Fix Leaking Portholes

By Nick Bailey

Notwithstanding the old joke about the boat being a hole in the water into which money is poured, we all know any boat with a cabin is just a floating conveyance designed to be wet on the outside and dry on the inside. It's the "dry on the inside" part that's hard to achieve consistently. One common leak source is the cabin window, or "fixed port," if you prefer.
Boat builders have devised many approaches to fastening and sealing windows but the outcome generally falls into two categories: the window lens is either mounted or retained by a frame or is fastened without a frame. In the case of the frameless window, the lens attaches directly to the boat with adhesive, mechanical fasteners or a combination of the two.
To repair leaky windows, it's important to understand the characteristics and most common failure modes of different window styles.

THE 'FRANKENSTEIN' WINDOW
It wasn't long after fiberglass became the material of choice for production boat building that acrylic (Plexiglas) or polycarbonate (Lexan) began to replace glass as the predominant material for boat windows. These plastics are workable with standard woodworking tools and, unlike glass, they are flexible enough to conform to the curved surfaces of a typical cabin trunk. A lot of the early plastic windows were off-the-shelf fixed ports with aluminum frames. These were only available in stock sizes so many boat builders, looking for more distinctive styling, began to forego the frame altogether and simply attached the plastic window lens directly to the cabin.
The first of the frameless windows mounted with mechanical fasteners was the Mark I or “Frankenstein” window. The window lens, usually a slab of acrylic 0.25-inch to 0.5-inch (6mm to 12mm) thick was mechanically fastened with screws or bolts over a gasket or bead of sealant laid between the lens and the boat structure. This simple installation was popular on boats built in the late '70s and early '80s but fell out of favor as maintenance problems persisted.
NICK BAILEYThe classic mechanically fastened "Frankenstein" window.
The lens color of choice was usually a dark tint, which absorbs sunlight, heats up and then expands, pushing hard against the fasteners. This causes a variety of problems: cracks originating at the fastener holes would quickly migrate across the lens; fasteners work loose; the sealant (many of which don't stick that well to plastic) loses its grip on the lens and the leaks commenced. A Band-Aid solution was to drill the fastener holes over-sized to allow room for the lens to expand. This reduced lens cracking but the amount of thermal movement, especially with the long narrow windows popular on sailboats, would eventually break the seal. Unfortunately, this style of window is always prone to leaks and is a good candidate for re-engineering the installation with the addition of an external frame. Nonetheless, good repair technique can maximize the leak-free service interval.
If the decision is made not to upgrade the window installation with a frame, this is just a reseal job. The fasteners are removed and, starting at the location of the leak, the lens is gently pried loose by slipping a putty knife between the lens and cabin trunk. Clean all the old sealant off both mating surfaces. Carefully check the lens for cracks. If cracked, replacement of the lens is mandatory. This is also a good time to replace a weathered or crazed lens for cosmetic reasons. Use the old lens as a template, trace the shape of the new lens onto a sheet of new plastic and cut out with a bandsaw or jigsaw. In many cases, the lens also needs a bevel or cove cut into an edge of the lens with a router or belt sander. Edges also usually need sanding with 120 grit paper to achieve a smooth finish. When a shiny gloss is required at the cut edge, this is quickly achieved by kissing the edge with the flame of a torch. Next, the fastener holes are drilled in the lens and it is at this point some minor engineering improvements can be made.
Drill fastener holes oversize as needed in the new lens. For example, if the original holes were drilled 5/32-inch (3.9mm) for a 1/8-inch (3mm) fastener, the new hole is enlarged to 1/4-inch (6mm). The limiting factor is the size of the washer used on the outside of the window. It should bridge the hole. Flathead fasteners with cup washers have the lowest profile and hold sealant best. Another technical upgrade is to reseal the window with an ultra-flexible professional glazer's silicone such as Dow Corning 795 or equivalent. These silicone sealants have the ability to stretch or compress 50 percent so they are flexible enough to maintain a seal despite the thermal movement of the lens.
NICK BAILEYThe fastener-free frameless window.
Now the new window is dry fitted. From inside the cabin, the outline of the opening in the cabin trunk is traced onto the protective paper on the inside face of the lens. The lens is lifted out and the protective mask is carefully cut and removed to expose the outer bonding ring of the lens. The exposed area is sanded thoroughly with 80 grit to ensure good sealant adhesion. To prepare for caulking, masking tape is applied to the cabin trunk about 3/16-inch (4.7mm) outside of the lens all around the outside perimeter of the window. The small, unmasked gap allows for a tidy external sealant bead when finished. Once the dry fit and masking is complete, the window is again removed and sealant applied to the cabin trunk in two or three fat concentric beads within the area in contact with the window. Special care is taken to make sure each fastener hole is well caulked. To make positioning of the window more precise, two or three fasteners are pre-fitted to the lens to act as registration pins. The lens position is fixed by engaging these fasteners into the correct cabin trunk holes just as the lens is lowered into place onto the fresh sealant.
