Mike Lehmkuhl, Renegade #3, ATHENE
I replaced mine with a Lexan knock-off about three years ago. I haven't really noticed them yellowing. Lexan is essentially a polycarbonate (as opposed to acrylic like plexiglass) and comes in a variety different makes (mine is Hyzod).
I used a clear polycarbonate because I do like to see who is bearing down on me when they are dragging anchor at night in a crowded anchorage when I'm half asleep!
The problem with clear polycarbonate is that it scratches very easily. But if you polish it once or twice a season (with a soft cloth-- no paper towels) and don't scrape it (yeah right), you should be fine.
Mine have a few big scratches in them, but compared to the crazed plexiglass mess they were 3 years ago, I can live with it.
I'm a little more than nervous using safety glass over such a big area as these port lights (same exact ports as the Renegade btw).
Dave Whyte, Mandara, #397
thought i'd throw in my 2 cents on the acrylic-vs.-polycarbonate debate...
acrylics ('pleiglas') can also yellow with age under some circumstances. more importantly, acrylics are somewhat brittle to begin with and continue to embrittle with age and UV exposure. under extreme loads, acrylics will fragment and rupture in an impressive fashion. scratch-resistance is not so great, either.
polycarbonates ('lexan') have a significant advantage when it comes to bearing up under load. while some yellowing can occur, my personal experience did not include anything. they are much less brittle than acrylic and generally do not fragment and shatter. under many conditions, 3/8 polycarb will stop a small-caliber projectile. because of it's properties to deform and contain impacting objects, it is used in aircraft and motorsport applications, safety eyewear, and as safety shielding around force presses. as to the scratching issue, tempered polycarbonate ('lexan II') has a surface treatment similar to the anti-scratch coating on eyeglass lenses and is much more durable in terms of optical clarity.
engineering data and even advice may be obtained for these materials by contacting the manufacturer. i've found it's usually best to call on the phone and ask to be put in touch with engineering for assistance with application data. if you make it sound important, you'll usually get around the reception department.
i haven't been involved much with motorsports since 1993, but at that time the SCCA (sports car club of america) GCR (general competition rules) read as follows in reference to the preparation of production-based cars "...windshields may be replaced with a suitable polycarbonate material..."
my favorite trick for cutting lexan is as follows (this works with plex, too, but some amount of gumming of the cutter takes place and it's yucky :(. ) take your original plex piece, use it as a guide pattern by sticking it down very securely with double-face tape onto your new material. use a 3/4" carbide router bit with a top-mounted guide bearing (usually about 25.00--high speed steel bits will instantly die. you will probably go through at least one carbide bit cutting out a full set of portlight glass for a triton.) the cuts are super clean and it is easily done. you can remove the tool marks from the edges if you like by "flame polishing" carefully with a propane torch.
Rob Squire, #96, Head Over Heals
Dave's comments are spot on for working with plastics, although I found when I made my acrylic portlights a lot of years ago, I used a variable speed saber saw, cut at a very slow speed with a coarse tooth blade and had non of the gumming Dave mentioned. I dressed the edge with a rasp and router. For west coast boats without frames, the acrylic edges can be "polished" with propane torch. As for strength and brittleness, I chose 3/8" material and only now after 10 years do I begin to see some slight crazing...maybe yellowing, but I don't think so. Another advantage to Acrylic over polycarbonate is cost. Both are great and either will work well....kind of a hard call.