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Leaking decks are perceived as a nuisance by some boat owners, who are often prepared to put up with them and turn a blind eye – “That’s boats for you!”. The reason for this might simply be explained because many boats are only used when the weather is fine.

If rain isn’t falling and water is not washing over the decks then a leaking deck can easily be forgotten about when the sun is shining, even if there are telltale signs of water ingress below in the cabin and bilges.

The problem here is that if the leaks are ignored a much more serious situation may well be developing, especially in the case of boats with balsa or plywood deck cores. Rot is likely to be occurring. Rot can’t be ignored on a boat, even on a fibreglass boat, as wood is used to strengthen the structure both of the hull and deck. So deck leaks do need to be investigated and dealt with.

Leaking decks

It can be quite a challenge to find the source of a leak. It helps if two people are involved in the search, with someone up on deck with a bucket of water or a hose and another down below looking out for the drips. Common sources of leaks include:

  • Deck glands – the fittings where wires and cables pass through the deck. These can fail because the rubber gaskets wear out, or sometimes the glands are not the right size for the wires passing through them. These can be fixed easily by replacing the rubber gaskets or by using a good quality silicone sealant such as Sikaflex 291i, which is ideal for bedding deck hardware.
  • Deck fittings – the average boat has a considerable number of deck fittings serving a variety of purposes. Each one of these could be the potential source of a leak. Removing them all and re-bedding them would be a major task, ideally done when the boat undergoes a major refit. It is best to start with the most likely culprits, those that are subject to heavy loads. These include mooring cleats, stanchion bases, genoa tracks, winch bases, toerails as well as pushpit and pulpit bases. The fact is that a lot of deck fittings are subjected to heavy loads and all have to endure a marine environment. 

Leaking decks

  • Windows – leaking windows are easy enough to detect but if in doubt the bucket of water test or hosing the window from outside will confirm if there is a problem. Window seals fail when the gasket or sealant hardens with age and no longer provides a flexible seal. The temptation is to try a quick fix by simply running some new sealant around the edge of the window – not the answer. Repairing a leaking window can be a time consuming job if it is done properly. The window needs to be removed completely in order to replace the seals. 
  • Chainplates – it is wise to do an annual check of the chainplates. Chain plates fail for the same reasons standing rigging fails – corrosion and metal fatigue. On many boats access to the bolts that secure the plates to the bulkheads or knees is difficult as they are buried out of sight behind bulkheads, or glassed in completely. If a bulkhead shows signs of damp, then a leaking chainplate could be the cause. A full inspection needs to be done, not only of the chainplate fixing itself but also checking for rot in the deck core and where the plate is attached. If you suspect this may be the case, then access is essential as the fitting will need to be cleaned up and re-sealed with a flexible non-setting sealant and possibly new bolts will be needed, depending on their condition. Stemhead chainplates which secure the forestay can also be difficult to access. Some are located in anchor lockers where they come into contact with salt water, which is not at all good. 

Leacking boat deck

  • Hull-to-deck joint – most GRP boats are constructed with separate hulls and decks. These can be joined together in a number of ways, with some having an inboard flange on the top edge of the hull onto which the deck is bolted; others having an outboard facing flange on the hull and deck; a third type have a joint where the deck fits over the hull – like the lid of a shoebox. All joint types are coated with sealant, bolted together and rubber or wooden strips are added to finish them off. These joints can fail after impact damage or when a boat has been subjected to the considerable stresses incurred when pounding through rough seas, causing the hull and deck to twist, distort and ultimately separate.

 Filling old screw holes

 When deck fittings are removed or upgraded the old screw holes need to be filled to ensure no water can find its way into the foam or wooden core beneath. The best way to do this is to clean the old hole up with acetone, or if necessary first enlarge it with a slightly bigger diameter drill bit and then clean and dry it. Next fill the hole with epoxy resin mixed with glass bubbles which results in a strong, lightweight filler that can be sanded and then painted.

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The gelcoat topsides of a GRP boat can be pampered and restored to their former glory relatively easily when it is ashore. Gelcoat is only a very thin outer layer of the hull, often less than 1mm thick, so you should avoid cleaning it with highly abrasive cleaners, or an-ything that could potentially damage its surface.

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