I have done the windows of a Cal 46 (A large pilot house sailboat) and a few small hatches. There is a lot to consider: First these hatches are the same ones you are going to find an any boat. They are subject to micro fracturing and UV degrading. The Acrylic is more UV and scratch resistant, the Polycarbonate is more impact resistant. Actually the acrylic is actually a better replacement. Polycarbonate (Lexan) is slightly more flexible than acrylic (Plexiglas), but because of the flexibility will sag and cause a slight depression in a large hatch. Polycarbonate is less resistant to UV light, scratching and chemicals. However, both of these plastics, as well as the frames have a great variation with temperature.
For starters, if a boat is kept in under cover the hatches will last much longer--also the reason I had acrylic covers for the hatches on my sailboats. Although a 1999 boat is only 14 years old, if it has been out in the sun, the hatch scratched, subjected to high temperature extremes or frequently stepped on (causing micro cracks on the edges), it will not last as long as one which is protected (I avoid stepping on my foredeck hatch, but it happens on occasion.
For replacement: Best to use the old one for a replacement pattern. Cut the sealant out carefully. You can either DIY, or send it off to any one of several outfits on the internet who will replace the "lens" at about half the price of a new hatch.
Cut the sealant with a utility knife, be careful to not cut or nick the soft aluminum frame. Clean the frame after the plastic is removed by first cutting out the sealant, then use a paint scraper, fine wire brush an finally sand paper. Clean the frame with an fine abrasive citrus hand cleaner and mineral spirits.
The plastic is difficult to cut, and can be easily damaged. For the large pieces I was more than happy to pay the plastic supplier the few dollars for him to use his high speed router (although I have one, plus sanders etc--he has the skills), to make sure the plastic is cut smooth, no burrs or cracks and has a slight arc to the edge.
To clean the acrylic only use isopropyl alcohol (2-propanol), pure petroleum ether (naphtha) or the Sikaflex cleaner below. Do not use other substances, unless you are sure it is safe, because many chemicals may craze or damage the plastic. (Same for cleaning--I only use soap and water and then plastic polish)
If you want to DIY--here is the official "plexiglas" bible:
http://www.plexiglas.de/sites/dc/Do...n/311-5-fabrication-tips-for-plexiglas-en.pdf
The Lucite web site also gives alternate sealants and the use of Butyl tape (but only if you use screws--which is not the case here)
Be sure and tape the edges of the frame and be sure that no material will drip into the cabin, by taping plastic beneath the hatch.
The best sealant is probably Sikaflex 295-UV. (I have also use Dow Corning® 795 Silicone) Sikaflex 295 is a black polyurethane sealant designed as a direct glazing adhesive for acrylic. Available from Jamestown Dist. Clean the edges of the frame with the Sika-Cleaner 226 and apply a light coat of Sika-Prima 209. Then clean the edges of the plastic with cleaner: Sika-Cleaner 226, and a primer, and apply Sika-Primer 209 to the plastic. Next run a good bead of the Sika-295 around the inside edge of the aluminum frame, and carefully drop the plastic in place. I used small spacers so I could get the acrylic even on each side and front to back, and then removed them as I put the bead of Sikaflex around
the periphery of the frame. Wear Nitrile gloves, and smooth the top of the bedding compound with a wet finger. You will be pushing it off onto the cover on top and the masking tape. I would remove the tape when the sealant is almost dry and wait until the sealant is dry for the top covering of the plastic sheet. On the inside I used an X-acto knife #11 to cut the small amount of sealant which had squeezed out on the inside, if necessary.