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Old 27-08-2014, 14:59   #16
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Re: Which Boat Window Materials?

It is important to keep the differences clear or you will wind up buying expensive wrong materials.

Perspex is an old British name for acrylic glazing, which in the US is generically called Plexiglass. Plexiglass is a trademark for the most common line of acrylic glazing products in the US. There are probably 24 different grades of Plexiglass and an equal number from each of two or three other makers.

Similarly Lexan is a trademarked brand of polycarbonate, and it also comes in different grades. Makrolon is a competitor's product, also in different grades.

For marine use, for outdoor use, you want to specify a UV-resistant and scratch-resistant grade (of whatever material) and that's not going to be stocked at the local hardware store, you probably will have to order in full sheets (about 4x8 foot) unless you find a true distributor who stocks the one you want. Pretty much all of them are available in different colors as well, i.e. smoke, bronze, translucent, clear and tints. That's why a local shop can't afford to keep them all in inventory.

Polycarbonates generally flex more than acrylics, and generally scratch easier. But they are also 10x-100x stronger, so if a spinnaker pole drops and spears your window, polycarbonate probably will hold when acrylic 4x thicker would shatter.

As to glazing, that's often caused by improper installation and maintenance, not just the material. Cleaning the glazing with anything that has petrochemicals (like common polishes) will eventually cause them to craze. Bolting the glazing in place, and not allowing it to move (yes, move) with thermal expansion will also cause crazing. Easy to blame the material but remember these materials are used for structural glazing in huge architectural applications without the failures you see on boats. Mainly because architects are better at following the instructions, which the makers of all these materials do provide.

Tempered glass has to be custom cut and then tempered. Once tempered, it cannot be cut or reshaped, so that's an expensive custom process. Safety glass still turns into lots of sharp gravel, so while a thick piece might be the most durable glaing to use on a boat, you still have to ask whether it is going to keep out a spinnaker pole--or the water breaking off a 30' wave--better than a hunk of polycarbonate. Which can be kept pretty scratch-free by proper polishing and maintenance.

All depends on your priorities and budget, but make sure you know what you are asking for, before you order it.
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Old 27-08-2014, 18:36   #17
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Re: Which Boat Window Materials?

So, what can I use for a large 2'x3' skylight that I walk on and abuse? I'm not really concerned with it staying clear, just strong.
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Old 27-08-2014, 21:49   #18
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Re: Which Boat Window Materials?

Possibly Lexan MR10 or Macrolon AU, both designed to be abrasion and UV resistant and available 1/2" thick. You'd need to get specs from the makers or distributors and since shipping can be expensive, look for a local source and find out what brands they carry. Then there's the question of whether that would be stiff enough, so it wouldn't bend or deflect under load across a 3' span. That might force you to a thicker material, or a brace under it.
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Old 27-08-2014, 22:13   #19
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Re: Which Boat Window Materials?

I agree about the bracing if you use Lexan. The only hatch manufacturer that uses Lexan is Bomar and their hatches have crossbars to prevent the Lexan from flexing, which would break the seal.

Every other hatch manufacturer uses acrylic (Plexiglas) both for its resistance to flexing and its superior longevity.
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Old 27-08-2014, 23:47   #20
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Re: Which Boat Window Materials?

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Originally Posted by mitiempo View Post
I agree about the bracing if you use Lexan. The only hatch manufacturer that uses Lexan is Bomar and their hatches have crossbars to prevent the Lexan from flexing, which would break the seal.

Every other hatch manufacturer uses acrylic (Plexiglas) both for its resistance to flexing and its superior longevity.
OK, so what would be the better material to use for Bomar extruded aluminum portlights, an acrylic or Lexan (or similar polycarbonate)? When I say 'better', I primarily mean for strength and durability.
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Old 28-08-2014, 00:00   #21
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Re: Which Boat Window Materials?

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OK, so what would be the better material to use for Bomar extruded aluminum portlights, an acrylic or Lexan (or similar polycarbonate)? When I say 'better', I primarily mean for strength and durability.
Though Lexan is stronger Plexiglas in a reasonable thickness is plenty strong enough. Lewmar Ocean hatches, as well as virtually all others, are Plexiglas. Lexan will craze in a few years and is easier to scratch. Plexiglass will look good many times longer.
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Old 28-08-2014, 00:28   #22
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Re: Which Boat Window Materials?

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Polycarbonates generally flex more than acrylics, and generally scratch easier. But they are also 10x-100x stronger, so if a spinnaker pole drops and spears your window, polycarbonate probably will hold when acrylic 4x thicker would shatter.
Polycarbonate is very strong particularly to small high speed missiles. However the high flexibility poses a lot of practical problems for boat use. It defects so much that there is real risk of popping out of the frame.
This flexibility also creates real problems maintaining an effective seal. Thermal expansion exacerbates the problems with seals. Polycarbonate is very soft and scratches easily even with an anticscratch coating.

Why don't more hatch makes use tempered glass? The glass does not have to very thick to be easily strong enough. It is more likely to leakproof, is uneffected by UV and is extremly scratch resistant compared to the plastic alternatives. If you are really concerned about strength the glass can be made as the same sort of strength as deck without getting to ridiculous thickness. This would be overkill, but it does show strength should not be a problem.
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Old 28-08-2014, 02:31   #23
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Re: Which Boat Window Materials?

