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formerly 'BoatyardBoy'
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Discussion Starter · #1 ·
I have two long fixed port lights on my boat. They were leaving so I removed them to reseal. Well one was already cracked so I'm deciding to replace them. Which should I use since they are so long and aren't being walked on? Also, they had screws about every 6 inches, should I have those redrilled or only half of them redrilled?

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Courtney the Dancer
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You could use either polycarb or acrylic. The poly will be stronger but will scratch easier, the acrylic will stay clear and is more resistant to scratches. The problem I see is how they are attached with just screws and no stainless or aluminum trim piece to evenly apply pressure around the entire perimeter, but if you use a good sealant like Dow Corning 795 and overdrill the screw holes (the plastic expands and contracts so if you tighten the screws you will have cracks) and use washers with flexible gaskets you should be OK.
 

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You could make the replacement in 2 pieces, inserting a piece of trim between them. That would reduce the stress on the parts - both from thermal changes and from mechanical stress.
 

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I'd use acrylic. True polycarbonate is stronger but acrylic is strong enough. What thickness?

As far as attachment holes and fasteners aren't needed. Dow 795 and 3M VHB tape is all that is required. The combination will hold through incredibly well and without holes cracks from stress are eliminated. I agree with smaller sections of glazing to reduce expansion problems.

This explains the method.





 

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formerly 'BoatyardBoy'
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Discussion Starter · #6 ·
Thanks yall, I am leaning to acrylic now.. It's half inch is what came off the boat. I just don't want it to look old and scratched in a few years, so I'm not doing lexan.

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The fastener holes can lead to cracks over time. If you can avoid using fasteners the results will both look better and last longer. I don't know of a manufacturer using fasteners.
 

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I'd use acrylic. True polycarbonate is stronger but acrylic is strong enough. What thickness?

As far as attachment holes and fasteners aren't needed. Dow 795 and 3M VHB tape is all that is required. The combination will hold through incredibly well and without holes cracks from stress are eliminated. I agree with smaller sections of glazing to reduce expansion problems.
I'm thinking of doing something similar (although my windows are considerably smaller and thinner). Is it really safe/effective to just use adhesives and not screw the windows down like they were originally? All of my leaks are from screw holes, but I'd hate to lose a window some day out sailing...

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Acrylic is harder, more scratch resistant. Polycarbonate can take more abuse, it's more impact resistant. It's what they use for hockey dashers, motorcycle helmets, visors and the like. Expansion isnt a problem until you get into big peices of dark material. We deal with poly sheets up to 39 feet long. Then it's an issue.
 

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formerly 'BoatyardBoy'
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Discussion Starter · #14 ·
Acrylic is harder, more scratch resistant. Polycarbonate can take more abuse, it's more impact resistant. It's what they use for hockey dashers, motorcycle helmets, visors and the like. Expansion isnt a problem until you get into big peices of dark material. We deal with poly sheets up to 39 feet long. Then it's an issue.
I got ya, well what about discoloration from not being uv stable?

And I read that ocean salt water can scratch lexan.. That's no good

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Acrylic will outlast polycarbonate by a very large margin. Every major hatch manufacturer uses acrylic and for good reason.
That's the balancing act. Poly is more expensive than acrylic but at the size they would be using, not a show stopper. They would then look at whether impact is less of a consideration than abrasion. In marine conditions, this is probably the case. Another issue might be that poly absorbs water, not like a sponge or anything, but enough to require 24 hours of drying prior to thermo-forming. A couple of years ago, we abandoned poly in lieu of impact modified acrylic. It's softer than standard acrylic so it can take a blow and not fracture. Not sure about abrasion resistance on that but I assume it's similar to poly. There is another product called co-polyester. We use it for hurricane units but I don't think the pigmenting (or lack ther of) is stable. Best bet is acrylic. What ever you do, don't use any type of urethane on it and clean with soap and water only. Window cleaner will cause crazing of the surface.
 

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boatboy, take a look online at what the plastics manufacturers have to say. PPG, Dow, GE, Rohm & Haas, whoever is in the game today.

Most of them have installation guides online which will show you the methods you can use. VHB tape is a great option, it holds windows in skyscrapers.

But on the glazing material itself, you don't just want to go to the local plastics place and buy "plastic". Every maker, every plastic (acrylic or polycarbonate) comes in different grades and unless you buy UV-resistant and scratch-resistant material, designed for use as exterior glazing, you will be disappointed in how the usual cheap stuff (intended for napkin holders and towel racks) holds up on your boat.

Buying the right stuff is going to cost more up front, but last way better on the boat.

VHB tape, like everything else, can be ordered discounted from Amazon, but some of the vendors will take a month to ship it, as they batch order it from 3M. Worth ordering ahead of time if that's what you need. And 3M also has web pages explaining the different colors, thicknesses, and widths of tape available.
 
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