On our Indian Ocean passage, we developed a deck leak. Whenever we took a big wave over the deck or when it was raining, we would get a slow, but steady drip from the header (ceiling) on the port side for an hour or so. It was particularly annoying when we were on a starboard tack, as this was when we use the port side settee as a sea berth. Each drip would land somewhere on the sea berth, until after an hour or so, the bedding would be sopping wet. This was bad enough, but probably one in three of these drips would splat right on the face of whichever of us was trying to sleep. All in all, it made for a very cranky crew.
The deck of Nine of Cups, like most fiberglass boats is constructed like a sandwich. The top and bottom layers, the bread of the sandwich, are fiberglass, while the filling of the sandwich is marine plywood. Attached to the underside of the deck is the header, which on Cups is constructed of wood panels. The top surface of the deck can develop a leak in a myriad of ways. All of the hatches, stanchions, brackets and deck fittings are attached by drilling or cutting holes in the deck. While every attempt is made to make these holes watertight, over time, some of them will begin to leak. In addition, cracks often develop due to the flexing of the hull. Once water makes it through the top layer of fiberglass, it eventually finds a path down through the core to the bottom layer. If it finds another opening or crack, it then drips down onto the header, and finally finds a screw hole or gap to make its escape into the interior – in this case, right above the sea berth.
Finding the source of a leak is important, not only to get rid of the annoying drip, but also because if enough water gets into the deck, the core will begin to deteriorate and/or lose its bond with the fiberglass layers. The result is a weakened, spongy deck.
Locating the leak is not always easy, however. The point of ingress through the top layer of fiberglass could be several feet away from where the drip materializes in the cabin. If the header can be removed, the spot that the water is exiting the lower layer of fiberglass can sometimes be found, narrowing the search somewhat.
The next step is to go topsides and reconnoiter the deck area in the vicinity of the leak below. The point of ingress will not necessarily be near the leak below, but it will most likely be uphill from it. Try to identify all the possibilities. The most likely candidates will be cracks or dings in the gelcoat or loose screws or fittings, but anything that pierces the upper layer of fiberglass is the possible source – no matter how watertight it appears.
The possible sources for the leak over our sea berth included, in the order of likelihood, the main salon hatch, which was close to and just uphill of the leak below, one of several blocks at the foot of the mast, the screws holding the track that secures the bottom of the dodger, or one of several cleats and clutches in the cockpit. Starting with the lowest possible source, inundate each candidate with water, one at a time and go below to see whether you've found the leak. Try to keep the water from running over other possible sources until they have been eliminated.
My leak turned out to be none of my candidates. I discovered, after an hour or so of searching, that if I poured water into the seam between the instrument cover and the cockpit coaming, the drip would appear below. I removed the instrument cover and hatch slider, and found – way, way in back, under the cockpit coaming, a small crack in the gelcoat. Sure enough, if I poured a cup of water onto it, the drip below would start and continue for half an hour or so. Other than the difficulty of reaching the crack, the repair was relatively easy – since it was hidden away under the coaming, the repair would not need to be faired and painted.
Don Casey's book, This Old Boat, is a great source of information on making repairs to the deck and rebedding fittings to prevent and repair leaks. Also, West System not only provides great epoxy, they are also are a wealth of information on making repairs.