Thanks for everyones comments.

Would be tempted to go for toughened glass as it doesn't craze or scratch but I think weight might be an issue as I need to replace 16 windows and my boat is already quite twitchy so want to avoid adding weight high up.

Does anyone know if glass does weigh alot more than acrylic and polycarbonate?

It seems acrylic doesn't have the strength and whilst polycarbonate is definitely a contender I am a bit concerned about the reports of it being very flexible and susceptible to scratching.
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Old 28-08-2014, 03:27   #24
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Re: Which Boat Window Materials?

According to google 10mm glass is 25kg per m2

So a large 500x500 hatch glass will weigh 6.25kg. Tempering does not change the weight. Laminated glass would be lighter.

Polycarbonate is the lightest and according to google 10mm it 12kg per m2 (I would have expected less) so the weight will be just under 1/2 @ 3 kg.

Acrylic will be a bit heavier than polycarbonate, but not much.
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Old 28-08-2014, 03:43   #25
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Re: Which Boat Window Materials?

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Originally Posted by noelex 77 View Post
Why don't more hatch makes use tempered glass? The glass does not have to very thick to be easily strong enough. It is more likely to leakproof, is uneffected by UV and is extremly scratch resistant compared to the plastic alternatives. If you are really concerned about strength the glass can be made as the same sort of strength as deck without getting to ridiculous thickness. This would be overkill, but it does show strength should not be a problem.
I think the Trend Marine is the only one using glass for hatches extensively.
They make also hatches in acrylic, but glass is their main material.
They provide real glass glazing for many top brands, Oyster between them.

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Old 28-08-2014, 04:31   #26
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Re: Which Boat Window Materials?

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I've been thinking automotive "safety glass", which is of course lamintaed glass. Tempered glass of course is used on all windows of a car except the windshield, tempered glass breaks into tiny chunks as opposed to a big piece and that is why it's used I believe, but in the windshield where absolute strength is required, I believe laminated glass is used.
I think laminated glass is a little more expensive, but much less likely to fail from an impact?
Actually, tempered is more expensive. They have to cut it and then it has to be fired. (Heat treated) I use both, safety glass in the port lights and port holes and 3/8 tempered in the overhead skylights.
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Old 28-08-2014, 04:43   #27
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Re: Which Boat Window Materials?

Tempered and laminated are both worth considering.

The big advantage of laminated is that if it does shatter. It mostly stays intact so the hatch/pothole is likely to still prevent most water entering the boat. With toughened glass its is likely to crumble and leave a large opening.

The big drawback of laminated is that in a marine environment the edges start to look very tatty, with moisture/sealant creeping between the layers. Generally toughened is preferred because it is likely to never need replacement.

Strength wise they are both similar, but toughened is slightly better. Laminated is a bit lighter and has better insulation for heat and sound.

It is a pity plastics are used so often they don't last long (both clarity and sealing) in the typical hot, sunny climates many of us prefer.
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Old 28-08-2014, 05:24   #28
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Re: Which Boat Window Materials?

I used tempered glass in my prior boat (luders 33) for the fixed portlights. I was going to replace scratched lexan for second time and hated the job. It came out great, and truthfully spending $60 per glass pane at a local glass shop (non marine!!!) seemed really cheap to not have to replace them again. The opening ports all had glass and to my knowledge were original to the boat.


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Old 28-08-2014, 05:41   #29
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Re: Which Boat Window Materials?

Coming from an aviation background, my experiences are mostly with that, but a lot of it carries over to boats.
Normally for smaller airplanes "Plexiglas" is normally used due to it's lightweight, relatively good scratch resistance and it's easily formed and is strong enough so that the wind doesn't blow it in, "Lexan" is used in some places like landing light covers for smaller panels for it's greater impact resistance, but it scratches more easily.
But on larger aircraft, when you start shooting chickens at windshields at 400 MPH, nothing but glass will hold, point is glass can be tougher than we may think, and it lasts a long time.
I hadn't thought that tempered glass would be a specialty order since the tempering has to be the last step, but it makes sense.
My port lights are all laminated safety glass and are in prefect condition at 27 yrs old, I intend to try to put laminated safety glass in my hatches too, any reason other than weight that this won't work? My biggest hatch will hold a 20" box fan without it falling through, so I guess it's maybe 18"?
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Old 28-08-2014, 06:50   #30
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Re: Which Boat Window Materials?

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Polycarbonate is very strong particularly to small high speed missiles. However the high flexibility poses a lot of practical problems for boat use. It defects so much that there is real risk of popping out of the frame.
This flexibility also creates real problems maintaining an effective seal. Thermal expansion exacerbates the problems with seals. Polycarbonate is very soft and scratches easily even with an anticscratch coating.
This may explain why my oem Bomar hatches came with Lexan, whereas my oem Bomar portlights came with acrylic. They are still made that way, btw, by Bomar/Pompanette. I suppose the acrylic portlights along the cabin sides would be subject to greater boat flex than Lexan hatches mounted on top of the deck. The Lexan deck hatches would also benefit from being stronger for standing on and deflecting falling objects & boarding seas. This also probably explains why the hatch frames have supports for the lenses which would presumably counteract Lexan's greater flexibility and possible failure of the seal you mentioned.

The mystery is finally getting solved -- thanks!
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