- Plexiglass is transparent to UV radiation. That means that anything inside the boat will be subject to UV degradation if the sun shines thru the window. That also means that UV radiation passes harmlessly thru Plexiglass without having any effect on it.
- Lexan is opaque to UV radiation. This means that it protects the boat interior from the ravages of UV. But because the UV radiation is stopped by the Lexan, that means the Lexan is subject to the damage that it is preventing on the interior. UV damage to Lexan causes it to turn yellowish brown and craze (millions of tiny surface cracks). The effect is that your view eventually is destroyed:
Lexan window after 7 years - Plexiglass eventually crazes too... But after a much longer time period. However it does not turn brown or discolor.
This Plexiglass port is 38 years old. - Lexan is often touted as the "bullet-proof plastic":
Sources:Property Unit Polycarbonate Acrylic Tensile strength σΜ at 23°C MPa 60-70 80 Flexural strength σbB MPa 90 115 Impact strength acU (Charpy) kJ/m2 35 15
- Lexan 9030 Sheet Product Datasheet
- Plexiglas GS Product Description
In tensile strength and flexural strength Plexiglass is stronger than Lexan. Plexiglass is weaker than Lexan only in impact strength (resistance to penetration by a quickly moving sharp object).
These comparisons are made on virgin material in both cases. I have no data, but all that surface crazing has to act as stress risers and therefore crack starters - much earlier for Lexan than for Plexiglass.
[This is no longer true. I now have data showing how little strength is retained in Lexan after a 6 year exposure in real world conditions.] - Lexan is two to three times more expensive than Plexiglass.
- Lexan is less scratch-resistant than Plexiglass
I will say tho, that for Eolian, we have chosen Plexiglass whenever it was available.
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7 comments:
Bob, a couple of questions, if you please :
1. we have severely crazed Lewmar hatch lenses on Starbright, undoubtedly PC, waaay too expensive to replace. Would you recommend plexi for "stand-on" hatches?
2. Two of our fixed plexi ports have a "cloudy haze" that won't buff out with plexi polish. Any more agressive compounds you would recommend?
Steve -
If it were me, I'd use acrylic for the hatch lens - it is actually stronger than Lexan for all uses except impact with a sharp object (eg a bullet). And it will survive far longer than Lexan in the sun.
If your fixed ports are old, they are probably acrylic. But if the haze is brownish-yellow in color, then for sure they are Lexan. If the problem is crazing (millions of tiny surface cracks), then no polish is going to fix it, unless you cut down past the depth of the cracks. We were faced with this problem on Eolian (she has 6 large 18" x 36" saloon iced ports), and finally made the decision to replace them. If you make this decision, I can't recommend Bomon Engineering (in Canada) highly enough. They will make ports to your EXACT specifications, for surprisingly affordable prices. If they are commercially available ports, your best bet is probably to replace them with new ones, if they are still available. Some manufacturers (eg Beckson) sell replacement lenses.
To keep our 11 Beckson opening ports (Lexan) clear, they get an annual treatment with Meguiars #17 Clear Plastic Cleaner, followed by Meguiars #10 Clear Plastic polish.
Does this help?
Bob
Steve, one more thought...
If you are at the "zero cost experiment" stage (that is, you're going to throw them out if nothing works), then go to an auto parts store and get a headlight lens restoring kit. It's probably worth a try...
Bob
All understood, Bob, and much appreciated. The 2 fixedd ports are hazed, not crazed, and am reasonably sure are acrylic as they are the cheap, factory flats bedded onto Dow Corning Compound that Hunter tends to do. I will try the headlight restorer, then move on to Bomon for the fixed as well as the hatches. Thanks!
Steve - If they are hazed and not crazed, then I would suspect that they are Lexan.
And Bomon will make you some fabulous aluminum-framed fixed ports with whichever glazing material you wish. Talk to Alain!
Bob
Steve -
Here is a report of my experience with Bomon:
http://windborneinpugetsound.blogspot.com/2009/05/weekend-report-531-replace-deckhouse.html?m=1
Steve -
Alternatively, you could buy some plexiglass/Lexan, cut it to fit, and glue it down using some fresh Dow 314 (? I'll check on that if your interested)
Bob
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