# Acrylic or Polycarbonate for windows



## jephotog (Feb 25, 2002)

I am looking to replace the windows on my boat. The opening is about 11x32" in the widest of the dimension of each window.

I have heard arguments for the two materials. Just looking for opinions here.

Also thinking of adding a piece of the plastics to the companionway for extra light when sleeping on board. I will use the boards underway and the plastic when sleeping. What are the recommendations there?

Thanks


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## mitiempo (Sep 19, 2008)

Use acrylic (plexiglass). Lasts the longest, uv less of an issue, and probably less expensive. All major builders use acrylic. Lexan is not apparently available in less than sheet size with uv protection, without which it crazes within a few years.
My neighbor on a Spencer 35 has smoked plexi and it still looks new. Ports are original (1966).
Ideally the installation will not require holes in the acrylic, but if it does make sure to overdrill to allow for expansion. Sealant used is important - this is the only time I would use silicone on a boat as acrylics aren't compatible with many other sealants. Dow 795 structural adhesive is probably the best choice - better than silicone from Home Depot.
Brian


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## mitiempo (Sep 19, 2008)

The two large ports in the pic I posted above are installed in a recess in the cabin side in sealant and held in place with the bronze frame that you see. The bronze frame is attached to the cabin side with the holes through the frame and cabin side only, not the plexiglass.
Brian


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## sailingdog (Mar 19, 2006)

Also, it doesn't have the dimensional stability issues that Lexan does.  

And you really need to use a structural silicone adhesive, rather than just a silicone sealant. Dow 795 is an excellent one and was mentioned by Mitiempo. 

Given the size of the ports, I would recommend going with at least 3/8" thick acrylic... 

For the companionway, you can use either lexan or acrylic. Acrylic is a bit easier to work with IMHO and will last a bit longer.


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## IslandRaider (Oct 6, 2006)

My 2 cents worth, as was demonstrated to me by a friend and former boat yard owner, regular plexiglass will shatter when impacted an lexan will not, and most glass companies will cut lexan to order.


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## sailingdog (Mar 19, 2006)

While this is true... the ocean rarely causes impacts of this variety. I'd also point out that UV is a greater danger and that NO hatch manufacturer uses LEXAN in their hatches. All of them use either tempered glass or acrylic. The real problem is that any strong impact that is less than capable of shattering an acrylic port will generally cause a lexan port to flex and foreshorten enough to break the seal. Ports leaking is generally a much more common problem on a boat than them breaking.



IslandRaider said:


> My 2 cents worth, as was demonstrated to me by a friend and former boat yard owner, regular plexiglass will shatter when impacted an lexan will not, and most glass companies will cut lexan to order.


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## jephotog (Feb 25, 2002)

*Glad i asked*

I was recommended elsewhere to use lexan but thought Plexiglass would be better as i am more likely to scratch the window where they are than hit it with a hammer. Glad i asked here. Figured on plexiglass for the hatch as it has the boards for backup.

Here is a picture of the current window. The actual opening is more like 10" by 30 inches tapering to 7". The plexiglass will be bolted to the outside of the opening and sealed.

Should I still use 3/8" in this configuration?

If I overdrill the holes, how do insure the bolt is centered? Will there be a risk if not?

If I use the Dow 795, would bolts every 4" be good?


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## Faster (Sep 13, 2005)

Agree with the above... acrylic (heavy smoked - looks best from outside and still provides plenty of light below) and DC 795 works very well. We've used it for 'screw-on' applications as well as in-frame windows with good results.


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## zz4gta (Aug 15, 2007)

Acrylic for me as well. The heavy smoke looks really nice. I tried to use LEXEL sealant first, and that didn't stand up to the heat. I have black paint where the windows seal, and the sealant 'melted' even though it said it was for acrylic and outdoor use. 

The acrylic cleaned up w/ mineral spirits and I'm using VHB tape this time around. Don't like the look of screws/bolts. We'll see how it goes.

Edit: Also keep in mind that almost no surface on a boat is perfectly flat. This was another issue I ran into, the through bolts should help it conform, but I'd use the thinnest sheet you feel safe with. The thicker the sheet, the less it likes to conform to the boat contours.


