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Old 26-04-2023, 07:39   #1
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Frameless Cabin Windows - Poly or Acrylic?

I know there are tons of threads on the age old debate on material selection for windows but I've yet to see one address any concerns with using acrylic for frameless cabin windows and being too rigid?

I'm planning to replace the lenses in my Bomar hatches which are obviously framed with 3/8" acrylic. While I'm at it I decided to replace the main cabin windows since they are scratched and have some fogging. Originally they appear to be 3/16" polycarbonate and are roughly 8" x 28" each. Acrylic seems to be the preferred material for all lenses / windows by consensus of other threads but it has me wondering if there are any stress issues with using acrylic since its more rigid. Will that lack of flex break the seals around the windows? Planning to install with VHB and 791 if that makes a diff.

What are the advantages for using polycarbonate?
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Old 26-04-2023, 12:23   #2
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Re: Frameless Cabin Windows - Poly or Acrylic?

The main advantage is that polycarbonate is more shatter resistant. It can also be drilled and fastened in place with screws, which acrylic generally cannot due to the likelihood of cracking.


My last boat was a Hunter H26 that had acrylic fixed portlights at least 30" long with a moderate curve. They worked out OK. Most silicone sealants have poor compatibility with acrylic glazing and will deteriorate and lose their bond after 10 years or so, not sure if 791 is one of the good ones that can withstand that or not.
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Old 26-04-2023, 12:53   #3
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Re: Frameless Cabin Windows - Poly or Acrylic?

The movement/flex of polycarbonate makes sealing more difficult. Sealing, and the chance of bending enough to pop right through the frame, is why deck hatches are in acrylic. There is no chance I'd do polycarbonate on my boat unless captured within a frame or with bolts.
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Old 26-04-2023, 13:14   #4
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Re: Frameless Cabin Windows - Poly or Acrylic?

Polycarbonate weighs less, it is less prone to breaking per thickness. So 1/4 inch polycarbonate and 1/4 inch acrylic are not anywhere near the same thing. The acrylic will shatter and the polycarbonate will just shake around and not break on impact.

Polycarbonate scratches easier. It also is not UV resistant unless you specifically buy the UV resistant ones. Poly carbonate doesn’t really craze. Acrylic seems to craze more.

For small pieces like you are looking at, the thermal expansion is not as big of a deal really. I have 48 x 96 sheets. For windows. My thermal expansion is a very big deal. So I need frames. I’m pretty confident you can float those smaller windows you are talking about nicely using 791.

But that’s pretty much it. It’s just a personal choice really. You will have to go thicker in acrylic then you do it in polycarbonate to get the same durability. And that is what will weigh more.

It’s definitely up to your personal choices of what you consider necessary or unnecessary
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Old 26-04-2023, 19:41   #5
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Re: Frameless Cabin Windows - Poly or Acrylic?

I’m probably echoing what’s already been said. The toughness/impact resistance/shatter resistance of polycarbonate is hard to overstate. They make motorcycle helmets out of it.

My understanding is acrylic is better in basically every other way.
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Old 26-04-2023, 20:30   #6
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Re: Frameless Cabin Windows - Poly or Acrylic?

FWIW I have acrylic windows on my boat about the same size as you. Not sure how old they are, but they don't leak, and are due for replacement due to minor crazing. I will use acrylic again.

They are thick. I'm guessing about half an inch. There is a channel routed around the edge, and sealant (I believe silicone/791) in that channel to glue it to the cabin side. The channel allows the acrylic to be flush/tight against the cabin side and still have a thick 1/4" bead of silicone. Additionally, there are screws every 6-8 inches around the edge. You *can* screw acrylic, but the holes must be considerably oversized, and don't torque them. The silicone is the main thing that holds them in place. The screws I think are just safety for heavy knockdowns etc. or to hold them in place while the silicone dries.
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Old 27-04-2023, 10:51   #7
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Re: Frameless Cabin Windows - Poly or Acrylic?

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Originally Posted by Chotu View Post
Polycarbonate weighs less, it is less prone to breaking per thickness. So 1/4 inch polycarbonate and 1/4 inch acrylic are not anywhere near the same thing. The acrylic will shatter and the polycarbonate will just shake around and not break on impact.

Polycarbonate scratches easier. It also is not UV resistant unless you specifically buy the UV resistant ones. Poly carbonate doesn’t really craze. Acrylic seems to craze more.

For small pieces like you are looking at, the thermal expansion is not as big of a deal really. I have 48 x 96 sheets. For windows. My thermal expansion is a very big deal. So I need frames. I’m pretty confident you can float those smaller windows you are talking about nicely using 791.

But that’s pretty much it. It’s just a personal choice really. You will have to go thicker in acrylic then you do it in polycarbonate to get the same durability. And that is what will weigh more.

It’s definitely up to your personal choices of what you consider necessary or unnecessary
Curious by your comment on thickness. My original cabin windows are in a 60 deg orientation (cabin slopes 60 deg from cabin top to deck), poly and are 3/16". Any thicker and it won't be flush with the cabin. Saying I can't use 3/16 acrylic?
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Old 27-04-2023, 11:37   #8
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Re: Frameless Cabin Windows - Poly or Acrylic?

