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Deck Leak Deliverance

By Tom Neale, 7/29/2004

Deck Leak Deliverance

Whether you have a runabout with decks around the sides or a full blown yacht, deck leaks can range from bothersome to extremely dangerous. Here are some special tricks for dealing with and fixing deck leaks.

Finding the Fickle Trickle

If you can’t locate the source of a leak, there is a trick that will almost always find it. All it takes is two people and a hose. First study the overhead layout around the area where the water is coming through below. Check out things like slope of deck around the area where you’re actually seeing the water below decks. Look for things like indentations in the head liner. Sometimes the manufacturer will mold in an indentation for stiffening, but this can also make a great little channel for water to run. Look out the portholes, if there are any nearby, to see what sources may be relevant on deck. These would include anything that pierces the deck. Stanchions and cleats are obvious, but also consider things like fuel and water filling ports and through deck wiring runs. When you’re looking out the porthole observe the slant in the deck. Remember that drips will often run down under the deck, clinging to its underside because of surface tension, before dropping off onto a liner to resume the journey.

Next have a partner go up on deck with a hose. Start running water from the hose, beginning at the lowest spot on deck that could possibly be relevant. Use the natural flow of the hose, don’t use a high pressure nozzle. The trick is to work your way up, possibility by possibility, until the person below sees water appear. It’s important to not move the stream to an area without first wetting all other possible sources below it. When you’re letting the water run on each spot, be patient. Don’t move to the next spot too soon. Sometimes it takes a while for the water to seep through and find its way down to where your partner is watching. But if you methodically move from possibility to possibility, going from the lowest ones and up, you’ll find the leak.

Extra Tricky Trickles

Sometimes, while a boat is being built, a worker will accidentally drill too far when installing a fixture, resulting in a deck hole that isn’t supposed to be there. Usually the worker will patch this hole. The patch may be done quickly with whatever is on hand, in an attempt to “fix it” before the supervisor sees it. These patches may fail later, resulting in a leak that’s hard to identify, perhaps in an area of open deck or cabin top. The spot will probably appear as a slight aberration in the deck top finish. Consider this possibility when you’re really stumped. We experienced this on a previous boat. They guy who had installed a cabin light drilled right through the cabin top. When the quick patch failed, the light fixture filled up with water in a rain storm.

Leaks where the hull and deck are joined are not uncommon, but should be taken very seriously. Both sailboats and fast power boats can stress this joint considerably. Usually these leaks are hard to repair, because of difficulty of access. Sometimes removal of the rub rail may reveal the problem, but not necessarily. Often you must pull out interior lining. Anytime you find water coming in here, slow down and do whatever else you can to relieve stress. It could be a sign of over stress with developing separation. Some hull deck joints are sealed in part by glassing over the joint, inside, after they are put together and before the interior of the boat is built or laid in. If done well, this is my favorite construction. But this method can allow water that leaks through one end of the boat to run long distances, between the glass tape and the joint, to come out in a void in the tape at a lower level. Consider this type of possibility if you’re exploring for leaks with a hose.

Deck Leak Dangers

When you’re installing wiring, always consider that deck leaks may be there but unknown, or may develop in the future. If, for example, you’ve used an open butt end connector to splice a wire together and pulled it into a wire run that goes under the deck, you may end up with a fire, or at least a shorted out wire. It’s important to use well insulated wire intended for the purpose, such as Ancor Boat Cable. It’s also important to not have any splices, or connections within these runs (or, for that matter, anywhere else that isn’t dry and that you can’t readily inspect).

A gravely dangerous leak area may be around the deck fill for your fuel tank. A small deck leak may, over the years, corrode the ground wire that should be fastened to the fill fitting. Even if the wire is still connected to the fill, it may be internally corroded enough to lessen its effectiveness. This is especially true if you boat in salt water. This wire is there to help prevent sparks and its integrity and that of its connections is very important. Regularly check this wire and be sure you don’t have leaks in that vicinity. If you see any green on the wire, remove it, check it, and refasten it. Then spray with a corrosion inhibiting product.

Many electronics installers go to great lengths (and you go to great expense) to install units so that you only see the pretty face, with the rest hidden behind a teak or fiberglass panel. Often these are installed where leaks are likely to occur, such as in electronics areas under T tops in open fishing boats, and in pedestals in the cockpit of sailboats. If your boat is like this, regularly inspect inside the cover to be sure all is dry.

Window Waterfalls

Sliding cabin windows almost always present problems in driving rain or spray. Generally, this type of window, while convenient and popular on many power vessels, just doesn’t do well in this type of heavy weather use and therefore, in weather bad enough, you may just have to lay out the towels. But there are things you can do to help. These windows are normally installed with channels at the bottom to drain water away to keep it from running over the inside frame lip. Check to see that the drain channels and the drain holes for these are large enough and not obstructed. Usually these windows have swipes or brushes of cloth or rubber to seal between the two panes of glass yet allow one pane to slide over the other, while keeping out some moisture, bugs, etc. When these become worn or brittle from age or UV exposure, they will allow water to be pushed in between the panes, particularly in high winds—as when you are running fast. Replace these regularly. Spraying them with a compatible lubricant such as silicone will help in a pinch, although it will probably smear the pane of glass if you try to open the window. On many boats with side windows, you can avoid most or all of the leaking if you have window covers made so that you can quickly snap them over the window when you expect heavy boarding spray or wind driven rain. If you want light and visibility while they are on, use clear vinyl framed in Sunbrella or Weblon. Be sure to seal the screws of the snap bases so that they won’t cause leaks themselves.

Portholes, whether made of plastic or metal, are almost always made of a different material than that of the hull or cabin side in which they are mounted. Because of this they often react differently to heat and cold, and respond differently to stress resulting from the boat working in seas. Thus they make a prime suspect area, not only between the frame and the side, but also between the glass or plastic and the frame of the port hole.

To learn more about hot to fix deck leaks, both on an emergency and on a long term basis, go to the Tom’s Tips section on

Copyright 2004-2005 Tom Neale