Worried about the vulnerability of his tempered
glass windows, the owner installed polycarbonate
(1/4” thick) over his tempered glass and sealed
the polycarbonate with silicone. At the time of
this photograph, the polycarbonate is only a year
old, but it leaks and its transparency has obviously
suffered—not so much because of UV-degradation but
because of the abrasive effects of a saltwater passage
from the eastern Canada to New Zealand. The
commercially-made hatch on the right is made of acrylic
(also 1/4")—it is two-years old and its transparency has
also suffered, but not as much as the polycarbonate.
This boat was knocked down in the south Atlantic and
the polycarbonate survived; but so did the hatch. So
too did the tempered glass, which was battered on the
inside by flying objects when the boat went over.
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)
These old acrylic windows are
crazed but still remarkably transparent.
Note the generous washers between the
fasteners and the plastic.
Installation Notes for Plastic Windows
The cracks in this window (they've
been "repaired" with black sealant)
all start at the fasteners,
which are too tight to accommodate
the material's expansion and flexing.
Using black sealant, which absorbs heat
and makes the expansion worse,