The remaining fasteners are now installed and the window is gently tightened using a sequence similar to torquing down an engine cylinder head. Care is taken to avoid over-tightening the fasteners, since the lens can crack easily and it's important to avoid squeezing out too much sealant. A thicker gasket of sealant has more compliance as the window expands and contracts. A bond line of about 1/16-inch to 3/32-inch (1.5mm to 2.3mm) thick is about right. Excess sealant that squeezes out is removed with a putty knife and the final caulking touch is achieved by smoothing a bead around the outside perimeter of the lens with a gloved forefinger. Masking tape is removed while the sealant is still wet. As soon as the caulking has cured, the protective mask on the lens is removed before it's exposed to rain, heavy dew or too much sun that render the masking painstakingly difficult to remove.
It's worth noting that some manufacturers have stuck with the mechanically fastened approach and have achieved a reliable window seal. A successful variant used by some boat builders uses screws and washers as retainer clips holding only the outside edge of the lens (e.g. no fastener holes through the window). The window sits in a recess and the perimeter fasteners are invisible, buried in the thick bead of black glazer's silicone that surrounds the window.
By the early '80s, boat builders, after many complaints, abandoned the Frankenstein frameless window to pursue the new technique of attaching a frameless window using advanced adhesives. Besides offering better leak resistance, boat designers also wanted to improve cosmetics and were glad to leave behind the fastener studded look.
(A) Example of Frankenstein Mark 11 window installation. (B) Typical installation of a frameless window without fasteners.
WINDOWS IN BONDAGE
A slick looking acrylic window glued in place with methacrylate adhesive is what I call the Mark II bonded window. This powerful adhesive supercedes mechanical fasteners and takes over the job of fastening the lens to the cabin trunk. The task of sealing the window is delegated to a bead of tough polyurethane sealant applied externally around the perimeter of the window. Particularly when flush mounted in a recess, this technique created a great looking, fastener-free window with what was supposed to be a bulletproof seal. Although this was usually the case in the early days (during the warranty period), as the years went by and these boats began to age, window problems developed.
Methacrylate is awesome stuff. It has an adhesive bond so strong that often the underlying laminate fails before the adhesive does and that is not a good thing. As the window lens expands in the hot sun, a powerful force is exerted on the bondline. Something has to give. First, the expanding window causes the cabin trunk to flex. In many cases the only indicator of the huge stresses involved are the “crow's feet” cracks radiating out from the corners of the window. Second, the adhesive can fail, or worse, the gelcoat tears away from the underlying glass laminate at the bond line. With time, it's not uncommon to see the window pop loose, but only in one corner. Special repair techniques are required to repair the problems created by the methacrylate bonded windows commonly found on older boats.
A leaking window of this type is first carefully assessed to determine exactly what repair is required. If the lens still seems to be well bonded, it may only be a simple matter of raking out and renewing the perimeter caulking. Nonetheless, if the lens is in any way debonded or loose, the perimeter caulking can't do its job of sealing the lens. If only one corner is loose, it may be possible to do a temporary repair by carefully installing one or two screws to secure the lens prior to raking and caulking the perimeter bead. A lens with more than a few inches of loose caulk requires complete removal and reinstallation.
The standard procedure is to first rake out the perimeter caulking and then carefully pry out the old lens, in one piece if possible. More often than not, however, the lens remains firmly bonded to some areas, so removal demands the use of a chisel or router. At this point, if removal might destroy the old lens, a cardboard template is made to preserve the pattern. After the lens pieces are removed, the residual adhesive is sanded away and any gelcoat or fiberglass damage to the bonding surfaces is repaired. If the old lens is salvageable, it's cleaned (or sanded) to prep it for reinstallation. If a replacement lens is required, it's cut following the instructions above.
Once the cabin surface and window is properly cleaned and prepped, a bead of methacrylate adhesive is applied to the cabin with a special mixing gun. The window is then immediately put in place and clamped firmly until the adhesive hardens. Methacrylate bonds and hardens almost immediately so the placement of the window is very much a one shot deal. The clamps, usually a few wood 2x4s jammed between the lifelines and window, are left in place for a few hours or overnight. To prep for the perimeter caulking the cabin and window are masked as outlined above and the sealant bead applied with a caulking gun. Theoretically, this style of window is not supposed to move around much so the less flexible polyurethane sealants (3M 5200, Sikaflex 252) can be used for the perimeter bead. In use, even a perfect repair may only last a few years before the window pops loose again.
(A) Alternate frameless installation uses fasteners, hidden by glazer's silicone, to retain outer edge of the window without drilling holes through it. (B) Sample add-on frame mounting.
