No boat owner should risk being complacent about deck leaks – a fibreglass deck will eventually become structurally compromised if water reaches the foam or balsa core, resulting in much more maintenance if they are left. In theory builders of such boats should remove core material from the location of any through-bolted fittings. However, although modern builders tend to be meticulous about this, it was by no means universally the case with older boats.

Fix leaking decks: How to bed fittings

Properly bedded deck fittings should be capable of decades of use without developing leaks.

In addition, while the foam used in the core of the deck should be fully wetted out with resin, there’s also no guarantee of this, particularly for boats built before vacuum bagging became a widespread construction technique at the start of the 21st century. The problem is that when the foam gets wet it quickly loses its bond with the fibreglass on each side of the sandwich, considerably reducing the stiffness of the structure. Boats with a balsa core deck tend to be even more prone to damage caused by water ingress. In addition, in extreme cases of deck leaks, water can also penetrate bulkheads, leading to further significant structural damage.

The heart of the problem

Very often the problem of deck leaks is caused by well-intentioned owners bedding new deck fittings down with a silicone sealant. The trouble with this type of sealant is that, to survive in a marine environment, a degree of adhesion to both surfaces is essential. Silicones, however, have a very low level of adhesion and will start to leak once fittings move under load, or the boat around them distorts, for instance when sailing in a big sea.

This leaves two main options for effective sealants – a marine polyurethane adhesive sealant such as Sikaflex 291, or a polysulphide such as Life Caulk. Once cured the former creates a permanent bond and is ideal for fittings that you never expect to remove. Polyurethane adhesives are available with different levels of adhesion – Sikaflex 291 and 3M 4200, for instance, have a lower level for use with general deck fittings, while Sikaflex 292 and 3M 5200 have a high level of adhesion for high load items.

Polysulphides never fully set, so they are ideal for fittings that you know may need to be replaced or repositioned at a later date. Like polyurethane sealants they stick tenaciously to almost any substrate, however, they are not suitable for use with some plastics as the solvents used may cause problems.

Teak decks

Leaks through a teak deck are particularly difficult to trace, but should be rectified as soon as possible and certainly before there’s any risk of water or frost damage lifting the timber from the deck below. Teak decks fitted in the past 10-12 years are likely to have been glued in place, however earlier versions relied on hundreds of screws to hold them in place. When the teak is nearing the end of its life these can be a major source of leaks, which in a number of cases allow water directly into the foam or balsa cores of the fibreglass sub-deck.

Surface preparation

Even the very best sealant is ineffective without proper surface preparation – this is the most important part of the job. Almost nothing will stick permanently to most silicones, so the first part of the preparation is to meticulously remove all traces of the previous sealant. This may involve a combination of scraping, sanding and the application of solvents, including proprietary silicone removers. Once the deck is free of silicone, a coarse abrasive paper can be used to help provide a mechanical key, before thoroughly cleaning the area with acetone or white spirit.

Making a neat job

One problem with any sealant is dealing with the excess – it's messy to work with, but looks untidy if left in place. Since bolt holes are the only route for water ingress into the boat, a single bead of sealant run along the line of the holes, and encircling each one, is all that's required for a successful outcome. For a larger job it’s possible to calculate the amount of sealant that will be needed to produce a uniform layer 2mm thick, leaving a margin for excess.

To make tidying up easier, where possible apply masking tape to both the deck and the fitting, so that sealant squeezed out of the joint can be easily cut away with a sharp knife once it has mostly cured – typically after 4-10 hours for polyurethanes, depending on the ambient temperature.

Fix leaking decks: applying sealant

Applying sealant around the bolt holes, after masking the deck around each fitting.

Fix leaking decks

The sealant comes away neatly, although it would have been even better in this case to allow it to cure for a little longer.

The easiest way of doing this is to dry fit the components first so that the deck around the new fittings can be masked accurately before applying the sealant. This technique works well and is a quick route to a professional looking result, providing the sealant is removed at the right time – too early and it gets really messy, too late and it can be awkward to remove.

Written by: Rupert Holmes
Rupert Holmes has more than 70,000 miles of offshore cruising and racing experience, in waters ranging from the North Sea to the Southern Ocean and Cape Horn. He writes about all aspects of boat ownership and marine travel, including destinations, seamanship and maintenance, as well as undertaking regular new boat and gear tests. He currently sails around 5,000 miles per year and in the past couple of seasons has cruised from the UK to the Azores, as well as winning his class in the 2014 two-handed Round Britain and Ireland Race. He also owns two yachts, one based in the Mediterranean and the other in the UK.