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Damaged cabin interior due to leak

Rotted deck core due to failed sealant

Drips coming into your cabin are not only an annoyance, but over time, can be quite detrimental to the cosmetic and structural integrity of the boat. The source of any leak should be found and fixed straight away. Fixing them is usually not too large of a problem. Finding them, however, is not always as simple as it may seem. Where water drips into the cabin can be very far away from the actual entry point thru the deck. Analyzing the obvious, and the not-so-obvious clues have led some to call us the “Sherlock Holmes” of deck leak investigations.


Why it Happens

The sealers used by boat manufactures on their ports, hatches, deck hardware, etc. installations will likely fail at some point. This allows water to find its way through, quite possibly dripping right into your dry, cozy cabin. This is not uncommon, even on relatively new yachts. On older boats, say over 15 or 20 years old or so, it is a fact of boat life. And one which must be attended to before too much damage is done not only to the area where the drip is settling inside the cabin, but more importantly to the areas of the boat in which the leak travels that can’t necessarily be seen. 


Just like your body, knowing when there is something wrong before it gets quietly much worse, can be a saviour. That drip into the cabin is an obvious indication that there is a problem. However, sometimes leaks don’t always make it through to the cabin, and this scenario could be far worse. Even when a leak does show itself by dripping into the cabin, it is quite possible that some internal structural damage to the deck or cabin sides has already started to develop. That being said, if there is any kind of obvious new leak that appears, it should immediately be rectified before any internal core material is affected.


How it Happens

Most modern production boat’s decks are made of a sandwich-type layup. (See illustration at left.) The deck surface---that on which you walk and to which you mount hardware---is normally made with layers of fiberglass built up to a certain mill-thickness somewhere around an 1/8 of an inch or so. Some decks are thicker, some thinner. The outside surface, or top surface, is usually finished with a thin layer of gelcoat which gives it a smooth, shiny look.  


The underside of the deck---the part facing the inside of the cabin---is also made of fiberglass layup like the top surface, but usually there is no gelcoat on the inside surface. 


Sandwiched between these two fiberglass layups is some sort of lightweight core material---the meat of the sandwich---usually about 3/8ths to 1/2 of an inch in thickness. This material is often made of a hard foam, some sort of marine plywood, or a very light balsa wood material. When the deck is manufactured, all these elements are strongly bonded together with epoxy or polyester resins and are 100% impervious to water intrusion. However, the moment a drill or saw goes through the deck to add a piece of hardware or cut out for a port, that 100% is greatly compromised. Though the the fiberglass top and bottom is still not very suseptable to absorbing moisture, the core material most certainly is. And this is where the problem lies. Once moisture gets in, it can continue to travel deeper within the core. Once it saturates the core, the adhesive holding the core to the fiberglass will begin to give up. The wetter it all gets the more damage can be done. 


Now, in theory, since a piece of hardware or a hatch or portlight is mounted at the factory using a sealant around the fastener holes and/or the cut out opening, it makes sense that no water should get to the core. And for the most part that is the case, at least initially. 


However, since a boat is constantly flexing and some tremendous stresses can be put on deck hardware, it is quite common for voids in the sealant to develop over time. Once moisture can get through that void, the problem has begun and it will continue to work its way deeper and deeper into the core. The deeper it gets the more damage can be done. One small leak at a chain plate cover on a sailboat could eventually grow into a large portion of surrounding deck being rotted out and structurally unsafe.  And it can happen over a relatively short amount of time. This can be an extremely expensive repair. 


The info offered here is not to scare you, but it is to warn you of the consequences. The message here is to:


Pay Attention to any leaks. Have their sources found and any resulting damage repaired as soon as possible! 


Why Are They So Hard to Find?

Leaks are often pretty easy to see where they enter the cabin, but are not necessarily as easy to know where they first make entry through the outside of the boat. As stated earlier, the source of a leak can be far from where the drip inside the cabin appears. Cabin headliners, for instance, can easily divert a leak through the deck up near the mast on a sailboat, and lead it down inside the liner and out in the form of a drip off the corner of one of the side portlights. This, of course, could lead one to believe that the port is leaking, when in fact the water is leaking in from two or three yards away. 


Another common scenario might be where water gets in through the sealant at the top skin of fiberglass, either around a fastener hole on a mounted piece of hardware or at a cut out through the deck for a hatch or port.  Some types of hardware are only screwed into the deck, instead of totally through the deck where they would have a nut installed inside the boat. If only screwed into the deck, any water that gets through the deck skin will travel down the screw. Since the screw does not go through the deck and into the cabin, that water intrusion won’t show up as a drip into the cabin, never alerting you to the fact that there is a leak. Instead, that water will be absorbed into the deck core, saturating it enough that the lower skin of fiberglass will separate from the core. This then will allow a clear path for the water to run anywhere and everywhere up inside the fiberglass skin until eventually finding a way out into the cabin. With any luck, it may show up in the form of a drip that you can see. At least then you’ll realize that there is an issue. However, the water may find its way over to the side of the boat and run into the boat along the insides of the hull, never giving you the opportunity to notice that there is a leak. At least, not until enough damage has been done to the core that it becomes clear. 


Leaks are tough. If you got some, Sherlock and the crew at DAKOTA MARINE are ready to crack the case.





For hardware mountings that are far less likely to leak, see our Proper Hardware Installations pages.

We’re the Sherlock Holmes of Cabin & Deck Leak Investigations


Serving Long Island Sound to the Caribbean and Beyond, since 2001