are long and oriented in precise directions. This allows the fibres to be oriented along the axes of greatest stress, resulting in better performing laminates. Three resins are most commonly used in the marine industry: polyester, vinylester and epoxy.

The vast majority of boats that sail have been laminated with polyester resin, as it is the cheapest and easiest to work with on large mouldings. Vinylester resin has very similar characteristics to the previous one, but has greater water impermeability, so it is sometimes used in the first layers to improve osmosis resistance, less commonly for the entire lamination. Epoxy resin is the most expensive, but it has the best mechanical characteristics, shrinks less during catalysing, and also has greater adhesive power, making it the most suitable for repairs. In fact, we must bear in mind that during lay-up, layers of resin-soaked fibres are overlaid in a shorter time than the time it takes for the resin to catalyse, so that very strong chemical bonds are created between the resin molecules.

When, on the other hand, we repair a laminate, we are dealing with already catalysed resin, so the adhesive power of the resin we are going to overlay is decisive for the success of the repair. Resins are a two-component product: the mixing ratio by weight or volume is indicated in the technical specifications. So we have to organise ourselves with volumetric dispensers (pumps like those found in soap bottles) or a good scale. Volumetric dispensers are very convenient when the quantity of resin to be mixed is small, but for large quantities it is more convenient to use a scale. Having said all this, let us now return to our project of plugging a hull hole in a GRP boat. We will use epoxy resin and biaxial fabric suitable for use with such resin.

Below is a series of illustrative images.

Properly installed sea inlet, with reinforcement plate and no signs of corrosion. This sea socket had not been used for several years, so we decided to remove it.

Once the hull gasket has been removed, we use chisels to separate the plywood plate from the GRP. An operation that is not always easy, as the polyurethane sealants with which the hull gasket is sealed have a strong adhesive power.

Using 40/60 grit sandpaper, we remove the gelcoat and fibreglass, creating a good virgin surface on which to layer.

Intake removed. The hull, from the inside, was sanded and prepared for lamination.
Sea inlet hole on the outside of the hull.

Once we have thoroughly cleaned and degreased with acetone, we are ready to laminate, an easy operation, but one that must be carefully prepared; once we have our hands in the dough, soiled with resin, we must have everything at hand, to avoid wasting time and smearing resin on tools and even worse on the boat.
In addition, we can cover the areas around the area where we are going to work with nylon, so that even an accidental spill will not turn into a disaster.
I also recommend setting up a workbench covered with nylon, a material to which epoxy resin does not adhere. On this table we will have the resin and hardener containers with their dispensers, a container in which we can mix the two resin components with the help of a ladle, the various fibreglass discs cut from the fabric which we will laminate, a brush with short bristles to soak the fibreglass with resin and finally a bubble breaker roller. On this table, once the resin has been mixed, with the help of the brush we can soak the fibreglass discs with the right amount of resin; being able to carry out this operation on a flat surface facilitates the resin-coating operations and above all it will be easier to do a clean job!

Resin dispensers: the mixing ratio may vary from resin to resin, so we must check that the dispensers are suitable for the resin we are using. Alternatively, we can take two identical dispensers, and with these, by counting the volumes (how many times we operate the pump) we can obtain the mixing ratio indicated in the technical specifications.

Placement of the first layer of fabric.

By adding fibreglass discs of gradually smaller diameter, we create a laminate of the same thickness as the hull laminate, both inboard and outboard.

Layered covered with nylon foil.

With the help of the bubble breaker we are able to remove any air bubbles that may be trapped between the sheets. During this operation, we apply strong pressure to the laminate while it is still fresh, which will tend to slide into the hole, so we have to organise ourselves to prevent this from happening, for example with the help of a helper who pads from the side we are working on, equipped with gloves and a piece of nylon.
To achieve a smoother surface and prevent dirt from settling on the fresh layer, we can cover it with peel-ply or simple nylon as seen in the photo.

Once the resin has finished curing, we proceed with sanding.


And eventual grouting. At this point, we have restored the integrity of the hull. This will be followed by another finishing sanding and finally we will treat this area like the rest of the living structure.

On the inside, we can worry less about the finish and proceed directly with a painting of the new laminate.

As we could see from the pictures, plugging a hole in our hull is a job that requires a little time and some preparation, but the satisfaction of sailing with one less possible waterway is priceless!

For further information, we recommend this link where you can download a simple and clear guide on repairing and maintaining fibreglass boats. Download here!