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How To Find, Stop & Seal Water Leaks

The problem was easy to see. Figuring out the solution was another matter.
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The Last Drop

The new owner of an older 60-foot fiberglass trawler expected to upgrade electronics and do a little varnish work on his first yard visit. Unfortunately, after the first real rainstorm, he found a few puddles inside. An inspection showed nothing obviously loose on deck, and no evidence of staining below—yet.

He knew the leaks must be found and stopped before they started doing damage. But finding leaks can be a task that’s easier said than done.

Leaks Already?

Industry estimates for the longevity of caulk on deck vary widely depending on the type of caulk and the quality of installation. By most accounts, 15 to 20 years is about the time when leaks first start appearing, even with high-quality caulk and the best installation practices. We call the various caulks and sealants, as well as the installation process, bedding.

Candidates for leaks include any piece of metal or wood attached to the fiberglass deck or cabin, and joints in the structure where two parts fit together. Most modern boats can have scores of potential leak points, and finding the source can be incredibly challenging.

There has been a leak here for long enough to stain the veneer. The leak may not be from the window and may actually come from a piece of gear on the cabin top above. Veneer damage can be costly to repair.

There has been a leak here for long enough to stain the veneer. The leak may not be from the window and may actually come from a piece of gear on the cabin top above. Veneer damage can be costly to repair.

Finding the Source

In a perfect world, we’d all have the money and time to have someone take every piece of gear off the deck and cabin, clean it up, and rebed it with fresh caulk every 15 years. In a more practical world, we’d like to find the specific gear that leaks and rebed that first. Then we can start a program of periodically rebedding the high-load gear first and the rest of the gear on a rolling schedule.

On most older boats, you can look up at the cabin top or at the underside of the deck and see the nuts and bolts that hold the hardware in place, making leaks easy to find. But modern production methods and aesthetics don’t lend themselves to exposed hardware. As a result, where the water enters your boat usually does not correlate to where it appears inside the cabin. The source of water might be 10 feet or more away from the drip you found on the settee.

Two different construction methods account for this reality. Many production boats utilize interior fiberglass liners. A liner can be a sole and furniture component, or a headliner and interior cabin side. These liners can look nice and be easy to keep clean. Depending on how they are installed, they may also add strength to the structure. These pieces often get installed early in the boat’s production and can limit access to gear behind them unless inspection holes are cut.

This fiberglass headliner has been nicely capped with teak and looks beautiful. There may be some access to the underside of the deck above if the lights are removed, but other than that, chasing a leak will be challenging.

This fiberglass headliner has been nicely capped with teak and looks beautiful. There may be some access to the underside of the deck above if the lights are removed, but other than that, chasing a leak will be challenging.

Not every boat has a liner, and many have removable overhead panels that give you access to the underside of the deck or cabin. Even without a liner, most modern fiberglass boats utilize some sort of coring material in their decks and cabin structures. This fiberglass-core-fiberglass sandwich makes the structure light, strong and stiff. The core material can be plywood, end-grain balsa or closed-cell foam.

Ideally, this core would be bonded to each skin, and all voids and kerfs (saw cuts that facilitate a stiff panel bending into a curve) would be sealed so that any water that happened to find its way into the core would just run down the fastener inside the boat.

In the real world, there are frequently long voids that can channel the water great distances. If the core is a wood product, then there is the extra detriment of the wood soaking up the water and retaining it until the wood rots.

And in any case, once water enters the core, where it exits can be anyone’s guess.

Targeted Testing

Let’s imagine that you take a hose and blast away at a given piece of hardware while someone watches below. After a few minutes, no leak appears. You move on to the next piece of hardware, and no leak appears. Then, while you’re hosing the next item, your partner sees water dripping into the cabin.

You found the leak, right?

Not necessarily. And this error can result in a lot of wasted time.

A simple leak would result in water appearing inside the boat immediately following exposure on the outside. But if you had a simple leak, you wouldn’t be reading this article. Liners and core materials can cause significant delays between the exposure to water and the evidence of a leak.

In the case of liners, the drip might run a good distance before it finds an exit hole. In some cases, the water must accumulate until it puddles over a rise and then spills into the boat. In the case of wet, cored structures, the passage of water through the kerfs can cause a delay. On balsa-cored structures, the core might absorb water at first, but not drip until it reaches a saturation point.

Sealing all potential leak sources but one will pinpoint the problem during a water test. Check for leaks below, but wait at least a day to give the water time to find its way out.

Sealing all potential leak sources but one will pinpoint the problem during a water test. Check for leaks below, but wait at least a day to give the water time to find its way out.

To determine a leak’s actual location, pick the gear on deck that’s most likely to be the culprit, and then apply plastic and tape to everything else in the area, temporarily sealing it. Soak the untaped gear with a hose until you’ve achieved the equivalent of a good, tropical downpour. Then, check below.

You may not immediately see a leak. But if you come in the next morning and find water, then you’ve found at least one source. If everything remains dry, then remove the tape from another item and soak it, and then check for leaks the next day. This process can take a while, but it is an effective way to isolate a stubborn leak.

Unsealed core hidden by the frame can also be a source of leaks for hatches, windows and ports. Since these items usually have a glass or plexiglass lens with a rubber or caulk seal and a bedded frame, it is important to water-test them to identify the specific reason for the leak. Hatches can leak between the clear lens and its sealant; between the hatch lid and the hatch frame; or between the hatch frame and the cabin top. Because most boat surfaces have some curve or crown in them, it is common to find that the straight metal of the window or hatch frame is not flush on all sides.

