Join Date: Jan 2011
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Re: Acrylic or Polycarbonate for windows
Acrylic expansion is approximately 9% greater than polycarbonate. And whatever figure you find on line, double it if you're looking at gray, bronze or another dark color. It's HARDER than poly so it's more scratch/abrasion resistant. Poly is softer so it will take a blow with a hammer or a flogging shackle but will scratch more readily. Both are/can be UV protected and will probably outlast your ownership of the boat.
This is the same joint I've used dozens of times at work. In the industry it's called a "four-sided flush glazed joint". I design this type of thing for a living, really. Similar to what others have said, shape the glazing, smooth the edges, make it look real nice. Be sure it overlaps the opening by about 1" on all sides. Apply structural glazing tape (3/16" minimum) to the opening so when the glazing is applied, the tape will be 1/4" from its edge. Be careful and make it round the corners, leaving the outside release tape on. Make it pretty. Now, fold down/up the corners of the tape so you can reach them and carfuly place the glazing. The little bit of exposed tape will grab quite well. Once you're satisfied with the arrangement, grab that tape and pull back at 45 degrees. It will unzip. Press HARD on the glazing, then use a soft cloth to protect the surface and beat around the tape with a rubber mallet. At this point, the tape should be 1/4" away from the edge of the glazing with no bubbles showing. Now mask around the edge of the portlight, on the boat and on the glazing. With a razor knife, trim a real nice perimeter 1/4" away from the edge of the glazing (on the boat). Also, trim the masking tape to the very edge of the glazing. Apply a bead of 795 around the edge, forcing it into the space created by the glazing tape. Use plenty. This is the most important part. TOOL the joint. Use a plastic spoon and a spray of water to make the joint look like a gasket. DO NOT SKIP THE TOOLING STEP. regardless of how good you think it looks. An untooled joint is a leaker. An unwarrantied leaker. Immediately remove the masking tape being careful not to get all that unused silicone all over everything. Your light should look like it's installed with a rubber gasket.
This is the same method they now install high-rise windows with. Please note there are no screws involved. They are not needed, add places for water to get in and stress to crack the glazing due to expansion cycles.
Have a great graphic to add but the site won't let me right now.