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## Faster (Sep 13, 2005)

jephotog said:


> If I use the Dow 795, would bolts every 4" be good?


DC 795 is a pretty good adhesive, so bolts every 4 inches should be fine. (see pic above in previous post)

If you're going to cut the plexi yourself, make a plywood template and use a router for a better edge. It's usually worth while to take a template to a plastics place and have them cut it for you ... the fees are modest and they'll probably do a better job.

I'd avoid the temptation to use flat head countersunk bolts - pan heads will work best. Do overdrill the holes. Do not over tighten to the point of causing localized deflection of the plexi at the bolt location. Let the sealant fill the space. If you're worried about the bolted "look", carefully paint out the boltheads in black afterwards.

Dry-fit your new ports and trace the outline on the cabinside in pencil and at the same time trace the INNER profile on the protective paper on the inside. Remove the ports and carefully cut the line you traced on the inside, and peel off the outer rim of paper/plastic (leave the center portion in place, it will protect the inner window from squeezed out sealant) . Rough sand this outer inside surface to improve the adhesion bond. Mask off the interior cabinside and then go back outside and carefully mask off a line 1/8 inch or so outside the traced line on the cabin.. Paint out the hull surface as zzgata suggests (we did that) or be sure to lay on the (black!)sealant in a continuous solid layer inside the masked off area (which I've done on a subsequent project) this makes sure that you can't 'see' the lighter coloured glass under the installed plexi.

Use plenty of sealant, and simply wipe away what squeezed out onto the masking tape inside and out. Finally, lay a bead of sealant around the edge of the newly installed plexi, and with a soapy gloved finger smooth it out so it covers the joint and the outside edge as well leaving a 'wedge' of sealant that goes to the outside edge. This makes a good "shingle" to shed water from the cabinside out to the outside of the window, discouraging any path between the plexi and cabin.

Once all the excess sealant is cleaned up, the golden moment: peel away the protective paper/plastic and masking tape and admire your smartly trimmed, clean installation!


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## sailingdog (Mar 19, 2006)

one more point on drilling holes in acrylic or polycarbonate... you should probably chamfer/countersink the holes on both sides to help prevent them from becoming a starting point for stress cracks.


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## tdw (Oct 2, 2006)

Fast,
We're just about to redo the Womboat's ports so thanks for all that. I've done this before on previous boats but hadn't used the "leave the backing paper intact" idea, wish I had. 
SD recommends 3/8" perspex. Is that what you used ? I'm wondering whether 1/2" would be overkill. By the looks of things our ports are about the same length but not as big top to bottom as yours. 

Now here's an interesting question....our existing ports are two piece covering a single opening that has a vertical support in the centre. My thinking was to do the new ones in single piece cos ironically enough the only leaks we have are where the two pieces of perspex butt up to each other. 

There is no great curvature involved. Can anyone see any good reason not to go one piece ? They'd end up being about a metre (three feet) long.


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## Faster (Sep 13, 2005)

td.. we had a considerable (compound) curvature and ended up using 1/4" - for the sailing we do I think it's adequate and they are not that large an opening really. 

Don't see any problem going to a single lens in your case - our forward lense is pretty close to a meter long and narrower than the aft one. That mid-port seam just has to be asking for trouble.. Since your boat is built from plate, I'd expect no problem whatsoever getting plexi to follow the same shape.


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## bmccuej (Nov 8, 2009)

*Just an FYI*

Lewmar, the port & hatch manufacturer associated with Sailnet, uses acrylic in their products.


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## mitiempo (Sep 19, 2008)

Virtually every major port and hatch manufacturer and boatbuilder uses acrylic (plexiglass) as the lexan is destroyed by uv in a few years.


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## poopdeckpappy (Jul 25, 2006)

What about Lexan MR-10, it has great UV protection, great scatch resistance and exceeds tempered in the impact resistance


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## mitiempo (Sep 19, 2008)

Acrylic us also less subject to scratching.