Long plexiglass or polycarbonate windows can also be leak sources. Because the plastic expands and contracts in the sun at a different rate than the fiberglass, the caulk is always in tension and may fail, especially if there is a curve in the frame.

The headliner has been removed for repair from damage caused by a leaking hatch. Both the lens and the gasket were compromised, and in this case, the hatch will be renewed before being rebedded.

The headliner has been removed for repair from damage caused by a leaking hatch. Both the lens and the gasket were compromised, and in this case, the hatch will be renewed before being rebedded.

If the water hose test doesn’t work, then for gelcoated or painted decks (no teak or other covering), a moisture meter can sometimes track a leak back to its source. A dry day following a rainstorm is best for using this type of tool—and the moisture meter may also find water trapped in the core.

If all else fails and you can’t find the leak, then you might have success using compressed air, carefully.

Install a regulator on a compressed air line, and then drill and thread a plumbing fitting into the core. Typically, the hole would be drilled beneath the location where a piece of gear was installed, to make the repair of the hole easier later. You need a regulator because you want to start with a minimum of air (just 2 or 3 psi) to prevent any damage to the core-fiberglass bond. Applying too much pressure can cause major debonding problems between the core and skin. Spray soapy water around deck fittings. Hopefully, you’ll see bubbles indicating the leak source.

Snaps for exterior canvas are notorious leakers. Many canvas installers don’t understand the nuances of core closeout (or have the cost of performing it built into their quotes) and many favor clear silicone as their bedding preference (we haven’t found this to be reliable). We’ve removed quite a few canvas snaps only to have water dribble out for several minutes.

And, be sure to pay attention to the core with every piece of gear removed. If the core has not been removed and backfilled with thickened epoxy, then it needs to be done for every hole before rebedding.

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Teak decks can dramatically complicate the search for a leak’s source. If the decks were installed the old-school way, with bunged screws every 18 inches in every plank, then every one of those screw penetrations is a potential leak source. As a teak deck wears, the seam compound will become proud. If it is not trimmed flush every few years, it becomes vulnerable to splitting. There is also a limit to how thin the teak can get before there isn’t enough groove to hold seam compound.

Replacement teak decks are not cheap, but most can be installed primarily without screw penetrations these days. The teak is bonded to the deck with epoxy, plugging all those old screw holes in the process.

Plugging the Holes

Once the leak source has been found, it’s time to remove the hardware. Let’s hope the fasteners turn and are accessible, and the caulk is dry and crumbly, so everything comes apart easily.

The frustrating reality is that a piece of hardware can be leaking and yet be tenaciously stuck. To remove a fastener, you might need impact wrenches or hammers, judicious use of heat or, as a last resort, center-punching and drilling out with cobalt drill bits.

Once the fasteners are out, pry bars, razor knives, wood wedges or saw blades might be needed to separate the caulk. Be careful to protect the fiberglass deck in any areas that won’t be covered by the hardware. Sometimes, if you can get a small separation going, one of the proprietary caulk-melting potions like Anti-Bond 2015 or DeBond 2000 will help break the adhesion.

With the part removed and the deck and part completely cleaned of old caulk, it’s time to pick your new sealant. For metal deck hardware, windows and hatches mounted on fiberglass, we’ve found GE’s SilPruf SCS2000 to be an excellent choice. It has a particular consistency coming out of the tube that fills and sticks to surfaces without creating air pockets. It also lacks overly aggressive adhesive qualities when cured.

When more adhesion is preferred, try Sikaflex-291, 3M 4200 or Pettit’s Anchortech. We tend to use these products more for under-waterline applications, but occasionally there may be an item such as a windlass or dinghy chock where the caulk adhesion would supplement the fasteners and be desirable. Many of these caulks have fast and slow versions that can work to your advantage if the weather is hot or cold, to allow more time before they start to skin over. Most cannot be used below 50 degrees Fahrenheit (for a more detailed discussion, see Passagemaker, October 2018).

The key to getting a good caulking bond is to prepare the location before you start the application.

Cut up several cotton rags into 12-by-6-inch strips (rags work better than paper towels because the sealant doesn’t soak through as easily, and rags don’t leave pieces of paper behind). The small size lets you throw away the rag as it becomes loaded with caulk, and there will always be a fresh one at hand to keep your fingers clean. Have an empty container ready to collect the messy rags.

Put on disposable gloves. Wipe down your fasteners (new fasteners may have machining oil on them) and your bedding surface with denatured alcohol and give it time to evaporate. Apply a liberal amount of sealant to the bottom of the hardware, and around the fastener shank and head.

You want a healthy amount of caulk to squeeze out all the way around the bottom of the hardware as you tighten the fasteners. Remove the excess with a tool, tongue depressor or popsicle stick. Techs do this all the time. The more caulk you remove with a stick, the less there is to smear with a rag.

Next, lightly wet a rag with mineral spirits and wipe away the remaining caulk. Do not use alcohol, as it can inhibit the sealant from curing properly. Pay attention to the manufacturer’s recommendation for cleanup. An incompatible solvent can damage the sealant.

Snug and Dry

Stopping deck leaks not only maintains the value of your boat by preventing damage, but also can be a safety item for the care of your crew. Slippery soles and wet bunks can turn a rough-weather outing into a dangerous situation. No one sleeps well with a drip on their forehead, and tired crew rarely make the best decisions.

We traced our owner’s leaks back to an overhead hatch and two stanchions. Fortunately, there was no core damage. After rebedding that hardware, we drew up a plan to attack the rest of the hardware during the next several seasons.

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