Acrylic or Lexan for fixed windows?

[RGN]

In the past everyone used Lexan (Yeah I know that is a trade name) for storm windows and fixed ports. I suspect it was the "In" thing. Now I am seeing many recommendations of using Acrylic instead, for a number of good reasons.


I have a four fairly large fixed windows on my boat and the existing acrylic is 20 years old and showing its age. I am planning on replacing them this fall but would like to get some of your thoughts on which glazing to use.


I am also going to change from four separate windows to one large, thicker exterior hung window on each side. I am thinking the double sided tape and Butyl caulk method of attaching the new windows.


The existing glazing is 1/4" thick which seems a bit thin to me (but then it has worked for 55 years!) I am thinking 3/8" of 1/2"


Each opening is roughly 200 in/2


The boat is used for open ocean passages.


As always thoughts, ideas and criticism (Gentle, my poor male ego and all ) is welcomed


Thanks folks

[Dulcesuenos]

My 2 cents,, I prefer Dow corning 795 it is a structural adhesive and great for your purpose.
I like lexan due to its impact resistance and will also make any slight curves you need without having to heat and mold it.

[tkeithlu]

Lexan is soft and strong. 1/2 inch Lexan (polycarbonate) will bend double before it cracks and is darn close to bullet proof. Lexan scratches very easily. Acrylic (Plexiglas) is harder and brittle, and nowhere near as strong. It shatters. It is cheaper than Lexan. I had 1/2" Lexan all the way around, but could not keep the windshields clear. I've replaced the two critical panels with two 1/4" layers of tempered glass with a layer of vinyl between. The rest is lexan.

That said, I would seriously consider a thick acylic for the not critical see-through-them ports.

[Annie in WA]

Since you are planning on redesigning from 4 panels to one you might consider checking with a naval architect on the change since you do go off shore

[Tupaia]

In the past Acrylic tended to be quite brittle and craze easily. By selecting "cold cast" Acrylic, stresses created during the old extruding process are essentially eliminated. So there is very little to chose now between polycarbonate and acrylic.

A word of caution about combining smaller windows into larger ones. Both materials expand and contract considerably due to thermal conditions the larger the pane the greater the expansion and hence they are much more difficult to seal.

GE4000 is a good alternative to the Corning 795. The joint surface of Acrylic if you decide to go that route should be abraded to provide a key to the adhesive.

[JPA Cate]

The other issue is that if the OP's plan is to remove the inner reinforcement the existing construction supplies to the deck house, it will be substantially weakened. However, he may actually be planning to place the acrylic or lexan on the exterior. Acrylic expands a lot with heat. He will have to solve the problem of keeping the larger panels in place while the glazing sets up. Screw holes need to be drilled more over size than you'd expect, because the metal and acrylic expand at different rates when exposed to warm air and sunlight.

Besides its higher cost, Lexan gets cloudy, and is a pain to keep see-through, although it is possible. Products from car shops for cleaning the plastic covers over headlights works. Also, as tkeithlu mentioned, Lexan is soft, it scratches very easily, and those scratches are time consuming and effortful to polish out.

I would think acrylic would do the job (it has for 20 yrs.). However, armored glass is also sometimes used, and it is very clear and strong. Heavy and expensive, though.

Ann

[RGN]

Hmm, I did not communicate clearly, yet again.


The window opening sizes will NOT be enlarged, except for minor smoothing of the Fiberglas edges.


There will simply be one large piece of glazing covering both window openings plus the area between the windows. The new glazing will also be an inch or so wider than the existing frames to allow for plenty of bedding area. The strength will be substantially higher than the existing windows.


The main reason for all this is to eliminate the leak prone bronze window trim/retainers, to allow for a thicker glazing and make the entire window stronger.



You are all quite correct about increasing the window sizes but that is not what is going to happen. Sorry I did not spell that out clearly.


So, cast acrylic for it's scratch and UV resistance seems to be ahead right now.


Thank you

[Marathon1150]

Cast acrylic is nearly as strong as Lexan but as noted less prone to crazing and scratching. The thermal expansion issue is significant so larger spans are contraindicated. I replaced all of our fixed port lights about 3 years ago and used marine silicone as a sealant. So far no leaks but each has a metal frame surround inside and out, through bolted. The friend who helped me with the replacement work, used Lexan and butyl tape on his boat. Crazing and leaking are the result.

[Bill whitmore]

Much experience with large plastic windows Lexan and acrylic- use GE1200 series silicone to handle expansion probs , every other solution I tried had issues

[PuttingDoctor]

Polycarbonate get’s my vote. I had plexi in the large hardtop openings that didn’t stand up well.
I used 1/8” on the outer 4 due to curves and 1/4” in the center to help support the opening hatch.
http://www.cruisersforum.com/forums/...ture30855.jpeg

[CHRIS V CLEMENT]

LEXAN MAR GARD is a product with an abrasive resistant coating available on both sides. Check it out...

[rbrentp]

2 cents: First went with Lexan, Two of my windows were about four feet long. the others about 2X2. Had to reseal about every two years in Gulf coast heat. 70 degree f temp range plus direct sun heating was more than any sealant I could find. Also, after 6 years, the surface had badly degraded. Just replaced with Acrylic. Made screw holes larger to accommodate expansion, Mounted where screw holes were with a marine grade butyl tape and then the outside half inch was Dow 795. In Florida, with three months of active rainy season. WIndows are great so far and dripless.

[tarian]

Avoided Carbonate , and used Acrylic for two large fron windows , got a nice blue tint to avoid glare, 7 mm fitted in with 3M marine sealant , no issues with leaks have taking some green water on the nose and windows no issues
The reason for acrylic is that you can polish out scuffs and scratches and will last longer in the Sun
Acrylic know is just as strong and can flex to a degree as my Windows had a slight curve in them

[Ken Pole]

I personally prefer polycarbonate, which I'm using on two deadlights (VHB tape and Dow 795 around outer edge) due to a slight bend. That said, poly evidently is only guaranteed for five years against UV -- so my supplier said.

[Pauls]

There is an important point that needs to be understood regards both your window design and the install. As others have mentioned, both materials have a high rate of thermal expansion. That means that the windows will brow in the heat and shrink in the cold, more than the structure you're bonding them to. The growth/shrinkage is directly proportional to the size of the windows, ie double the window size and the growth/movement of the window doubles. This movement is a cause of sealant failure. The sealant is forced to stretch with every temperature change. There are 2 things you can do to minimize the forces trying to tear your sealant off. One is to keep the windows smaller. Your original small windows will have half the thermally driven growth as the single large window. The other thing to do, and this is important, is to keep the sealant thickness between the window and the structure thick. A thick layer of sealant has the ability to stretch and accommodate movement. A thin layer cannot stretch much and so fails. Windows are glued onto modern skyscrapers. Window sealant thickness is commonly required to be 1/4" plus. Accomodating thermal expansion is why.