FREE-FLOATING FRAMELESS WINDOW
In the long term, any attempt to restrain the thermal motion of a large plastic window creates problems. The window eventually leaks or cracks or the cabin trunk cracks and the window comes loose. In the last 10 years or so boat builders have recognized this and have taken a different tack. Many boats are now built with frameless window installations where the plastic lens is free to expand and contract. This is achieved by using the aforementioned high flex glazer's silicone to both seal and bond the window in place. This technique, similar to the way the lens in a framed deck hatch is installed requires the window to be mounted in a wide recess and works best when the window is nearly flat or is pre-molded to the shape of the cabin. No mechanical fasteners are used and the window is allowed to float in a thick silicone gasket. Here it can expand and contract freely without breaking the seal or coming loose. This installation technique is difficult to duplicate in the field, as the window must be fixed in place and left completely undisturbed for the two days (more or less depending on the temperature) required for the sealant to cure completely.
FRAMED WINDOWS
Framed windows, either off-the-shelf or custom, don't suffer the same kind of problems as outlined above but unfortunately are not 100 percent problem free. The advantage of a framed window is that the frame itself handles the job of fastening the window to the boat and the lens is freed from all those stresses. The flexible rubber gasket that secures the window in the frame also allows the lens to expand and contract. The disadvantage is the rubber gasket that eventually degrades and leaks. Many older boats have off-the-shelf fixed ports with aluminum frames. Excellent replacement ports are still made today by all the major hatch manufacturers but are only available in certain sizes. Rubber gaskets in such framed windows slowly deteriorate from exposure to sunlight and pollution, especially ground level ozone. Occasional gasket leaks can be dealt with by working Permatex penetrating windshield sealant (available at automotive shops) under the edge of the gasket. The sealant between the window frame and the boat occasionally leaks for some of the same reasons as outlined above with respect to frameless windows. When you cannot stop the leaks, you have no choice but to replace the gasket, if one is available.
A gasket replacement usually requires removal of the entire window assembly. Fasteners are removed, the frame is gently pried away from the cabin trunk and the sealant cut with the edge of a putty knife. Once the window assembly is on the workbench, the old gasket is peeled out and the lens freed. Each make of window has its own tricks. The one shown below was made by the Beclawat Company in Belleville, Ontario, and was supplied by the thousands to Canadian boat builders during the '70s. It uses a unique rubber gasket (still available) to seal the outside of the lens against the frame and double-sided foam adhesive tape to bond the inner side of the lens to the frame's inner flange.
NICK BAILEYWhite painted aluminum add-on frame goes over top of flush mounted window.
After removing the lens, all the old tape must be scraped off and the frame cleaned with solvent or a citrus cleaner capable of dissolving gummy residue. The old lens (if reusable) is also cleaned and polished before reinstallation. Acetone or lacquer thinner is never used to clean plastic lenses but stove alcohol is okay. If the lens is in poor condition, a new one is cut. The new gasket is measured and cut about 1” (25mm) longer than the old one to allow final trimming during installation. New double-sided foam adhesive tape is cut (3M Scotch- Brite VHB tape works well for this), the protective backing peeled off and the tape firmly applied to the frame's inner flange. The protective mask is peeled from the front of the tape and the lens is carefully placed onto the sticky surface and immediately pressed firmly to maximize the bond. Starting at the bottom of the frame, the new outer gasket is pushed in between the frame and the lens until the detent cut into the gasket “snaps” onto the frame. To enhance the seal, a little clear silicone is smeared onto the sealing surfaces of the gasket as it's pushed into place. Next, the window assembly is refastened to the cabin with fresh sealant (either polyurethane or silicone works fine in this application).
THE ULTIMATE FRAMELESS SOLUTION
When a frameless window installation is repeatedly defeated by the problem of expansion, a completely different method of securing the window is needed. Here is where a retrofitted external frame offers a permanent solution. The addition of an external frame does two important things. First, it mechanically fastens the frame to the cabin and takes over the job of securing the lens in place. No fasteners go through the lens and no ferocious bonding adhesives are required to retain the lens. This equates to less stress on the lens and the boat. Secondly, the lens, now that it's free of being rigidly fastened, remains restrained by the frame and is caulked with flexible glazing silicone allowing it to expand and contract freely without problems.
Frames are made of polished stainless, painted or anodized welded aluminum or wood. In many cases, a troublesome window doesn't need to be removed. The frame, together with a generous bead of caulking is simply installed overtop of the existing window. For maximum durability, the new frame is usually thru-bolted and acorn nuts are used on the inside of the cabin for best cosmetics. Many chronic window leaks have been banished forever by retrofitting a frame. (Including those on my own boat.) The only minor drawback is that an external frame doesn't appeal to everyone aesthetically. Compared to the way many windows look after repeated amateur attempts to seal leaks, an add-on frame is a big improvement.

A version of this story ran in DIY Boat Owner magazine.