When replacing their windows, most DIY boat owners use plastic (either acrylic or polycarbonate). Plastic is easy to work with and produces seaworthy results. Plastic can be used to make curved windows; it can be fastened directly to the cabin trunk or fit into frames. There is much debate (often rancorous) about which is better: polycarbonate (i.e Lexan) or acrylic (i.e. Plexiglas). While we prefer polycarbonate for certain purposes, generally speaking we think that acrylic is better. In many cases, however, the best choice is actually tempered glass, but we'll have that discussion later.
Polycarbonate (i.e. Lexan)
DIY boat owners often get very excited about Lexan (which is the proprietary name for polycarbonate made by SABIC, formerly GE). Polycarbonate has gained a reputation for being virtually indestructible because it is used to make bullet-proof glass. Other characteristics of polycarbonate, however, are less appealing. Polycarbonate's strength is best understood in terms of impact resistance--polycarbonate doesn't break because it flexes. But this flexibility also makes it difficult to seal; polycarbonate windows are notorious for leaks. Sure, your Lexan may keep out bullets (and this may be important, depending on how you use your boat); but it may not be able to keep out the water. Polycarbonate is relatively soft, which helps it absorb a bullet’s energy but also means that it is easily scratched, even by salt water spray (which is much more prevalent at sea than flying lead). Friends of ours replaced the Lexan windows on their catamaran after only a couple of years of cruising because saltwater spray had rendered them almost opaque. Polycarbonate is also vulnerable to UV degradation. While you can purchase polycarbonate with a UV and abrasion resistant film, its effectiveness on a boat window is limited; curving the polycarbonate, moreoever, will compromise the film. Keep in mind, too, that polycarbonate is much more expensive than acrylic and, if you like a spiffy boat, needs to be replaced more often. That said, polycarbonate is excellent for storm windows, which don't need to be waterproof--just make sure that you leave enough room between the polycarbonate and the actual window so that when the polycarbonate flexes it does not break the window. Its is also a good idea to have a few pieces of polycarbonate on the boat for emergency repairs. On Momo we use a 10mm piece of polycarbonate for our companionway door. Given how much the material flexes, however, it would be a good idea to stiffen the polycarbonate with strips of teak--I don't think the polycarbonate will ever break, but I can imagine that if struck with enough force the door could blow out of the frame.
Acrylic (i.e. Plexiglas)
The question about the difference in strength between polycarbonate and acrylic needs to be kept in perspective. Unless you're sailing in a war zone, acrylic (often known by the proprietary name of Plexiglas) is strong enough for boat windows and hatches. Lewmar and Bomar use acrylic in their deck hatches; the Comex Deep Star tourist submarine uses acrylic for its pressurized hull and hasn't lost a tourist yet. Non-marine uses include protective glazing in hockey rinks, helicopter windows, and the walls and tunnels in large public aquariums. Other factors besides strength also play a role in determining which material is best suited for boat windows. Compared to polycarbonate, acrylic is less vulnerable to UV-light, resists scratches better, and is easier to seal. It is also less expensive. Momo's hatches (the largest of which is 30" x 32") are made of 3/8" acrylic, supported by crossbars. They are badly crazed after 30 years of relentless exposure to the sun, but they are very strong and don't leak. They are as firm under foot as any other part of the boat and an ideal place to set up the sewing machine when we need to repair a sail. The fact that we use our hatches as a convenient workspace has added a few scratches, but they would be much worse if the material was Lexan.
Installation Notes for Plastic Windows
The plastic windows we've seen on blue water boats are usually between 5/16" and 3/8" thick. While plastic is easy to work with, it needs to be treated carefully. To prevent cracks, mounting holes should be substantially over-sized (i.e. a 1/4" fastener should have a 7/16" hole). Screws should not be countersunk; rather, their heads should rest flush on the surface of the plastic. Better yet, cushion the screws with a decent rubber washer. Avoid the use of black sealants because they absorb heat and exacerbate expansion and contradiction. When choosing sealants, avoid polyurethane adhesives like 3M 5200: the outgassing of the plastic will affect the seal and then you'll have windows that not only leak and but are also very difficult to take apart. Sika, 3M, and BoatLife all make products that supposedly work well with polycarbonate and acrylic. But it should be noted that polycarbonate and (to a lesser extent) acrylic are difficult to seal. Thus the directions for Sika's 295 UV (which is specifically meant for plastic windows) explicitly state that Sika "does not guarantee the performance of Sikaflex® 295 UV on acrylic, polycarbonate, or any other form of organic windows." Good results have been reported from silicone-based products used for glazing purposes in the commercial building industry, like GE's Silproof SCS 2000 and Dow 795. We've used Dow 795 successfully for many different projects (you won't find it at Home Depot; you'll have to hunt it down). One advantage is that it is easy to clean up and the residue is not as tenacious as regular silicone.
