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jephotog 11-10-2009 01:32 PM

Acrylic or Polycarbonate for windows
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?


mitiempo 11-10-2009 02:40 PM

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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.

mitiempo 11-10-2009 02:43 PM

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.

sailingdog 11-10-2009 04:42 PM

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.

IslandRaider 11-11-2009 06:32 AM

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.

sailingdog 11-11-2009 07:18 AM

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.


Originally Posted by IslandRaider (Post 540751)
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.

jephotog 11-11-2009 12:17 PM

Glad i asked
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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?

Faster 11-11-2009 12:18 PM

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.

zz4gta 11-11-2009 04:02 PM

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.

Faster 11-11-2009 04:39 PM


Originally Posted by jephotog (Post 540851)
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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