Replacing the windows on your boat is a labor-intensive undertaking. But you'll save a lot of money by doing it yourself, and following a few solid strategies will make the labor less tedious. Knowing your materials is one of the keys to a successful project.
Removing the Old Windows
When taking your old windows apart, pay attention to how they were installed. Boat windows can be either curved or flat; they can be fastened directly to the cabin trunk or installed in frames. The nature of your windows will determine the kinds of material you can use to replace them. Clean your frames and cabin trunk thoroughly. Any sealant residue may affect the quality of the new seal. Rub away the residue with paint thinner and alcohol and then abrade the surfaces with sandpaper.
Glazing Materials -- Introduction
When replacing boat windows, you can choose from three glazing materials: polycarbonate (Lexan), acrylic (Plexiglas), and tempered glass. The windows on offshore boats vary from between 5/16 inch to 3/8 inch thick. As a point of reference, the large pilot house windows on a Nordhavn 43 are 10mm (about 3/8 inch) thick. Smaller windows can use thinner glass.
Glazing materials -- Polycarbonate
Polycarbonate is very strong. But it is relatively soft and thus easily scratched (even saltwater spray will scour a polycarbonate window). Its flexibility makes it difficult to seal and it is vulnerable to UV light. Polycarbonate is also fairly expensive. Its advantages are that it can be easily tooled, used for curved windows, and fastened directly to the cabin trunk. Polycarbonate is also an excellent material for storm windows or emergency repairs.
Glazing Materials -- Acrylics
While not as impact-resistant as polycarbonate, acrylic is sufficiently strong for most marine applications. It is less expensive than polycarbonate, and more resistant to scratching and UV light. Like polycarbonate, it can be tooled by the average handy-person, used for curved windows, and fastened directly to the cabin trunk.
Glazing Materials -- Tempered Glass
Tempered glass is, in many respects, the best material for marine windows. It is strong, difficult to scratch, impervious to UV light, and easy to seal. A severe impact, however, can still break tempered glass, so it is not the best choice for an unprotected hatch. Other disadvantages are that tempered glass cannot be used for curved windows. Nor can the average person cut tempered glass; it must be ordered from professionals.
Polycarbonate and, to a lesser extent, acrylic windows are notoriously difficult to seal. When choosing a sealant, check the manufacturer's recommendations: whether a product is "marine" grade is less important than whether it is designed for the surfaces in question. The best choices for polycarbonate and acrylic are silicone/polyurethane hybrids. Some good results in sealing polycarbonate and acrylic have come from using silicone-based products meant for installing windows in the commercial building industry, like GE’s Silpruf SCS2000 or Dow 795 Silicone Building Sealant. These products can also be used on tempered glass.
Reassembling your Windows
When reassembling your windows, keep in mind the following. Any holes drilled into polycarbonate or acrylic should be substantially oversized. The screws should not be countersunk; rather, screw heads should rest flush on the surface. Clean the areas that will be sealed by wiping them down with paint thinner. When applying sealant, run a continuous bead along the surfaces that will make contact with the glazing material -- it is better to use too much sealant than not enough.
Make sure not to squeeze the sealant out by overtightening the new windows or the frame (use shims or spacers if necessary); keep a generous layer of sealant between the window and the other surfaces. Finally, if using frames and fasteners made of different types of metal (i.e. aluminum frame and stainless fasteners), coat your fasteners in a corrosion inhibitor like Tef-gel or Lanocote.
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