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## GaryHLucas (Mar 24, 2008)

I am making a new set of bolt in place windows as we speak. The old ones were leaking, I removed them last week. I can see why, they were sealed with butyl tape. The plexiglas was bowed in at the bolts while the fiberglass bowed out at the bolts, leaving gaps beteen bolts. The Pan head bolts though had rubber washers under the heads, which seemed like a good idea.

I am most concerned about the seal around the windows though. In a previous career I researched sealing thing that have to have lots of room for expansion (aluminum against concrete) Everything I could find on the subject from the manuafacturers was moslty concerned with having the sealant THICKER than the motion needed. For silicone the sealant can take a motion equal to its thickness in any direction without it breaking loose. So my plan is to pad the windows out away from the hull using 1/8" rubber washers. Then I'll run a nice bead all the way around the plexiglass to form the seal. This should alow the window to move yet never break the seal. A fillet bead around the edge of a plexiglas sheet doesn't work because when the plexiglass moves it shears the sealant off.

I'll let you all know in about 5 years if it works!

Gary H. Lucas


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## mitiempo (Sep 19, 2008)

Gary
Remember to overdrill the holes. A neighbor in my Marina has redone most everything on his Beneteau including the windows. He is using a soft washer under the bolts that is "T" shaped when looking fron its side. The hole in the plexi is just large enough for the washer and the bolt a good fit inside the washer. I asked where he got them and he said they were original from Beneteau, but it looks like a good idea, must be able to source them somewhere.


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## GaryHLucas (Mar 24, 2008)

I'm using the old windows as a template, just route around around the outside with a flushcut bit and drill through the existing holes. All the existing holes were properly oversized. McMaster Carr sells ruber washers in any size or elastomer you could want, neoprene, EPDM, even Viton. EPDM has the best weather resistance I believe.

Gary H. Lucas


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## Incarnate (Dec 7, 2009)

*Use of Lexan*

I just wanted to post a quick note about the use of Lexan. We replaced the large-area ports (windows?) on our Fantasia 35 back in the mid 80s, using half-inch Lexan. The material came from the bulletproof windows in a bank, but since it was surplus or reclaimed, I don't know any more details on the exact type. We used those windows, full-time liveaboard, for maybe another 5 years, and never saw a single stress crack or any crazing.

The boat has been up on the hard since '91, in the direct Florida sunshine, and I visited it last year. The windows are still solid, and I didn't see any evidence of cracking or crazing, and that's with 20+ years of weathered exposure. The Lexan has some patchy tan discoloration from the many stainless through-bolts rusting lightly, but I don't expect that would be hard to clean up.

Perhaps we were just lucky on the cracking thing, or perhaps it's the really heavy thickness we used. Lexan definitely does scratch easily, and that was the biggest downside to our use. After a few years they were good for letting light in, but not for seeing through very clearly. I'll probably try polishing them when I get back to working on the boat.

For what it's worth..


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## poopdeckpappy (Jul 25, 2006)

> Lexan definitely does scratch easily, and that was the biggest downside to our use.


That why I mentioned the MR-10, it's superior to std lexan in both scartch & UV protection, I'm using it on our forward hatch and it's been there 2 yrs now


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## Faster (Sep 13, 2005)

Incarnate said:


> .....The boat has been up on the hard since '91, in the direct Florida sunshine, .....


Say that's a typo..... otherwise your boat's been on the hard for close to 20 years??? yoiks!


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## Incarnate (Dec 7, 2009)

Faster said:


> Say that's a typo..... otherwise your boat's been on the hard for close to 20 years??? yoiks!


Sad, but true . We lived on her from '81 to '91, but since then she's been about 1200 miles away from me. I hope to either move down there and fix her up, or find someplace to store/work on her up here, but for the moment she's still sitting.


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## jacaranda01 (Nov 27, 2014)

Which expands more in hot climates, acrylic or polycarboante??


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## mitiempo (Sep 19, 2008)

Acrylic does expand more that polycarbonate with temperature increases but doesn't scratch as easily nor craze as quickly. A good acrylic port installation is good for decades while polycarbonate needs to be replaced often to maintain optical clarity. That is the reason that almost all major hatch manufacturers use acrylic - the exception is Bomar on some models.


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## DonScribner (Jan 9, 2011)

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.

Installation:

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.


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