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Curious by your comment on thickness. My original cabin windows are in a 60 deg orientation (cabin slopes 60 deg from cabin top to deck), poly and are 3/16". Any thicker and it won't be flush with the cabin. Saying I can't use 3/16 acrylic?


yes. You are correct. If the original equipment was poly carbonate and you change to acrylic of the same thickness, you are not getting the same impact strength at all. However, maybe it’s not that important. It really depends if you are going to see some impact to those windows or not. Acrylic is more brittle. With the same thickness, the same impact it will crack. Poly carbonate is floppy, for use of a better word. It basically flops around a little bit upon impact instead of breaking.

That’s why bulletproof glass is actually poly carbonate. When the bullet hits it, it absorbs the impact and flops around and shakes. If it were acrylic, it would just shatter into 1 million pieces.
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Old 27-04-2023, 12:45   #9
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Re: Frameless Cabin Windows - Poly or Acrylic?

For most boat glazing the required thickness of acrylic and polycarbonate is similar. While sheets of polycarbonate are more impact resistant than acrylic, retaining this material is difficult because it is so flexible.
If the glazing material flexes excessively the glazing will pop out of the frame or overwhelm the glue joint, not good on a boat.

Generally for marine glazing the minimum required thickness for polycarbonate (governed by the the maximium amount of permitted flex) and acrylic (governed by the required impact resistance) is similar. Don’t think changing from acrylic glazing to polycarbonate will enable savings in the required thickness because this is usually not the case despite the strength advantage of polycarbonate.
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Old 27-04-2023, 13:44   #10
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Re: Frameless Cabin Windows - Poly or Acrylic?

Quote:
Originally Posted by Cdn_bacon View Post
I know there are tons of threads on the age old debate on material selection for windows but I've yet to see one address any concerns with using acrylic for frameless cabin windows and being too rigid?

I'm planning to replace the lenses in my Bomar hatches which are obviously framed with 3/8" acrylic. While I'm at it I decided to replace the main cabin windows since they are scratched and have some fogging. Originally they appear to be 3/16" polycarbonate and are roughly 8" x 28" each. Acrylic seems to be the preferred material for all lenses / windows by consensus of other threads but it has me wondering if there are any stress issues with using acrylic since its more rigid. Will that lack of flex break the seals around the windows? Planning to install with VHB and 791 if that makes a diff.

What are the advantages for using polycarbonate?
Hey I used half inch poly carbon routered 45 angles and with some elbow grease and progressive finer wproof sand paper to high gloss attached with 3m double side butyl tape ( check age) machine screws every 10 inch
since my windows are dark tint , expansion can be a problem if windows are very long,ike mine initially (6foot) so I redid in 2 sections so far so good no leaks
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Old 27-04-2023, 14:00   #11
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Re: Frameless Cabin Windows - Poly or Acrylic?

Best way to mount:

- route a rebate around the window on the inside surface for holding sealant.
- paint a black frame around the window on the inside and also the cut edge if it’s mounted proud (so that UV can’t enter the material there)
- study required thickness of adhesive for dimensions of the window. Heat expansion dictates this. Find the correct thickness 3M VHB tape or even use two layers if needed.
- mount rubber tape of adhesive thickness to the outside surround of the opening. This will close the gap from adhesive thickness on the inside.
- mount some boards to step the window on while mounting so that it is aligned correctly. Use hot glue etc.
- mount with VHB tape, which is hidden from view by the painted frame
- caulk the bead around the window for sealing it.
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Old 27-04-2023, 14:56   #12
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Re: Frameless Cabin Windows - Poly or Acrylic?

This is a very helpful and timely thread! Can you tell from the attached photos whether these are polycarbonate or acrylic? They are badly crazed and I want to find replacements. Also, has anyone used UPD Plastics to make new windows for their sailboat? They are in Ft Lauderdale.

https://www.updplastics.com/windshield-manufacturers/
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Old 29-04-2023, 09:42   #13
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Re: Frameless Cabin Windows - Poly or Acrylic?

I’ve found this blog to be helpful. From an expert on this:

https://hatchmasters.com/acrylic-vs-polycarbonate/
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Old 29-04-2023, 11:57   #14
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Re: Frameless Cabin Windows - Poly or Acrylic?

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I’ve found this blog to be helpful. From an expert on this:

https://hatchmasters.com/acrylic-vs-polycarbonate/
Thanks for the link. I think many boat manufacturers could benefit from reading (and putting into practice) the advice:

"Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficient of thermal expansion for Acrylic and Polycarbonate is in the neighborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job."

Leaking boats (especially saltwater) are not luxurious no matter how many extra gizmos are installed. Unfortunately many offshore cruising boats leak.
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Old 29-04-2023, 12:34   #15
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Re: Frameless Cabin Windows - Poly or Acrylic?

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Originally Posted by noelex 77 View Post
Thanks for the link. I think many boat manufacturers could benefit from reading (and putting into practice) the advice:

"Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficient of thermal expansion for Acrylic and Polycarbonate is in the neighborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job."

Leaking boats (especially saltwater) are not luxurious no matter how many extra gizmos are installed. Unfortunately many offshore cruising boats leak.

Which I learned the hard way.

Except, you actually can do 8 feet. But it has to be in a frame. That’s the only way for it to work.
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